Aristotle's Meteorology and Its Reception in the Arab World: With an Edition and Translation of Ibn Suwār's Treatise on Meteorological Phenomena and Ibn Bājja's Commentary on the Meteorology (Aristoteles Semitico-Latinus)

ARISTOTLE'S METEOROLOGY AND ITS RECEPTION IN THE ARAB WORLD

ARISTOTELES SEMITICO-LATINUS founded by H.J. Drossaart Lulofs is prepared under the supervision of the ROYAL NETHERLANDS ACADEMY OF AR TS AND SCIENCES as part of the CORPUS PHILOSOPHORUM MEDII AEVI p r o j e c t o f t h e UNION ACADÉMIQUE INTERNATIONALE.

The Aristoteles Semitico-Latinus project envisages the publication of the Syriac, Arabic and Hebrew translations of Aristotle's works, of the Latin translations of those translations, and of the mediaeval paraphrases and commentaries made in the context of this translation tradition.

General

Editors

H. DAIBER and R. KRUK Editorial

Board

H.A.G. BRAAKHUIS, W.P. GERRITS EN J. MANSFELD, C.J. RUIJGH, D.TH. RUNIA

VOLUME

/6 8 ^ '

10

ARISTOTLE'S METEOROLOGY AND ITS RECEPTION IN THE ARAB WORLD With an Edition and Translation

of Ibn Suwār^s

Treatise on Meteorological Phenomena and Ibn

Bājja's

Commentary on the Meteorology

BY

PAUL L E T T I N C K

'68^'

BRILL LEIDEN · B O S T O N · K Ö L N 1999

Library of Congress Cataloging in Publication Data is also available

D i e D e u t s c h e Bibliothek - C I P - E i n h e i t s a u f n a h m e Aristotle' s Meteorology and its reception in the Arab world : with an edition and translation of Ibn Suvvâr' s Treatise on meteorological phenomena and Ibn Bājja' s Commentary on the meteorology / by Paul Lettinck.- Leiden.;Boston ; Köln : Brill 1999 (Aristoteles Semitico-Latinus ; Vol 10) ISBN 90-04-10933-1

ISSN ISBN

0927-4103 90 04 10933 1

© Copyright 1999 by Koninklijke ΒήΙΙ NV, Leiden, TL· Netherlands All rights reserved. No part of this publication may be reproduced, translated, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission from the publisher. Authorization to photocopy items for internal or personal use is granted by Brill provided that the appropriate fees are paid directly to The Copyright Clearance Center, 222 Rosewood Drive, Suite 910 Damiers MA 01923, USA. Fees are subject to change. PRINTED IN T H E NETHERLANDS

CONTENTS ACKNOWLEDGMENTS

VII

SCHEMATIC SURVEY

VIII

INTRODUCTION

1

I.

STRUCTURE OF THE ATMOSPHERE; THE DOUBLE EXHALATION . . .

32

EL

PHENOMENA IN THE UPPER ATMOSPHERE

66

III.

PHENOMENA IN THE LOWER ATMOSPHERE DUE TO MOISTURE . . .

IV.

RIVERS AND THE SEA

97 120

V.

WINDS

156

VI.

THE INHABITABLE REGIONS OF THE EARTH

194

VII. EARTHQUAKES

209

VIII. THUNDER, LIGHTNING, HURRICANES, WHIRLWINDS AND

IX. X.

THUNDERBOLTS

225

HALOES, RAINBOWS, MOCK SUNS AND RODS

243

EXHALATIONS WITHIN THE EARTH; BOOK IV OF THE METEOROLOGY

301

SUPPLEMENT 1 : IBN SUWĀR - TREATISE ON METEOROLOGICAL PHENOMENA SUPPLEMENT 2 : IBN BĀJJA - COMMENTARY ON THE METEOROLOGY

315 . .

383

BIBLIOGRAPHY

482

INDEX LOCORUM

488

GREEK-ENGLISH-ARABIC GLOSSARY

495

INDEX OF NAMES AND SUBJECTS

498

ACKNOWLEDGMENTS I would like to thank Prof. H. Daiber (Amsterdam, Frankfurt) for his supervision, his willingness to make his extensive private library available for consultation, and his checking the editions of the Arabic texts and suggesting solutions for parts that made difficult reading in the manuscripts. Also, the corrections and additions that were suggested by him and Prof. R. Kruk (Leiden) have improved the text of this book to a considerable extent. I am also much obliged to Peri Bearman (Brill, Leiden) who corrected a large part of the English text. In addition, I am grateful to Prof. Jamil Ragep (Norman, Oklahoma, U.S.A.) for his interest in this work and his discussions with me. He suggested solutions for passages in Ibn Bâjja's text concerning astronomical subjects that were obscure at first reading. Part of the work was done during a nine month's fellowship at the Institute for the History of Science of the University of Oklahoma. I thank the staff of the Institute and its Library for giving me the opportunity to work with them. The research for this book has been made possible by funding from the Netherlands Organization for Scientific Research (NWO) and from the Rockefeller Foundation.

SCHEMATIC SURVEY We present here a schematic survey of the works on meteorology that are discussed in this book. An asterisk means that the work is not extant. Between brackets are the names of authors that influenced the work. Ibn al-Bitriq was used in all works mentioned under 'Arabic discussions of meteorology'.

1. Versions of Aristotle's text on meteorology Aristotle: Meteorology

Hellenistic version *

1 Hellenistic compendium * Syriac version * Syriac version * Ibn al Bitriq: Arabic version Ibn Suwār: Arabic version * (chapters on the halo and rainbow)

Hunayn Ibn Ishāq: Compendium

incorporated in Ibn Rusd's Middle Commentary

Gerard of Cremona: Latin translation

Ibn Tibbon: Hebrew translation, incorporated in Otot ha-Shamayim

2. Greek commentaries on Aristotle's Meteorology Alexander of Aphrodisias-j Arabic translation * Olympiodorus (Greek or Syriac) paraphrase Arabic translation * Hunayn ibn Ishāq: Olympiodorus' commentary on Aristotle's Meteorology (= Pseudo-Olympiodorus) Philoponus

3. Arabic discussions of meteorology Al-Kindl: letters on precipitation and wind Ibn Suwār: Treatise on Meteorological Phenomena (on the halo and rainbow) (Theophrast, Alexander, Olympiodorus, Pseudo-Olympiodorus) Ibn Sīnā: Κ. aš-Šifā', part of Tab. 4, Tab. 5 (Pseudo-Olympiodorus) School of Ibn Sīnā: Bahmanyār: at-Tahsīl Abū 1-Barakāt: al-Mu'tabar Fakr ad-Dīn ar-Rāzī: al-Mabāhit_ Ibn Bājja: Commentary on the Meteorology (incomplete) Ibn Rušd: Short Commentary al-Haytam, Ibn Bājja) Ibn Rušd: Middle al-Haytam)

(Alexander, Pseudo-Olympiodorus, Ibn

Commentary (Alexander, Pseudo-Olympiodorus, Ibn

INTRODUCTION

L Meteorological treatises in the Arab world Meteorological phenomena have aroused the interest of Arabicspeaking people because they were part of their daily life. The reflection of this 'practical' interest is found in the works of poets, grammarians and lexicographers. Various aspects of this Arabic meteorological tradition, such as astro-meteorological prognostication and the views on seasons, winds, clouds, precipitation and weather-signs have been investigated by Sersen in the first part of his dissertation on Arab meteorology. He also discussed superstitious beliefs connected with certain natural phenomena that were reported in cosmographical literature, such as miraculous springs and beliefs inspired by whirlwinds and waterspouts. Besides this practical tradition there also existed a theoretical one that considered meteorological phenomena within the framework of a theory intending to explain them. This tradition links up with Greek philosophy, especially that of Aristotle. The discipline dealing with meteorological phenomena from this point of view is known as the 'science of the upper phenomena' ('ilm al-āt_ār al-'ulwiyya) and appears under this name in the enumerations of sciences, such as Ihsā' al-'ulūm by a1-Fārābī (t 950), Mafātīh al-'ulùm by a1-Kwārizmī (//. 980) and the Rasâ'il of the Ikwān as-Safā' (end 10th century).1 The literature concerned consists of paraphrases of and commentaries on Aristotle's Meteorology and other treatises on meteorology inspired by Aristotle, written by philosophers such as al-Kindl (t ±873), Ibn Sīnā (t 1037), Ibn Bājja (t 1138) and Ibn Rušd (t 1198). The subject was also treated, with a different approach, by cosmographers, geographers, encyclopedists and by writers of adab literature ('belles lettres') and even in heresiographies and books on medicine. Authors belonging to this category are 'All ibn Rabban at-Tabari (t ±860) (Firdaus al-hikma), an-Nāši' (t 906) (Al-kitāb al-awsat), a1-Mas'ūdī (t 965) (Kitāb at-Tanbīh), al-Maqdisî (//. 960) (Kitāb Bad" al-kalq wa-t-ta'rîiç), Ibn al-'Amld (t 976) {Ar-rasā'il al-Amīdiyya ilā 'Adud ad-Dawla and Risāla fī l-humra fī l-jaww), Ikwān as-Safā' (end 10th century) (Rasâ'il), at-Tīfāší (t 1253) (Surūr an-nafs bi-madārik al-hawāss al-kams), a1-Qazwīnī (t 1283) 1

See Gardet, Anawati 106-111 for the lists of sciences by these and other authors.

(Kitāb 'Ajâ'ib al-maklūqāt), al-Watwât (t 1318) (Manāhij al-fikar wa-mabāhij al-'ibar\ ad-Dimašqî (t 1327) (Kitāb Nukbat ad-dahr fī 'agâ'ib al-barr wa-l-bahr), Najm ad-Dīn (//. 1332) (Jāmi' al-funūn wa-salwat al-mahzūn) and an-Nuwayrī (t 1333) (Nihāyat al-arab). These authors will not be our main interest in this book. Some information on what they wrote on meteorological subjects is to be found in Sezgin, GAS VII (e.g. on Jābir ibn Hayyān, 'All ibn Rabban at-Tabari, al-Mas'üdi, al-MaqdisI, Ibn al-'Amid and Ikwān as-Safā'), in Daiber's edition and commentary of Hunayn ibn Ishâq's Compendium of the Meteorology 2 (e.g. on 'All ibn Rabban at-Tabarī, an-Nāši', al-Maqdisi, Ikwān as-Safā', al-Qazwïnî, ad-Dimašqī and an-Nuwayrī) and in his edition and commentary of letters by Ibn al-'Amid.3 Sersen discusses a1-Mas'ūdī, al-Maqdisi, the Ikwān as-Safā', al-Qazwïnî, Najm ad-DIn and an-Nuwayrī. He investigates their accounts of the wind and of clouds and precipitation and concludes that they were all (except al-Maqdisi) influenced by Theophrastus' meteorological doctrines rather than those of Aristotle. Whether this is true or not will be discussed below pp. Ill and 176-177. Our main concern in this book will be the treatises on meteorology written by the philosophers that were inspired by Aristotle's Meteorology. These works include a number of letters by al-Kindï, the chapters on meteorological phenomena from the Kitāb aš-Šifā' by Ibn Sīnā and the Short and Middle Commentary on the Meteorology by Ibn Rušd. Two other works belonging to this category will also be discussed, namely the Treatise on Meteorological Phenomena by Ibn Suwār ibn al-Kammâr (t ±1030) and Ibn Bâjja's Commentary on the Meteorology. They have not been published before; they form important additional material for the study of how the Meteorology was received in the Arab world and have in their turn influenced other Arabic writers on meteorology. Therefore we have included an edition and translation of these works in this book. Furthermore we shall consider the meteorological sections of the encyclopedic works of Bahmanyār ibn al-Marzubân (t 1067) (Kitāb at-Tahsīl), Abū 1-Barakāt a1-Bagdādī (t 1165) (Kitāb al-Mu'tabar) and Fakr ad-Dīn ar-Rāzī (t 1209) (Kitāb al-Mabāhit_ al-mašriqiyya). We have grouped these authors under the heading 'School of Ibn Sīnā', because their books were inspired by and modelled on Ibn Sînâ's Kitāb aš-Šifā'. This does not imply that they always follow Ibn Sînâ's doctrines. It is well known that Abū 1-Barakāt and Fakr ad-DIn differ

2 3

Daiber 1975. Daiber 1993.

from Ibn Sīnā in various respects. Bahmanyār and Fakr ad-Din keep close to the text and phrasings of the Šifā'; Abū 1-Barakāt deviates from it to a large degree. The work of Ibn al-Haytam (t ±1040) is important for one special subject of the Meteorology, to wit the halo and rainbow, especially their geometrical description; much of his account of these phenomena has been adopted by Ibn Rušd. Therefore we shall consider his work and also later developments (after Ibn Rušd) in the research of the rainbow (see below pp. 285-287 and 298-300). We shall also discuss the Arabic paraphrase of Aristotle's Meteorology by Yahyā Ibn al-Bitrlq (t ±830), the compendium by Hunayn ibn Ishāq (t 876) and the work entitled Olympiodorus' Commentary on Aristotle's Meteorology, translated by Hunayn ibn Ishāq and revised by Ishāq ibn Hunayn. The latter work is not an Arabic translation of Olympiodorus' Greek commentary.4 Many parts appear to be a paraphrase and systematization of Olympiodorus' commentary, but it also contains features that are not in Olympiodorus. It is quite possible that what Hunayn translated was a Greek or Syriac work that was largely, but not exclusively, a paraphrase of Olympiodorus' commentary. The author of the work translated by Hunayn ibn Ishāq has been called Pseudo-Olympiodorus in this book. Yahyā ibn al-Bitrlq is mentioned by Hājji Kalîfa as translator of the Meteorology. The Fihrist of Ibn an-Nadīm mentions another translation, done by Ibn Suwār ibn a1-Kammār. Indeed, Ibn Suwār writes in his Treatise on Meteorological Phenomena that he has translated Aristotle's text from the Syriac, this translation forming the second part of a work, the first part of which is the treatise just mentioned.5 Probably Ibn Suwār did not translate the entire Meteorology, but only the part concerning the subjects dealt with in his treatise (the halo and the rainbow). The Fihrist also mentions Arabic translations of the Greek commentaries of Alexander (//. 200) and Olympiodorus (//. 540), neither of which are extant; we know that they were used by the Arabic authors on meteorology. It seems that the commentary of Philoponus (t ±570), which only comments upon Book 1,1-12, was not known in the Arab world.5 The Meteorology such as it appears in the Aristotelian corpus consists of four Books. There is no unanimity concerning the question whether Book IV of the Meteorology was written by Aristotle or not, 4

See Zimmermann, Brown. Ibn Suwār, Treatise on Meteorological Phenomena, see below p. 384,10-13. 6 See Peters, Aristoteles Arabus 39-40 for further data on the Arabic versions of the Meteorology and other meteorological treatises. 5

although most modern commentators take the view that it is genuine. If so, it was written before Books Ι-ΙΠ. The subject has no connection with the subject of Books Ι-ΠΙ: these Books treat the phenomena that occur due to the two exhalations that are dissolved from the earth by the sun; Book IV treats various properties of matter and homoiomerous bodies, and has rather a connection with De Generatione et Corruption than with the preceding Books of the Meteorology, as has been remarked by Alexander and Ibn Rušd (see below pp. 310 and 312). The exhalations are not mentioned in Book IV (except in one place, that is probably a later addition). Book IV may be considered as a separate treatise by Aristotle that was later added to the Meteorology. Because of this special relation of Book IV to the rest of the Meteorology we shall not discuss it. Instead, we shall pay some attention to what Ibn Sīnā and his school say about minerals (see next paragraph), a subject announced by Aristotle at the end of Book III. One would expect a discussion of this subject in what follows on Book III rather than what is now in Book IV. In fact, there is no treatise on minerals by Aristotle. The Meteorology (Books I-III) was translated into Latin by Gerard of Cremona (t 1187) from the Arabic version of Ibn al-Bitriq in the second half of the twelfth century (vetus translatio). The fourth Book had already been translated before, by Henricus Aristippus (t 1162), from the Greek. A new translation of the whole book, directly from the Greek, was made by William van Moerbeke after 1268 (nova translatio). Often three chapters were added to copies of the Latin versions under the title De Mineralibus·, these chapters also occur in manuscripts as a separate treatise. Their subject matter is the formation of stones and mountains and different kinds of minerals, discussed under the titles De congelatione et conglutinatione lapidum, De causis montium and De quattuor speciebus corporum mineralium. These chapters were ascribed to Avicenna, Geber or Aristotle. They were translated around 1200 by Alfred the Englishman (Alfred of Sareshel), who also wrote a commentary on the Meteorology. In 1910 Stapleton discovered that they were in fact translations of parts of Ibn Sînâ's Kitāb aš-Šifā', and the Arabic text was subsequently published and translated into English by Holmyard in 1927. A partial Italian translation was given by Baffioni.7 Samuel Ibn Tibbon (+ ±1230) has translated Ibn al-Bitriq's version of the Meteorology into Hebrew under the title Otot ha-Shamayim. His book is not a mere translation, but he adduced other works, such as the commentary of Alexander and the Short Commentary of Ibn Rušd, in 7

Baffioni 1980.

order to elucidate obscure passages of Ibn Bitrlq's text.8 Ibn Tibbon's work, recently edited and translated by Fontaine, is useful for the emendation of some of the corrupt passages in the extant text of Ibn al-Bitriq's version. The Syriac and Arabic translations of Theophrastus' Meteorology 9 were edited, translated and commented upon by Daiber. A survey of everything that is known of Theophrastus' works on natural philosophy, including meteorology, was given by Steinmetz.10 Moreover, all texts in which Theophrastus has been quoted have been edited.11 Theophrastus' views on cosmological and meteorological questions differ from those of Aristotle in many respects. The above-mentioned works make a survey of Theophrastus' doctrines on meteorological phenomena superfluous here. We shall occasionally pay attention to his views if they are brought up by other authors. The present book contains a survey of the contents of the abovementioned works. This will make clear how various scholars influenced each other and also what each particular author added to the work of his predecessors. It will show, in short, how Aristotle's Meteorology was received and transformed by these scholars. As was said above, two treatises are edited here for the first time as supplements to this book. They form important and interesting additional material for the study of the reception of the Meteorology in the Arab world. It will become clear, for instance, how Ibn Suwār gave a survey of views of Aristotle, Theophrastus and the Greek commentators and added some original features; furthermore, how Ibn Bājja rejected an Aristotelian theory which was contradicted by observed facts and transformed it to a new theory which was better adapted to the facts and how Ibn Rušd adopted Ibn Bäjja's views. Ibn Suwār ibn a1-Kammār was a pupil of Yahyā ibn 'AdI in Baghdad (second half of the 10th century). Later the ruler of Khwārizm took him to his court and when a new ruler took power, he was taken to Ghazna (beginning of the 11th century). He studied logic, medicine and philosophy,12 and was also concerned with meteorology. He translated Theophrastus' Meteorology from the Syriac (edited and translated by Daiber in 1992), and wrote a work on meteorological phenomena 8

See Fontaine X-XI and XXXIX-LXXI. 'Daiber 1992. 10 Steinmetz 1964. 11 See Theophrastus of Eresus. Sources for his life, writings, thought and influence, ed. by W. Fortenbaugh, P.M. Huby, R.W. Sharpies, D. Gutas, 2 vols, Leiden 1992, 19932. Ibn Suwār is known as the editor of the Arabic translation of several of Aristotle's logical works, see Walzer, Greek into Arabic 66-113; see Daiber 1992 220-221 for some more information about him, with the relevant references.

caused by watery vapour such as the halo an rainbow. The latter work consisted of three parts: a general discussion of these phenomena, a translation (from the Syriac) of Aristotle's text dealing with these phenomena, and a detailed commentary on this text. Only the first part of this work has been preserved and that is the treatise edited and translated in Supplement 1. Ibn Bājja (Avempace) was the first commentator of Aristotle in Spain. He had an important influence on Ibn Rušd (Averroes), who was considered to be The Commentator by the Latin scholastics. Apart from commentaries on logical works of a1-Fārābī which in turn are treatises on Aristotle's logical works, Ibn Bäjja wrote treatises on Aristotle's Physics, On Generation and Corruption, Meteorology and On the Soul, and he further wrote the biological treatises Book of Animals and On Plants. A bibliography of Ibn Bäjja's work, including a list of sources which give information about his life and work, was composed by al-'Alawî (1983). He also lists the contents of the manuscripts that contain Ibn Bäjja's works. As an addition to the above-mentioned bibliography we mention a new edition, with Spanish translation, of Ibn Bäjja's Kitāb al-kawn wa-l-fasād (Book on Generation and Corruption) by Puig Montada (1995). His Commentary on the Meteorology, which is edited and translated below in Supplement 2, confirms his importance as a critical interpreter of Aristotle and as a precursor of Ibn Rušd. See further my book on Aristotle's Physics for some more information on Ibn Bājja, the manuscripts that contain his work, etc.13

2. Authors on meteorology What follows is a brief description of the commentaries, criticisms and additions that were written in connection with Aristotle's Meteorology by the scholars studied here. It gives a survey of how each author contributed to the reception and transformation of Aristotle's Meteorology in the Arab world. The contents of these contributions, ordered by subject-matter, will be summarized in the next section of the Introduction. See also the schematic survey on pp. VIII-IX. 2.1. The Greek commentators Alexander of Aphrodisias mostly gives a faithful rendering of Aristotle's text. His main point of criticism is Aristotle's explanation why wind moves horizontally along the surface of the earth. He adopts 13

Lettinck 6-8.

Theophrastus' view on this subject (see below pp. 161-162). He also uses Theophrastus' explanation of the formation of precipitation (see below p. 100) and of the halo (see below p. 252). Olympiodorus' commentary tends to be a systematization of Aristotle. The result contains additions to and differences with Aristotle. Some differences are due to certain special interpretations of the text, others are points of criticism against Aristotle's opinion. Examples of the former concern the questions why the sea is salt (see below pp. 130-133) and why wind moves horizontally (see below p. 163). An example of the latter is Aristotle's view that the Milky Way is a phenomenon caused by ignition of smoky exhalation in the upper atmosphere (see below p. 74). An addition is for example Olympiodorus' explanation of certain questions concerning the halo and the rainbow that were left unexplained by Aristotle (see below pp. 256-257 and 261). The commentary on earthquakes, thunder, lightning and hurricanes is lacking, that on whirlwinds is incomplete. Philoponus' commentary, as was noted above, only covers Book 1,1-12. He criticizes Aristotle's view that the sun is not hot and that it heats the earth by the motion of its sphere, through friction with the sublunar world. His view is that the sun itself is hot and heats the earth by means of its rays (see below pp. 42-44). He also criticizes, like Olympiodorus, Aristotle's view of the Milky Way (see below pp. 73-74). 2.2. Ibn al-Bitriq and Hunayn ibn Ishāq Ibn al-Bitriq's text is a distorted, incomplete and sometimes confused version of Aristotle's text, as was already noticed by Ibn Tibbon.14 Also, it contains passages that do not correspond to any text of Aristotle, but are a kind of commentary or explanatory additions; such passages sometimes give a special interpretation of Aristotle's text or even show a different point of view. For instance, Ibn al-Bitriq explicitly states that the Milky Way is a phenomenon belonging to the celestial world, contrary to Aristotle's opinion (see below pp. 76-77). Furthermore, he distinguishes three exhalations, instead of Aristotle's two (see below p. 46). Endress shows in his review of Petraitis' edition of Ibn al-Bitriq's version of the Meteorology that such differences with Aristotle's text indicate that a Hellenistic author has adapted and transformed Aristotle's text according to his own views.15 It was this Hellenistic text that was translated, probably into Syriac first, and then into Arabic. Other differences with Aristotle seem rather due a to misunderstanding 14 15

Ibn Tibbon, Otot ha-Shamayim 1,17-23. Endress 506-509.

or misinterpretation of the text, as occurs for instance in the accounts of the saltness of the sea, earthquakes, and the halo and the rainbow. Sometimes mistranslations give rise to statements that differ from Aristotle. There is no final answer on the question how the original Aristotelian text became what is Ibn al-Bitriq's version. A comparison with Aristotle's Greek text shows that the transformation is partly due to misunderstandings and mistranslations and partly to commentatorial additions and differences of opinion, as was said above, and it is clear that Ibn al-Bitriq did not directly translate or paraphrase Aristotle's text. It is probable that a Greek (Hellenistic) treatise existed based on Aristotle's text, which contained additional explicative passages and in which the author adapted the text according to his interpretation and views. This treatise will have been the text that possibly was first translated into Syriac and then into Arabic by Ibn al-Bitriq.16 It is also possible that a Syrian author was responsible for the adaptation of Aristotle's text. This could explain the deviations that are apparently due to an inaccurate or wrong reading of the Greek text. The distortions, additions and other deviations from Aristotle survive in works that are related to or mainly based on Ibn al-Bitriq's text. This holds for Hunayn's Compendium, which is probably based on the same Greek treatise as Ibn al-Bitriq's version, as we shall see in the next paragraph; it also holds for Ibn Rušd's Middle Commentary, which for a large part consists of a more or less literal rendering of Ibn al-Bitriq's version, and for Ibn Tibbon's Otot ha-Shamayim, which contains the translation of Ibn al-Bitriq's version into Hebrew. Ibn Rušd and Ibn Tibbon had other copies of Ibn al-Bitriq's text than the ones that are extant now; they contain additional passages that are lacking in our copies. In some cases Ibn al-Bitriq's extant text may be improved using Ibn Tibbon's translation. The understanding of Ibn al-Bitriq's text also profits from Ibn Rušd's Middle Commentary, but emendation of the text is not possible, because Ibn Rusd's rendering of Ibn al-Bitriq's text is not perfectly literal and because he inserts explanatory remarks of his own between the text, without specifying where his own commentary starts and Ibn al-Bitriq's text ends. Ibn Tibbon, on the other hand, clearly distinguishes between his own remarks and the translated text of Ibn al-Bitriq. Hunayn ibn Ishäq's Compendium at first sight seems to be an abstract from Ibn al-Bitriq's version: often the same formulations and expressions are used, also in passages that do not correspond to any text in Aristotle. However, it cannot be a mere abstract from Ibn al-Bitriq: 16

See Daiber 1975 15-17.

the order in which the subjects are treated is different, the phraseology is often different, sometimes the contents are different and a different opinion is propounded by Hunayn. Although he is generally shorter than Ibn al-Bitriq, he also has passages that do not correspond to any text in the latter's work. The comparison between Ibn al-Bitriq and Hunayn in this book confirms what Daiber found from his comparison of both versions, namely that Hunayn's treatise is a translation of a shorter (Greek) version of the Greek treatise that formed the basis of Ibn al-Bitriq's version.17 Furthermore, Daiber has shown that Hunayn's text existed in a Syriac version in the second half of the ninth century, as the text was used by Mose Bar Kepha (t 903). It is possible that Hunayn was the Syriac translator and that someone of his school made the Arabic version.18 2.3. Pseudo-Olympiodorus Pseudo-Olympiodorus' treatise (translated by Hunayn ibn Ishāq) is neither a translation of, nor merely an excerpt from Olympiodorus' commentary; 19 many parts are a paraphrase and systematization of Olympiodorus' commentary, but it also contains features that are explained in another way than by Olympiodorus and Aristotle or that do not occur in their works at all. The result is a systematic account of the Meteorology, in which Olympiodorus' commentary has been used extensively, but not exclusively. This is clear, for instance, from the account of the sea (see below pp. 138-141) and of the halo and rainbow (see below pp. 267-274). Possibly the work translated by Hunayn was a Greek or Syriac work that was largely a paraphrase of Olympiodorus' commentary, with material taken from other sources as well. 2.4. Al-Kindī Al-Kindï's contribution consists of some letters on a couple of selected subjects, namely precipitation and wind. His views on these subjects are different from those of Aristotle and Theophrastus and demonstrate an independent way of thinking (see below pp. 107-111 and 176). 2.5. 1kwān as-Safā' and al-Qazwīnī The views of these authors on precipitation and wind will be treated in the present book, because it has been a subject of discussion whether 17 18 19

Daiber 1975 6-17. Daiber 1991 47-49. See Zimmermann, Brown.

they were influenced by Theophrastus or by others like al-Kindi. We shall see that an influence of Theophrastus cannot be demonstrated, whereas an influence by al-Kindi is quite probable (see below pp. Ill and 176-177). 2.6. Ibn Suwār ibn al-Kammār Ibn Suwâr's Treatise on Meteorological Phenomena discusses the halo, the rainbow, mock suns and rods. He also translated Theophrastus' Meteorology, edited and translated by Daiber in 1992. The explanations in his Treatise are mostly Aristotelian, but his text contains several features that do not correspond to any text in Aristotle and are taken from Alexander, Olympiodorus and Theophrastus. These authors are mentioned by him, but it appears that he also knew the commentary of Pseudo-Olympiodorus (see below p. 276). A few features could not be traced back to earlier commentaries. The text we have of this Treatise is the first part of a more extensive work, of which the other parts are not preserved. Ibn Suwār announces that the second part is his translation of the Aristotelian text from Syriac and the third part a detailed commentary on the text. Probably he means a translation and commentary on the part of the Meteorology dealing with the halo, rainbow, etc. 2.7. Ibn Sīnā Ibn Sînâ's views on meteorological phenomena are basically Aristotelian, although he has deviating views on several subjects. His account in the Š i f f f is quite different from Aristotle's account; the organization of the material, the division into chapters and the phraseology all deviate from Aristotle. Many passages do not correspond to any text in Aristotle at all. From a few instances it appears he knew Ibn al-Bitriq's version. A more important source was the treatise by Pseudo-Olympiodorus: there are numerous correspondences between this text and that of Ibn Sīnā. On the whole, Ibn Sînâ's work gives the impression of a scholar with an independent mind, who relates observations he made himself and is willing to admit that he does not know how to explain a phenomenon rather than follow an opinion of which he has seen the impossibility (e.g. about the colours of the rainbow; see below pp. 281-283). Ibn Sīnā criticizes Aristotle, for example concerning the heating of the earth by the sun: this does not occur by the motion of the sun's sphere, but by means of the sunrays that have a heating power (see below p. 56). Furthermore, he disagrees with Aristotle about the possibility of inhabitation of the tropics: he thinks that the climate

is moderate there, whereas according to Aristotle that region is uninhabitable because of the heat; in connection with this, he presents a theory explaining why different latitudes of the earth are differently heated (see below pp. 196-198). He also criticizes Aristotle's explanation of the colours of the rainbow, admitting that he does not know a correct explanation. He presents additional explanations for the formation of wind (see below pp. 177-178), earthquakes (see below p. 218) and thunder (see below pp. 234-235). 2.8. School of Ibn Sīnā Bahmanyär's account is mostly an abstract from Ibn Sînâ's Šifā'. Abū 1-Barakāt has some quotations from the Šifā', but on the whole he differs from Ibn Sīnā in formulation and phrasing, often also in opinion. He says that phenomena such as shooting stars, comets, the halo and the rainbow cannot be explained by terrestrial causes only, but that in fact they are caused and maintained by celestial forces (see below pp. 83-85). The same holds for wind: it is air moved by celestial forces (see below pp. 183-184). Furthermore, he disagrees with Ibn Sīnā and Aristotle about the explanations of the formation of hail (see below pp. 114-115), rivers (see below pp. 146-147) and thunder (see below p. 237). The Milky Way is a phenomenon of the celestial world, not of the upper atmosphere, as Aristotle thinks (see below p. 85). On the whole, the account of Abū 1-Barakāt is quite original and independent.20 Fakr ad-Dîn ar-Rāzī follows the discussion of the Šifā' and gives many quotations from it. On the other hand, he clearly disagrees on certain matters, for instance on the way Ibn Sīnā determines the wind-directions (see below p. 186) and on the statement that the temperature of the region near the equator is moderate (see below pp. 203-204). 2.9. Ibn al-Haytam Ibn al-Haytam differs from the other authors considered here because his works cannot be considered to be commentaries on or paraphrases of Aristotle. He is included here because he wrote a treatise on the halo and rainbow that was used by Ibn Rušd. His geometrical treatment of these phenomena is an improvement compared to that of Aristotle, although it did not contribute to a correct explanation (see below pp. 285-287). 20

See for Abú 1-Barakāt's originality in other fields: S. Pinès, Studies in Abu l-Barakāt al-Bagdādī, Physics and Metaphysics, The Collected Works of Shlomo Pinès, vol. I, Jerusalem, Leiden 1979.

2.10. Ibn Bājja Ibn Bäjja's Commentary on the Meteorology is not a commentary in the sense that Aristotle's text is followed and commented upon wordby-word, such as the Greek commentaries on the Meteorology of Olympiodorus and Philoponus and such as the 'Long Commentaries' of Ibn Rušd. It is instead comparable to Ibn Rusd's 'Short Commentaries', which are a kind of paraphrase: the subjects brought up by Aristotle are discussed by the author in his own formulation and style, with his own examples and with digressions that contain discussions that are not in Aristotle at all. Indeed, there are several passages in Ibn Bäjja's Commentary on the Meteorology and Ibn Rusd's Short Commentary on the Meteorology that are similar in structure and formulation (see below pp. 91-92). However, Ibn Bäjja's treatise is mostly less systematic than that of Ibn Rušd and seems to suppose that Aristotle's text is already known to the reader (see below p. 58). Also, what is extant of Ibn Bäjja's Commentary on the Meteorology is very incomplete. The only complete account is that of the Milky Way. According to Ibn Bäjja's theory, this phenomenon is caused by a combination of a celestial and a terrestrial cause: the light of the Milky Way comes from stars in the heaven; that it appears as an elongated patch of light is caused by refraction of this light in the upper atmosphere with its fire and smoky exhalation (see below pp. 86-88). This theory is adopted in Ibn Rusd's Short Commentary. It appears that Ibn Bājja used Ibn al-Bitriq's version of the Meteorology. 2.11. Ibn Ru'sd Ibn Rušd first wrote a Short Commentary and later a Middle Commentary on Aristotle's Meteorology. The former treatise is a kind of paraphrase, the latter follows the text (in Ibn al-Bitriq's version) closely.21 Both commentaries are based on Ibn al-Bitriq's version and from both commentaries it appears that he knew and used the commentary of Alexander, the treatise of Pseudo-Olympiodorus and Ibn Sînâ's work. Sometimes opinions expounded in the Short Commentary are renounced in the Middle Commentary. For instance, the theory on the Milky Way in the former commentary, which he adopted from Ibn Bājja, is criticized in the latter commentary. As for the Short Commentary, it is clear that he used Ibn al-Bitriq. For instance, Ibn Rušd also distinguishes three exhalations (see below

21

For a discussion of Ibn Rusd's different kinds of commentaries and their relation, see Gätje 1985 23-31.

p. 62) and also locates the origin of the light of the Milky Way in the celestial world. He notes that Alexander (in fact: Aristotle) had a different view on the Milky Way, namely that it is an ignition of smoky exhalation in the upper atmosphere. In fact, Ibn Rušd adopts Ibn Bäjja's theory of the Milky Way and his account is very similar to that of Ibn Bājja (see below pp. 90-92). This view is expounded and then renounced in the Middle Commentary (see below pp. 94-96). Ibn Rušd criticizes Ibn Sînâ's opinion that the area near the equator has a moderate climate and presents an extensive refutation of Ibn Sînâ's arguments (see below pp. 205-206). As for the geometrical explanation of the halo and the rainbow, he follows Ibn al-Haytam; he defends Aristotle against Ibn Sînâ's criticism concerning the colours of the rainbow (see below pp. 289-290 and 291-292). The Middle Commentary is largely a rendering of Ibn al-Bitriq's version, with explanatory remarks and commentaries inserted between Ibn al-Bitriq's phrases; also whole sections are added that are not in Ibn al-Bitriq. The additions and commentaries are often taken from Alexander's commentary, which gives a more faithful version of Aristotle's text than Ibn al-Bitriq. Some additions are from PseudoOlympiodorus. The general result is more comprehensible and more in agreement with Aristotle than Ibn al-Bitriq's text; on the other hand, sometimes Ibn Rušd is still misled by the misunderstandings that made Ibn al-Bitriq's text corrupt and confused. When Ibn Rušd inserts commentary of his own between phrases of Ibn al-Bitriq's text, he does not distinguish between this text and his own additions. Therefore it is not possible to decide whether more correct readings in Ibn Rusd's text are due to the fact that he had a better copy of Ibn al-Bitriq's text than that which is known to us or to improvements which he made himself, possibly on the basis of his knowledge of Alexander. Anyway, the copy which Ibn Rušd used was another than those which are extant now. Ibn Rušd found discrepancies between Ibn al-Bitriq and Alexander. In such cases he often does not decide between them; he accepts them as two possible explanations, or tries to give an explanation that combines both. This occurs for example in his treatment of winds (see below pp. 191 and 193) and of earthquakes (see below p. 223-224).22 We finally remark that a Latin translation of the Short Commentary exists in the Iuntas edition of the 'Works of Aristotle with the commentaries of Averroes'. Of the Middle Commentary only a few sections are included in this edition; they are enumerated in the following table. 22

These cases are also discussed in Fontaine, Otot ha-Shamayim LXX.

Arabic

Latin

Arabic

Latin

43,5-49,19 51,8-62,2 89,5-18 100,20-104,11 108,7-110,5 not in Ar.

408F-409K 411M-414C 427F-H 435C-436B 436B-I 436I-437K

120,2-14 110,13-1183 140,7-146,14 149,9-150,15 16045-163,9 158,10-160,12

437K-438A 439M-441F 451A-452C 458E-H 458H-459G 459C-L

2.12. Ibn Tibbon Ibn Tibbon has translated Ibn al-Bitriq's version of the Meteorology into Hebrew, with remarks and commentary of his own added to iL23 He used a copy of Ibn al-Bitriq's text that was different from the copies we have. This text may be improved on the basis of Ibn Tibbon's translation. Ibn Tibbon, unlike Ibn Rušd in his Middle Commentary, clearly distinguishes between his translated text and his own remarks and commentary. He has compared Ibn al-Bitriq's text with the commentary of Alexander and also used the Short Commentary of Ibn Rušd; a few times he refers to the Šifā' of Ibn Sīnā. Ibn Tibbon often found Ibn al-Bitriq's version confused and defective and he noticed discrepancies between this version and what he found in Alexander's commentary. These discrepancies are due to misunderstandings and mistranslations and also to differences of interpretation and opinion that have arisen in the course of the process that turned the original Aristotelian text—of which Alexander gave a faithful account—into that of Ibn al-Bitriq. At other occasions Ibn Tibbon just reproduces the distorted or defective version of Ibn al-Bitriq without further comment, e.g. the incomplete account of the comets. Ibn Rušd gives a complete account of this subject in his Middle Commentary. We cannot decide whether his copy of Ibn al-Bitriq's text was more complete than that of Ibn Tibbon and the extant copies or whether he took his more complete text from Alexander (see below pp. 75-76).

3. Subject-matter of the Meteorology This section presents a summary of the discussion of the various subjects of the meteorology by Aristotle and later authors. It gives a survey of the content of their contributions to the reception and transformation of Aristotle's Meteorology in the Arab world. 23 Ibn Tibbon's version was edited and translated by Fontaine in 1995. What is said in this paragraph is extensively explained in the introduction of her book.

3.1. Structure of the atmosphere; the double exhalation The meteorological phenomena discussed by Aristotle in his Meteorology are the phenomena that occur between the earth and the lowest celestial sphere (of the moon); some phenomena on and below the surface of the earth are also included. Therefore, he first investigates what exactly occupies the space between the earth and the sphere of the moon (= the atmosphere). He states that the solar heat dissolves two exhalations: a vapourous, moist, cold exhalation from the water on the earth and a windy, smoky, hot, dry exhalation from the earth itself. Both exhalations move upward; first, they remain together and their combination forms the layer of air, but then the smoky exhalation, being hot, rises above the heavier vapour; this exhalation fills the layer adjacent to the celestial sphere; it is an inflammable material (ύπέκκαυμα), a kind of fire. In this way the atmosphere is filled by these two exhalations; they are the material source of all meteorological phenomena and the principle that unites them, from comets to earthquakes. The vapourous exhalation is considered hot by most commentators. Aristotle is not completely consistent about it; for instance, he says that vapour is hotter than water, because it still contains the heat by which it was dissolved and that makes it rise, but on the other hand he also says that air is moist and hot, as it is composed of moist, cold vapour and hot, dry smoke. One might ask whether Aristotle with this description of the structure of the atmosphere means that there is nothing but moist and dry exhalation in the atmosphere and thus renounces the existence of air and fire as independent elements, or considers the exhalations to be different from and existing besides the usual air and fire. Most commentators adopt the latter, traditional view, but certain passages in Aristotle support the former view. Anyway, Aristotle clearly states that the upper layer of the atmosphere, adjacent to the sphere of the moon, is not real fire, but inflammable hot, dry exhalation (ύπέκκαυμα). Aristotle brings up the subject of cloud formation and this leads to a further division of the stratum of air: the air that directly surrounds the earth is hot due to the reflection of the sunrays against the earth; no clouds are formed here. Then follows a cold layer where the reflection of the sunrays has no effect anymore. Clouds may be formed here. This layer reaches to the summits of the highest mountains. Higher up the air is moved along with the daily motion of the celestial sphere and no clouds are formed there. Going further upwards one will find that the air will become warmer as one approaches the ύπέκκαυμα, the stratum

of inflammable hot, dry exhalation that is adjacent to the lowest celestial sphere. A fortiori no clouds are formed here, nor in the ύπέκκαυμα itself: the latter does not even contain moist exhalation. The ύπέκκαυμα is hot due to the presence of the hot, dry exhalation and due to its ignition by the motion of the adjacent celestial sphere. The sun is not hot itself, but confers heat to the earth by the motion of its sphere, by means of the friction of that sphere with the upper layers of the atmosphere. Another cause of heating is that parts from the hot ύπέκκαυμα are scattered downward by the celestial motion. According to Olympiodorus vapour and smoke are intermediate stages between the elements, e.g. vapour is hotter than water and cooler than air. They exist in the atmosphere besides the usual air and fire. Philoponus explicitly says the same about vapour only. As for the heating of the earth by the sun, Alexander wonders how the motion of the sun's sphere can heat the atmosphere by friction, when there is the sphere of the moon between them. However, he adheres to Aristotle's view, saying that the moon's sphere merely passes on the effect, without being affected itself, like a burning glass passes on sunrays—which are able to burn a body—without becoming hot itself. Philoponus raises the same objection against Aristotle as Alexander and other objections as well. He concludes that the sun is fiery and that its heat comes together with its light. Olympiodorus adopts Aristotle's explanation of heating by the motion of the sun's sphere, but he adds heating by sunrays as a second cause, otherwise he cannot explain why more heating takes place during the day than at night and why heating varies in accordance with different seasons. The ambiguity in Aristotle concerning the question whether the moist exhalation is hot or cold makes itself felt in Ibn al-Bitriq's version: he distinguishes three exhalations: a moist, cold one, a moist, hot one and a dry, hot one, that rise into successive layers of the air, the moist, cold one rises into the lowest layer and may turn into rain; the moist, hot one rises into the next layer and turns into air; the dry, hot one rises into the highest layer and may be inflamed, as it is close to the fire and the celestial motion. The exhalations exist besides the already existing air and fire. Above the layer of air into which the hot, dry exhalation rises, there is still the layer of fire, differently from Aristotle. The three exhalations are not consistently maintained throughout Ibn al-Bitriq's version: in other places just two exhalations are distinguished. Hunayn follows Ibn al-Bitriq's account, but omits the layer of fire. The systematizing tendency of the account of Pseudo-Olympiodorus appears from the fact that he states that either exhalation occurs in a

thin, rare and in a thick, dense version, so that four different exhalations may be distinguished, each giving rise to a specific phenomenon. According to al-Kindi the exhalations arise besides the usual elements. They are hotter than the elements from which they arise; this heat is conferred to them by the sun, adheres to them by accident and that is what makes them rise. Ibn Suwār ibn a1-Kammār, like Olympiodorus, states that the exhalations are intermediate states between the elements. According to Ibn Sīnā the two exhalations arise besides the usual elements and are an intermediate stage between the elements. The element fire remains pure: nothing is mixed with it. The exhalations are mixed with the element air. The result is that air consists of various layers: a vapourous layer, a layer of pure air and a smoky layer. Within the vapourous layer one may distinguish a lower part that is hot due to heating by reflected sunrays and an upper part that is cold; that is where clouds may be formed. The smoky layer is adjacent to the layer of pure fire. The sun itself is not hot; heating by the sun occurs because the sunrays have a heating power; the sunrays are not corporeal and are not hot or fiery, but they heat the body on which they shine. Heating does not occur by motion of the sun's sphere. Ibn Bājja distinguishes, like Pseudo-Olympiodorus, a thick and a thin version of both exhalations. The dry, thin exhalation is a kind of fire, the dry, thick one is smoke; both versions of the moist exhalation are called vapour. He also distinguishes air below the mountain tops and above them; the latter moves together with the daily celestial motion, which makes the matter thinner. Heating by the sun occurs by a combination of the motion of the sun's sphere and reflection of the sunrays, similar to Olympiodorus' view. Ibn Rušd in his Short and Middle Commentary follows Ibn al-Bitriq in distinguishing the same three exhalations, although in other places he just mentions two exhalations. The heating by the sun occurs in the way mentioned by Olympiodorus and Ibn Bäjja: motion of the sun's sphere combined with reflection of the sunrays. In the Middle Commentary he states that the sunrays have no heating power by themselves. The heating actually occurs by the motion of the sun's sphere and the emission of light accompanies the process by accident. The heat and the rays of light behave in the same way; for instance, if the rays are reflected or concentrated by a burning mirror, the same occurs for the heat.

3.2. Phenomena in the upper

atmosphere

These phenomena include, according to Aristotle, burning flames, shooting stars, torches, 'goats',24 comets, the Milky Way and certain colour effects in the sky (blood-red colours, chasms and trenches). All these effects arise when hot, dry, fiery exhalation in the upper atmosphere, adjacent to the celestial sphere, forms clusters and these clusters are ignited by the motion of the celestial sphere. The result depends on the form and density of the ignited material. For instance, if the material is scattered, not dense, we see a shooting star: the ignition starts in a certain place, the burning material ignites the material adjacent to it, and is subsequently extinguished; then the next adjacent material is ignited and so on; this gives the impression of a moving star, whereas in fact different parts of inflammable material are ignited and extinguished successively. If the material is denser, the ignition remains stationary; then we see a comet that stays in the same place for a while. Sometimes the material is gathered and ignited under a star and the ignition follows the motion of that star, so that a comet is formed, of which the fringe is something in the upper atmosphere, whereas the star itself is in the heaven. Something similar occurs for a collection of stars in a certain part of the heaven, a part where the stars are large, numerous and close together. These stars collect fiery exhalation in the upper atmosphere below them; the ignition of this exhalation is seen as the Milky Way. Aristotle's theory of the Milky Way is criticized by Philoponus and Olympiodorus (the latter followed by Pseudo-Olympiodorus). They point to the fact that the Milky Way appears as something that is always in the same state. It is difficult to see how this could occur if it consists of ignited exhalation, for the dissolution of exhalation will depend on the season (in summer more exhalation is dissolved). How could the distant stars constantly draw up so much exhalation? Moreover, the Milky Way should be subject to parallax, i.e. have a different position in relation to the fixed stars for observers in different places on earth; this is not observed. These commentators do not explicitly state their theory of the Milky Way; they most likely concluded that the Milky Way is the light of a collection of stars. Ibn al-Bitriq's account of the comets is very incomplete. Ibn Tibbon reproduces this incomplete version, but Ibn Rusd's Middle Commentary gives a more complete version. It cannot be decided whether he had a more complete copy of Ibn al-Bitriq's text than that of Ibn 24

'Goats' are a kind of meteorites; their appearance resembles the flocks of wool that hang down from a goat; see below p. 67 and Olympiodorus, in Meteor. 38,2.

Tibbon and our copies or whether he got his additional text from Alexander. About the Milky Way Ibn al-Bitriq explicitly states that it is a phenomenon in the celestial world. The light of the Milky Way is, in fact, the light of the stars and it does not have its origin in the upper atmosphere. We get the impression—but this is not unambiguously stated—that the ignited exhalation also plays a part: it causes the light of the stars to become a continuous patch of light. Ibn Sīnā elucidates the process of ignition occurring in the sphere of fire: the pure fire itself, which is mere heat, does not emit light. It is only when inflammable matter, i.e. smoke, is in contact with the fire that light is emitted. It is extinguished when the ignited matter gives out; then only pure fire is left. In general, a burning fire may also be extinguished when its principle (the heat) is changed by cold or moistness, but this does not occur in the upper atmosphere. Ibn Sīnā does not mention the Milky Way among the phenomena that are caused by ignition of smoky material. He probably considers it a phenomenon in the celestial world. Abū 1-Barakāt says that shooting stars, comets and other phenomena like mock suns and rods occur by ignition of smoky, dry exhalation in the upper atmosphere. The forms of these phenomena are stable for a certain period of time, which can only be explained if one assumes that the ignition is caused and retained by celestial forces. Also the halo and the rainbow are caused by celestial forces working in a cloud when the light of the sun or the moon shines on it. According to Abū 1-Barakāt and Fakr ad-Din, the Milky Way cannot be a phenomenon in the upper atmosphere, because then it would be subject to parallax. Instead, it consists of starlike bodies that are too small to be seen separately. The Milky Way is the only subject from the Meteorology that is completely and properly treated by Ibn Bājja. He criticizes Aristotle's view with objections similar to those presented by Olympiodorus and Pseudo-Olympiodorus. Then he says, following Ibn al-Bitrlq, that the light of the Milky Way originates from a collection of stars, but the upper atmosphere with its fire and smoky exhalation also has a share in its formation: the light of the stars is refracted in these upper layers of the atmosphere and that causes the light to form a continuous patch. Ibn Rušd in his Short Commentary adds his own objections against Aristotle's view of the Milky Way to those already mentioned by Ibn Bājja and adopts Ibn Bäjja's explanation of this phenomenon: light from the stars is refracted (or reflected—this is not unambiguously clear from Ibn Rusd's text) in the upper layers of the atmosphere. The latter explanation is what Ibn Rušd considers to be Aristotle's explanation. Aristotle's actual view, which Ibn Rušd criticized, was known to him

from Alexander's commentary. In the Middle Commentary the view adopted in the Short Commentary is expounded and then criticized, for Ibn Rušd does not see how light from the stars can be reflected in the fiery layer of the upper atmosphere, because reflection only occurs against dense matter. He gives three other possible explanations, but says that he cannot decide between them, since the essential nature of the Milky Way is not evident to us. 3.3. Phenomena in the lower atmosphere due to moisture When moisture rises into the lower atmosphere, different kinds of precipitation may be formed. This occurs, according to Aristotle, when moist exhalation rises and cools down in the cold region of the atmosphere which is no longer heated by the sunrays that are reflected from the earth. The exhalation is densified into clouds and condenses into rain or freezes into snow. The same occurs for exhalation that during the day does not rise very far and cools down during the night. Then dew and hoarfrost are formed. Hail is formed when drops of water freeze in a cloud or during their fall as rain, due to cold that is concentrated by surrounding heat; this occurs when the cloud is not very high and the season is not very cold (spring or autumn). The process in which a quality is concentrated or driven away if it is surrounded by or confronted with the contrary quality is called άντιπερίστασις. It also explains why rain falls in summer in hot places like Arabia and Ethiopia. Theophrastus adduces compression as another cause of condensation besides cooling. Therefore, precipitation may ensue if wind drives clouds together or pushes them against a mountain. Alexander combines άντιπερίσιασις and compression to explain the rain in Ethiopia: the clouds are formed elsewhere and the wind drives them together against the mountains, so that they densify; the άνιιπερίσχασις changes them into rain. Olympiodorus explains the rain in Ethiopia in a similar way, although elsewhere he explicitly denies the possibility of condensation by compression; there he argues that rain occurs in Thebes, where it is hot and where there are no mountains; cooling and άντιπερίσχασις are the only causes of rain. Pseudo-Olympiodorus explains rain in Ethiopia by compression only, not by άντιπερίστασις. Ibn Sīnā, too, recognizes compression as a second possibility for condensation and follows Pseudo-Olympiodorus in his explanation of rain in Ethiopia by compression. On the other hand, for the explanation of hail Pseudo-Olympiodorus and Ibn Sīnā follow Aristotle in using άνιιπερίστασις.

A1-Kindī describes the formation of precipitation in different ways. (1) There is a horizontal motion of exhalation (moist and dry) or air, which is caused by expansion due to the sun's heat: the air will flow from the place where it is expanded to a cooler place where it contracts; this horizontal flow is wind and when it arrives in a cold area, it densifies and the moist part turns into rain. (2) There is a vertical motion of exhalation that rises from the earth to cold layers of the atmosphere. This follows Aristotle's description. Both Ibn al-Bitrlq and al-Kindi give Aristotle's explanation of hail in spring and autumn, but add that hail may also be formed in clouds in cold strata of the atmosphere; then the drops of water are frozen by the surrounding cold. This possibility is also mentioned by Pseudo-Olympiodorus and Ibn Sīnā. Abū 1-Barakāt has a special explanation of hail: it is formed when falling snowflakes are pushed together by wind and coalesce into circular hailstones. 3.4. Rivers and the sea Aristotle says that rivers are fed not only by precipitation, but also by water formed by condensation of subterranean moist exhalation that rises in hollow places inside the earth. The condensed drops of water gather and eventually emerge from the earth as a spring. Most authors considered here follow Aristotle. Hunayn ibn Ishāq, however, does not mention subterranean condensation; according to him, all the water of rivers comes from the usual precipitation that collects below the surface of the earth and emerges from it when its quantity has sufficiently increased. Abū 1-Barakāt's view also is that there is no subterranean condensation; the water of rivers originates from precipitation that collects below the surface of the earth and forms a supply to keep rivers flowing, also in dry periods. The sea is, according to Aristotle, the element water in its proper place, existing eternally together with the rest of the world. The saltness is due to an admixture of earthy particles that are carried up in the dry exhalation. When this dry exhalation rises together with the moist one, it will be mixed with the condensed moist exhalation that falls down as rain. In this way the earthy particles arrive in the sea. They are salty, because they are like ashes, sweat or urine: a residue of a process of combustion, sc. the process that occurs when earth is heated by the sun and dry exhalation is dissolved. The saltness remains constant because salty particles are drawn up with the evaporation of sweet water from the sea.

Olympiodorus interprets Aristotle's text in a different way; he says that two processes contribute to the saltness: evaporation of the light, sweet part of the seawater, so that the dense, salty part remains, and the dissolution of dry exhalation from the earth. The latter is either directly mixed into the sea or rises with the vapour and then descends, after condensation of the latter, mixed with rain. Ibn al-Bitrlq gives a rather distorted and confused version of Aristotle's text. According to his text, light, moist exhalation is evaporated from the sea and dense, hot moistness is left behind. When heat affects what is left behind, it becomes salty. One must assume that Ibn al-Bitriq means that dry exhalation is mixed with the water that is left behind after evaporation of the light part, but this is not explicitly stated and it is not made clear where this dry exhalation comes from. The lack of clarity of this version makes itself felt in the Compendium of Hunayn ibn Ishāq and the Middle Commentary of Ibn Rušd, which are based on the version of Ibn al-Bitriq. The views of Pseudo-Olympiodorus and Ibn Sīnā are clearer: according to Pseudo-Olympiodorus, two causes contribute to the saltness: Aristotle's cause, sc. dry exhalation that carries earthy particles, and earthy particles by themselves. Ibn Sīnā only mentions admixture of (burnt) earthy particles, not dry exhalation. Ibn Rusd's Short Commentary follows Pseudo-Olympiodorus when he gives smoky exhalation and earthy particles as the substances that contribute to the saltness. 3.5. Winds According to Aristotle, wind is not air that is in motion, but dry exhalation that is dissolved from the earth by the heat of the sun. Moisture of the earth supports such dissolution, therefore wind starts to blow especially after rainfall or melting of snow. The wind moves horizontally because the exhalation is moved along with the motion of the upper air, which in turn moves along with the circular motion of the celestial sphere. None of the later authors will follow this explanation of the horizontal motion. Alexander justly remarks that this would mean that all winds have the same direction. Therefore he preferred Theophrastus' view, which holds that besides light, dry exhalation also heavy, moist exhalation is a constituent of the wind. The combination of the upward motion of the former and the downward motion of the latter results in a horizontal motion. Actually, Theophrastus also holds that wind is air that moves back to the place from where it was driven away under the influence of the sun's heat. Olympiodorus rejects Theophrastus' view. He adheres to Aristotle, but, in order to meet

Alexander's objection, he says that Aristotle means that the rising dry exhalation hits upon the moving upper air, is thrust back, and thereby gets a horizontal motion, not necessarily in the direction of the moving air. Pseudo-Olympiodorus adds another explanation: the rising exhalation is thrust back by air that contains much moist exhalation; on its way down it meets other rising exhalation. The combination of descending and rising exhalation gets a horizontal motion. Al-Kindfs theory of wind is neither Aristotelian nor Theophrastian. In fact, he gives two explanations of the origin of wind. (1) When the sun is above a certain place and heats it, the air expands; then the air flows from that place to a cooler place where it contracts; this flowing of air is wind. (2) Exhalation rises vertically from the earth; when it reaches cold layers of the atmosphere, it is densified; the moist exhalation becomes water (rain and other precipitation), the dry exhalation becomes earth; these earthy particles push the air by their weight and this motion of air is wind. Ibn Sīnā follows Aristotle insofar as he states that the matter of wind is dry, smoky exhalation. The explanation of the horizontal motion is inspired by Pseudo-Olympiodorus, with some additions. The rising exhalation gets a horizontal motion because it is thrust back by the circular motion of the upper air or because it arrives in a cold layer, becomes heavy and descends again; when it meets other rising exhalation, it will get a horizontal motion. Sometimes wind is formed before the smoky exhalation has reached a cold region or the moving air; it gets a horizontal motion when it hits upon other winds. When air is occasionally moved, e.g. when it is expanded by the sun's heat, this is not wind in the proper sense. Abū 1-Barakāt rejects Ibn Sînâ's theory of wind with respect to what its matter is as well as to how it is moved. Abū 1-Barakāt says that wind is moving air, but he rejects al-Kindî's view that it is moved because of to expansion by the sun's heat. He says that the motion of the wind is caused by celestial forces, not by cold or heat on earth. Ibn Rušd in his Short Commentary agrees with Aristotle that wind is smoky exhalation. As for its horizontal motion, Ibn Rušd adopts one of the solutions mentioned by Pseudo-Olympiodorus: when the exhalation arrives in a cold, moist area of the atmosphere, it cools off, becomes heavy and descends; it joins the rising exhalation and the combination results in a horizontal motion.

3.6. The inhabitable regions of the earth Aristotle claims that there are two inhabited regions on earth: the region between the tropic of Cancer and the parallel circle that is the projection of the circle delimiting the circumpolar stars in the northern hemisphere and the corresponding region in the southern hemisphere. The other regions are too cold (near the poles) or too hot (the tropics) for habitation. Ibn Sīnā has quite a different opinion. He says that the region near the equator is a moderate region, not extremely hot, whereas the regions near the tropical circles suffer from extreme heat and cold. The reason is that the heat in a certain place is determined not only by the position of the sun in relation to the zenith, but also by the period of time the sun remains in this position. If one considers a place near the tropic of Cancer, then the sun, when it is at the summer solstice, spends a longer time near the zenith in that place than when it is near the zenith at the equator. Moreover, the days are longer in that place, so that the atmosphere is heated up more. The inhabitable area extends at least until the equator. South of it there must be mostly sea, for the surface of the sea must exceed the surface of the land.25 Abū 1-Barakāt says that the heat in a certain place on earth is primarily determined by the duration of the sunshine, or in other words, by the length of the day. An additional reason is the distance of the sun to the zenith. The heat depends on these two factors combined. One of the results is that the area near the equator is moderate. Fakr ad-DIn disagrees with Ibn Sīnā and Abū 1-Barakāt on the climatic condition of the region near the equator: if indeed the heat is determined by the duration of the sunshine combined with its position in relation to the zenith, then the area of the equator must be very hot, because the sun is never far from the zenith during the whole year; the sun is also near the zenith in places near or north of the tropic of Cancer, but this occurs only around the time of the summer solstice; for the rest of the year the sun is far from the zenith. Ibn Rušd in his Short Commentary disagrees with Ibn Sīnā: the area of the equator is extremely hot and uninhabitable. The heat at a certain latitude is determined by the height of the sun, but that is not the only factor. An area remains hot during a certain time after the sun has reached its highest position, e.g. for a period of three months in al-Andalus. The more to the south, the longer this period, because the heating by the sun is more intense to the south. Thus, at the equator 25 There must be more water than earth in the world, for the quantities are such that if all earth turned into water, this would have to result in the actual amount of water, and a certain volume of earth turns into a larger volume of water.

the heat must be extreme, because three months after the sun has left the zenith it starts to approach it again. Ibn Rušd agrees with Ibn Sīnā about the regions in the southern hemisphere. That area must be water, otherwise there would be more land than water in the world, for the tropical region is land anyway, due to its dryness. 3.7. Earthquakes They arise, according to Aristotle, if dry exhalation does not become a wind above the earth, but turns inside the earth and becomes a subterranean wind which shakes the earth. All other authors considered here follow Aristotle's view; Ibn SInä adds that an earthquake may also be caused by flowing water or collapsing caverns. Ibn al-Bitriq gives a distorted version of Aristotle's text. He adds that earthquakes especially occur at times of rain, because then the earth becomes densified and clogged up, so that the exhalation cannot escape from inside the earth. This relation between moist earth and earthquakes is also mentioned by Pseudo-Olympiodorus, Ibn Sīnā and Ibn Rušd. 3.8. Thunder, lightning, hurricanes, whirlwinds and thunderbolts All these phenomena are related in Aristotle's theory. Hot, dry exhalation, when it rises together with vapour, may be caught in the cooling vapour when it becomes a cloud. When the cloud condenses, the hot exhalation is ejected and strikes against the surrounding clouds, thereby producing a noise, which is thunder. The ejected exhalation usually becomes inflamed and this is lightning. A hurricane is a wind that emerges from a cloud, similar to the wind that causes thunder; the difference is that in the case of a hurricane the wind forms a denser and more compact body. When the wind that is formed in a cloud does not emerge from it and collides with other winds in the cloud, it will get a circular motion. Finally it will descend, dragging the cloud with it. This is a whirlwind. When the constitution of the wind emerging from a cloud is rare, it becomes inflamed. This is a thunderbolt. In the version of Ibn al-Bitriq the wind that causes thunder does not emerge from the cloud, but moves about within the cloud. The noise that occurs when it strikes against the moist parts of the cloud is thunder. When it catches fire, we see it as lightning. Lightning may contain earthy parts, therefore it may descend as thunderbolt. Al-Kindi's account resembles that of Ibn al-Bitrlq: wind moves about inside the cloud and shakes it; that is the noise of the thunder. The moving wind inflames the earthy, lower part of the cloud and that is lightning. If the impulse of the wind is so strong that it reaches the earth, a thunderbolt

has arisen. Ibn Sînâ's account is similar to those of Ibn al-Bitriq and al-Kindi. He adds that the extinguishing of inflamed wind inside a cloud may also cause the noise of thunder. Abū 1-Barakāt does not mention the theory of Ibn Sīnā or Aristotle; he says that maybe the old theory is correct that claims that clouds knock against one another and strike fire. Ibn Rusd's Short Commentary follows Aristotle's view of dry exhalation that is ejected from a cloud. The sound that accompanies the ejection is thunder. 3.9. Haloes, rainbows, mock suns and rods These phenomena arise, according to Aristotle, when the light of the sun or the moon is reflected against, respectively, particles of mist and waterdrops in a cloud.26 A halo is formed when mist is located between the light source and the observer; a rainbow arises when a cloud with its waterdrops is located opposite the light source in relation to the position of the observer. Colours arise when rays of light are weakened because they have to travel a long distance or when the colour of a dark reflecting surface is mixed with the colour of the incident light. Therefore colours arise in the rainbow and not in the halo, because the rays have to cover a longer distance in the case of the rainbow than in that of the halo and also because the reflection occurs against (dark) waterdrops, not against (light) particles of mist. The more light is weakened, the darker the resulting colour will be. The different colours of the rainbow are explained as being due to different degrees of weakening of light, but Aristotle is not able to give a consistent explanation of their order, in particular when also the order of colours in the secondary rainbow is considered. Aristotle's geometrical proofs that the halo and rainbow must be circular and that the rainbow always appears as half a circle or less, depending on the height of the sun, are correct, but he cannot explain why reflection apparently occurs in certain places of the mist or cloud only, forming a ring or a part thereof; in other words, he cannot explain why only rays are reflected that are at a specific angle with the axis connecting the observer with the light source. The problem mentioned in the previous phrase is dealt with by Theophrastus in his theory of the halo. He says that when the light of the moon falls on the mist vertically below, the particles are swept sideways equally in all directions and gather along a circle, as if they were dust that is blown away by someone blowing through a pipe. 26

Actually, Aristotle does not use rays of light, but visual rays, i.e. rays that extend from the eye to the object that is seen.

Alexander presents a similar solution: vapour that is vertically under the light source becomes rarefied and thin under the influence of the light; this occurs equally in all directions until a certain distance from the axis that joins the observer and the light source. Particles of mist gather in a ring perpendicular to this axis, against which rays of light are reflected. Olympiodorus does not use a process in the mist itself, as Theophrastus and Alexander did. He says that rays close to this axis are more powerful and therefore are not reflected, but continue straight through the mist. Rays that make a certain angle with the axis are weaker and are reflected, thus forming the halo. Pseudo-Olympiodorus and Ibn Suwār quote the solutions of both Alexander and Olympiodorus. Ibn Suwār mentions Theophrastus' theory of the halo as being the 'physical explanation' of the circular form, besides Aristotle's 'geometrical explanation'. Olympiodorus notes that Aristotle's explanation of the order of colours in the primary and secondary rainbow is not consistent. He adduces a solution, which is in fact the same one as the one he adduced for the question why the halo is a ring, not a disc (see the previous paragraph): if we look at the double rainbow, the axis of the cone formed by our visual rays falls between the two rainbows. Along this axis the rays are most powerful; they are not weakened, therefore no colour is seen there. Weakening occurs for the rays that make an angle with this axis; the larger this angle, the greater the degree of weakening. Therefore the colours red, green and purple successively arise in regions of the cloud that are more and more removed from the point where this axis intersects the cloud, both below (the primary rainbow) and above (the secondary one) that point. Pseudo-Olympiodorus brings forward Olympiodorus' solution, but on the other hand also states that the secondary rainbow is a reflection of the primary one, something that was explicitly denied by Olympiodorus. Ibn Suwār follows Olympiodorus' solution. Ibn al-Bitriq's version of Aristotle's account of the halo and the rainbow is very distorted and confused. Sometimes the text may be improved on the basis of Ibn Rusd's Middle Commentary and Ibn Tibbon's Otot ha-Shamayim. According to Ibn Sīnā, the rainbow is formed by reflection of light, not in a cloud, but in air that contains a spray of small waterdrops, with some dark object as background; this background could be a cloud, a mountain, etc. Ibn Sīnā reports that this is confirmed by his own observations: he has seen rainbows at places where there was no cloud, but a mountain as background. He rejects Aristotle's explanation of how the colours arise and why they are ordered as they are; he

admits that he does not know how this should be explained. Abü 1-Barakāt does not have a physical explanation for the colours of the rainbow either. He considers the halo and the rainbow to belong to the same class as shooting stars and comets: light effects that are formed and retained by celestial forces. If these forces work in clouds, the halo and the rainbow may be formed by means of the light of the sun and the moon. Ibn al-Haytam's theory of the halo and the rainbow is Aristotelian insofar as he claims that they occur by reflection against moist particles in a cloud. He solves the above-mentioned problem why reflection apparently only occurs at certain parts of the cloud by specifying the plane against which the reflection occurs and by applying the law of equal angles that holds for reflection. In this way he shows that these phenomena must be a circle, i.e. that reflection does not occur against all points of the cloud, but against the points of a circle only. The colours arise because reflection occurs against layers of the cloud that are at different depths inside the cloud. Ibn Rušd in both his commentaries follows the geometrical explanations of Ibn al-Haytam for the halo and the rainbow. In his explanation of the colours of the rainbow he defends Aristotle against Ibn Sînâ's criticism. His explanation is mostly Aristotelian, but the details are rather confused due to the confusion in Ibn al-Bitriq's text. The correct paths of rays of light in raindrops that are responsible for the formation of the rainbow were found in the 14th century by Kamäl ad-Dīn a1-Fārisī. Simultaneously, but independently, the same was found in the West by Dietrich von Freiberg. The quantitative calculation that explains the radius of the rainbow and an explanation of the colours would later be given by Descartes and Newton. 3.10. Exhalations within the earth; Book IV of the Meteorology Aristotle concludes the discussion of phenomena that arise due to the exhalations with some remarks on what is formed within the earth: if dry exhalation affects earthy matter, then minerals like stones, sulphur, etc. are formed; if moist exhalation is compressed and solidified by dryness, then metals (meltable and malleable substances) are formed. Metals contain an admixture of earthy matter. Aristotle announces a further discussion of metals, but a treatise on this subject by him does not exist. As was noted above (p. 4), Ibn Sīnā wrote about minerals (including metals) in his Kitāb aš-Šifā'. He divides minerals into stones (neither malleable nor meltable), metals (malleable and meltable), sulphurs (not soluble in water) and salts (soluble in water). He counts

mercury among the metals, because when a metal is melted, mercury is the result. If mercury is mixed with sulphur, it solidifies and turns into one of the six metals (gold, silver, copper, iron, tin and lead), depending on the purity of the components. Ibn Sīnā denies the possibility of alchemy: he thinks it is impossible to produce the metals from mercury and sulphur artificially, as we do not know the essential properties that characterize the different metals. Abü 1-Barakāt denies that mercury is a constituent of metals, because it does not mix with other substances and is not affected by fire. He denies, with Ibn Sīnā, the possibility of alchemy. Fakr ad-DIn thinks that alchemy is possible and refutes a number of objections that were raised against it by others. As for the place of Book IV of the Meteorology in the corpus of Aristotle's works, Alexander, Ibn Sīnā and Ibn Rušd considered it to connect to De Generatione et Corruptione. Olympiodorus, PseudoOlympiodorus and Ibn Bājja considered it to be a sequel of the previous Books I-III of the Meteorology. According to Ibn Bājja and Ibn Rušd, a treatise on minerals should follow Book IV.

4. Conclusion The following conclusions can be drawn from our study: The Arab scholars considered in the present book knew Aristotle's text via the paraphrase by Ibn al-Bitriq. This paraphrase is a far from faithful rendering of the Greek text. It appears, however, that several other texts were available to them: the commentaries of Alexander of Aphrodisias and Olympiodorus, the treatise of Pseudo-Olympiodorus and Theophrastus' Meteorology. Their names are explicitly mentioned by Ibn Suwār, except, of course, that of Pseudo-Olympiodorus, as this treatise was known as Olympiodorus' Commentary'. Ibn Rušd has compared Ibn al-Bitriq's text with Alexander's commentary and mentions the latter's name whenever he finds discrepancies between these texts, such as in the sections on the Milky Way and the winds. In fact, these discrepancies are due to the deviations of Ibn al-Bitriq's text from that of Aristotle: the latter's text is more faithfully rendered by Alexander than by Ibn al-Bitriq. The treatise of Pseudo-Olympiodorus is never explictitly mentioned, but it had a considerable influence, especially on Ibn Sînâ's discussions of meteorological phenomena. For the rest, Ibn Sīnā is basically an Aristotelian, but he is quite willing to observe the phenomena under discussion himself; this sometimes brings him to criticisms against Aristotle, e.g. in the explanation of the rainbow.

Also, because they realized that the observed facts were not in accordance with Aristotle's view on the Milky Way, the scholars under consideration here (except Alexander) rejected Aristotle's view that the Milky Way is a sublunar phenomenon. Those who expressed themselves on this subject considered it to be a phenomenon which is formed by stars that are closely together. Ibn Bājja and Ibn Rušd tried to combine both theories, saying that the Milky Way is caused by the celestial stars, but that the upper (sublunar) atmosphere plays an essential part in the explanation of its appearance. Ibn Rušd reconsiders this view in a later stage of his life. A similar adaptation of Aristotle's views occurred with his opinion that wind is dry exhalation that rises from the earth and then gets its horizontal motion because it is moved along with the circular motion of the upper atmosphere. The majority of Aristotle's successors retained the view that wind is dry exhalation, but his explanation of the horizontal motion was unanimously rejected (also by Alexander) as being in clear contradiction to what is observed. Several devices were invented to explain the horizontal motion, which may have various directions. Here again the treatise of Pseudo-Olympiodorus had a considerable influence. Apart from the views adopted from or inspired by Aristotle and his Greek commentators the Arab scholars present some contributions that can not be traced in previous works and may be considered their original contribution to meteorology. We mention, for instance, al-Kindi's explanations of precipitation and wind, Ibn Suwär's comparison of the rainbow with the lunar phases, Ibn Sînâ's view that the tropics are the most moderate region of the world and Abū 1-Barakāt's explanation of several phenomena by spiritual celestial forces. The scholars studied here, who have received and transformed Aristotle's Meteorology, generally contributed little towards a correct explanation of the various meteorological phenomena. Progress towards such an explanation can be found, for the particular subject of the rainbow, in another tradition: the one starting with Ptolemaeus' Optics, going via Ibn al-Haytam, with his experimental research of optical phenomena, to Kamā1 ad-Dīn a1-Fārisī, who did experiments on the course of rays of light in a transparent sphere.

5. Structure of the present book In each chapter of this book a group of meteorological phenomena is treated that belong together from a certain point of view. The chapters

do not correspond to chapters in Aristotle's Meteorology. First an account is given of the contents of Aristotle's text on the subject concerned, in order to provide the reader with the necessary basic information to understand what commentators and other authors on meteorological phenomena have written. Then follows an account of what the Greek commentators have written, insofar as they clarified Aristotle's text, added interesting examples or presented opinions different from those held by Aristotle. Sometimes we have included the clarification of Aristotle's text by the commentators in the account of Aristotle's text itself. The next sections in each chapter deal with the Arabic versions of the Meteorology by Ibn al-Bitrlq and Hunayn ibn Ishāq, and with the Arabic version of the paraphrase by PseudoOlympiodorus. The remaining sections deal with the Arabic commentaries and other treatises on meteorology, such as those by al-Kindi, Ibn Sīnā, the school of Ibn Sīnā, Ibn Bājja and Ibn Rušd.

CHAPTER ONE

STRUCTURE OF THE ATMOSPHERE; THE DOUBLE EXHALATION

L Aristotle Chapters 1,1 and 2 of Aristotle's Meteorology are introductory. In U Aristotle determines the place of the book in relation to his other books on natural philosophy. He first mentions the subjects treated in his previous books on natural philosophy (Physics, On the Heaven, On Generation and Corruption). Then he states that his book on Meteorology deals with those natural phenomena that occur with a regularity which is less perfect than that of the heavenly phenomena. The places where these phenomena occur are between the earth and the lowest heavenly sphere (i.e. the sphere of the moon), on the surface of the earth, and also within the earth. He enumerates the subjects to be treated in Books I, II and III. Some of these subjects are explicitly mentioned (the Milky Way, comets, shooting stars, winds, earthquakes, thunderbolts, whirlwinds and fire winds), others are described in a more vague manner, so that it is not always clear which subject exactly is referred to. The enumeration is not an exhaustive table of contents. Chapter 1,1 concludes with the announcement that the program for the books coming after the Meteorology will be the discussion of animals and plants. At the end of Book III (378a12 ff.) Aristotle announces that, after having studied the phenomena above the earth, he will now study what is formed in the earth: minerals and metals (τά ορυκτά καί ιά μεταλλευχά, lit.: what can be quarried and what can be mined). This promise is not fulfilled in the known works of Aristotle. It is not the subject of Book IV of the Meteorology. We shall return to this question below in Chapter 10, pp. 310 ff. The next chapter (1,2) summarizes some of the results from the previous books on natural philosophy: the heaven and the heavenly bodies are made up of one element, sc. ether, and their motion is circular and eternal; the sublunar world is made up of the four elements fire, air, water and earth. These elements are constituted by some material substrate having two of the primary qualities hot, cold, dry, moist, as follows: earth is cold and dry, water is cold and moist, air is

hot and moist, fire is hot and dry.1 Their natural motion is rectilinear and finite, either up, or down, i.e. from or towards the centre of the world. The province of the phenomena under discussion in the Meteorology is the sublunar world; thus, the four elements are to be considered the material cause of these phenomena. What occurs in the sublunar world is influenced by the celestial world—in particular by the course of the sun—as all motion and change originates from the primary eternal motion; thus, the motion of the celestial bodies must be considered the effective cause of the meteorological phenomena. Before turning to the treatment of each individual phenomenon (from chapter 1,4 on), Aristotle presents another preliminary chapter (1,3), in which he looks for an answer to the question what exactly occupies the space between the earth and the sphere of the moon, this being the space in which most meteorological phenomena occur. The answer seems already known from previous books: the earth is surrounded by water, air and fire. These elements each have their natural, proper place—earth is at the centre of the world and is surrounded by water, which is surrounded by air, which is surrounded by fire—and they occupy their natural place when they have not been forced to move to somewhere else. If an element is not in its natural place, it will move to it if not hindered by some obstacle. Furthermore, the elements may change into one another, as Aristotle repeats (339a36-b2) from previous books;2 this occurs when one of the primary qualities that characterize an element changes into its contrary. The effective cause of such a change is the sun in its course along the ecliptic. Thus, when part of an element changes into another element, this quantity of matter will move to another place, namely the natural place of the element into which it has changed. For instance, solar heat changes a quantity of water into air (vapour); this air will move upward, cool down when the sun moves away, and change into water, that falls as rain. One would expect that this is the way in which Aristotle explains the meteorological phenomena. However, in the Meteorology the elements and their transformation into one another retreat to the background. Instead of these the principal role is played by two exhalations, the moist and the dry exhalation, that are dissolved from the earth by the sun. The space between the earth and the sphere of the moon is filled by these exhalations and they are the material

1

De Generatione et Corruptione 11,3. The elements constituted this way are an abstraction: what one usually calls earth, water, etc. is a mixture of all these elements, as all natural bodies are a mixture of these elements (De Generatione et Corruptione 330b21 ff. and 334b31 ff.). 2 De Caelo 111,6 and 7; De Generatione et Corruptione 11,4 and 10.

source of all meteorological phenomena. It is this principle of double exhalation that unites different phenomena such as comets, rain, wind, thunder and earthquakes and justifies them to be treated in this single book. In the discussion of 1,3 these exhalations are mentioned in passing. They are explicitly introduced in 1,4 and are brought up again in several other places. We shall give here a survey of what Aristotle says about them. When Aristotle introduces the exhalations in 1,4, he states that the solar heat dissolves from the earth two exhalations (άναθυμίαοις), a vapourous (άιμιδώδης) exhalation from the water on and within the earth, and a windy (πνευματώδης), smoky (καπνώδης) exhalation from the earth itself. The former is moist, the latter is dry and hot. The exhalations move upward; the windy exhalation, being hot, rises above the heavier vapour. The sublunar stratum adjacent to the celestial sphere is filled with the hot, dry exhalation, and this what we call fire; it is an inflammable material (ύπέκκαυμα), which is easily ignited. Below that comes air (341b6-18). In 1,3 the subject is discussed as follows: The sublunar matter is ordered around the centre of the world in the following way: earth is at the centre and is surrounded by water; earth and water are surrounded by air, and air is surrounded by the so-called fire. It is not really fire: real fire is an excess of heat, a kind of ebullition (ζέσις). Of the atmosphere the lower part is moist and hot (and this is air); it contains vapour (άτμίς) and dry exhalation (άναθυμίασις) 3 from the earth. The vapour is moist and cold,4 the dry exhalation is dry and hot. Vapour is potentially like water, dry exhalation is potentially like fire. The upper part of the atmosphere is hot and dry: it contains the dry exhalation only, which is a sort of fire (340bl9-29). The subject of the exhalations is brought up again in the discussion of wind (11,4). Aristotle says that the exhalations are dissolved by the The word άναθυμίασίς is used here (340b26) to refer to the hot, dry exhalation; in all other places it refers to both exhalations. 4 Most MSS have; 'moist and hot' and this is adopted by the Greek commentators and by Gilbert, Strohm and Steinmetz; Lee, Webster and Düring have: moist and cold. At 360a23 vapour is said to be moist and cold. Olympiodorus in his commentary on this statement notices the difference with Aristotle's text in 340b19 ff. He says that vapour is moist and cold by nature because it arises from water; it is moist and hot by accident because heat is added to it from outside. See Olympiodorus, in Meteor. 172,6-lL Alexander does not comment on the difference between both paragraphs. Both in his comment on 360a22 ff. and on 369a12 ff. he says that vapour is moist and cold, see Alexander, in Meteor. 90,31-32 and 12633. According to Steinmetz the ascription of different qualities to vapour here and in 360a23 is an indication that chapters 1,1-3 of the Meteorology are a later addition, not written by Aristotle. The different use of α ν α θ υ μ ί α σ ί ζ (see previous note) could also be such an indication, see Steinmetz 1969 236 ff. 3

internal heat of the earth and by the sun's heat (360a6-10). The moist and dry exhalations are different in kind, but they do not exist separately: in each one of them there is always an admixture of the other. They are called moist and dry according to the dominating quality (359b28-34). Air is a mixture of both exhalations. Thus, air is moist and hot, as it is composed of vapour, which is moist and cold, and smoke, which is hot and dry (360a21-27). More dry exhalation is dissolved from dry bodies that have become moist to a certain extent than from bodies that are just dry, like moist (green) wood gives off more smoke when it is burned than dry wood (361a16-19, 362a10). In other places aspects of the doctrine of exhalations are repeated, or some properties are specified: The hot and dry exhalation (the so-called fire) forms the upper stratum of the sublunar world, and is adjacent to the lunar sphere (344a8-10, discussion of comets). The moist exhalation is warmer than water, as it contains the fire that causes it to rise (347a24-25, discussion of dew and hoarfrost). The moist and dry exhalations are mixed in the lower atmosphere (where they form air) (358a21-24, discussion of the saltness of the sea). The two exhalations are both potentially contained in their combination (369al2-15, discussion of thunder), which is air, i.e. air is a mixture of the moist and dry exhalation. To sum up, Aristotle arrives at the following arrangement of matter in the sublunar world: Earth and water are in and around the centre. Two exhalations are dissolved from them by the heat of the sun: a dry exhalation (from the earth) and a moist exhalation (from the water). Both move upwards; the dry exhalation moves further upwards than the moist one. Around the body of earth and water is air, which is a mixture of dry and moist exhalation. This is the region where clouds, rain, etc. (from the moist exhalation) and winds (from the dry exhalation) are formed. Around the air is the so-called fire (not real fire, but some inflammable material), which is dry exhalation; this is the stratum adjacent to the lowest celestial sphere, and that is where comets, shooting stars, etc. are formed. With this the question posed at the beginning of 1,3 has been answered. We shall now see how Aristotle arrives at this result in 1,3, and which other preliminary questions are treated in that chapter. As has been stated above, the subject under discussion is the arrangement of the elements in the sublunar world. As for water, says Aristotle, it is clear that its place is around the earth (sea, rivers and any subterranean water), as it is not observed to exist anywhere else (339b9-13). In the discussion of the arrangement of the other elements between the earth (with its surrounding water) and the lunar sphere Aristotle

includes the question of the nature of the element in the heavenly sphere. This element is ether, and 'ether' does not mean fire, as Anaxagoras thought (339bl6-24). The view that the stars and the space between them consist of fire, whereas the space between the earth and the stars is filled with air, must be dismissed, because in that case the proportion between the quantity of fire on the one hand and air and the other elements on the other hand would be so large that fire would destroy all other elements, for the earth is very small compared to the heavenly space (339b30-340a3). The view that the whole space above the earth, including the heavens, is filled with air alone must be dismissed too, for then there would be a quantity of air disproportionately large compared to the other elements, whereas we observe that, for instance, the proportion between a quantity of water and the air which arises from it when it evaporates, is not large to such an extent (340a3-13). Thus, the stars are not fire, and the heavenly space is not filled with fire, nor with air, but the element of the heavens is ether. The question remains how the sublunar elements fire and air are ordered and another question arises, sc. how the stars (esp. the sun), not being hot themselves, confer heat to the earth; it is this heat which acts as effective cause for the meteorological phenomena. Before answering these questions Aristotle considers another problem: why are clouds not formed in the upper atmosphere? It will appear that the solution of this problem also provides the answer to the questions mentioned above. One would expect formation of clouds just in the upper atmosphere because the temperature is lowest in that region: it is far from the earth, so that the heat of the rays reflected from the earth is not noticeable anymore, and also still far from the heat of the stars. After some confusing preliminary remarks (340a32-340b3) Aristotle answers the question about the formation of clouds as follows:5 (a) The circular motion of the heaven dissolves (διακρίνω) 6 and 5

The interpretation of passage 340b6-10 is doubtful. It is unclear to which region Aristotle refers here, to the heaven or the sublunar region, and what is meant with the 'varying purity and quality' of the material. The Greek commentators interpret it as referring to the heavenly region, followed by Strohm and Lee; Webster thinks the sublunar region is meant. 6 διακρίνω refers to the process in which an element is transformed into another, less dense element—e.g. water -* air or air -» fire—or in which an element is not transformed into another, but becomes less dense. An appropriate translation in these cases would be 'evaporate' or 'rarefy*. We adopt the more general 'dissolve' from Webster's translation. The process is induced by heat (e.g. solar heat), or by motion causing friction, which is also a source of heat. Therefore dissolution often induces inflammation. Cf. 340a10, 340a29, 340b3, 341a17, 345a8, 346b22, 354b30.

inflames (έκπυρόω) the sublunar matter directly under it, and so causes heat in the upper atmosphere (340bl0-14). (b) The lower part of the atmosphere contains air, which consists of both moist and dry exhalation from the earth; the upper part contains so-called fire, which is hot and dry exhalation only (340bl5-30: the theory of the two exhalations). These two reasons are adduced to explain that the upper atmosphere is hot: the motion of the heaven, and the hot, dry exhalation from the earth. Clouds cannot be formed in that region because of this heat, and because it contains dry, hot exhalation ('fire') rather than air. Then, a third reason is brought forward: the circular motion of the heaven drags along with itself the adjacent stratum of 'fire' and the layer of air under it, until the level of the mountain tops; the air in the valleys between the mountains cannot be dragged along with this circular motion. Thus, these layers of the atmosphere are in circular motion, and this prevents the formation of clouds. The strata of air and fire are not static: parts of fire may cool, become heavy and sink down, whereas other parts of fire move upward with the smoky exalation (340b32-341a9). The way heat is conferred to the earth from the sun—which is not hot itself—has already been stated above: it is the (daily) motion of the sun in its sphere that causes heat, through its friction with the upper layer of the sublunar world, just as we observe that fast moving objects become hot and heat the surrounding air. To be sure, the moon is closer to the earth, but it does not cause heat, because its motion is so much slower than the sun's motion. It is the solid body of the sun itself that causes the underlying regions of the sublunar world to be heated by its friction. Therefore, there is no heating by the sun at night (34M7-30).7 Another way of transmission of heat to the earth is that the celestial motion causes the adjacent 'fire' to be scattered downwards (341a30-31). We see that the introduction of the exhalations leads to a view of the structure of the sublunar world which is rather different from what we know from Aristotle's previous books On the Heaven and On Generation and Corruption. Indeed, earth and water are surrounded by air and 'so-called' fire. However, this air is a mixture of moist and dry exhalation and this fire is dry exhalation and is not real fire; real fire does not occur as a stratum of the sublunar world. Thus, we get the impression that air and 'so-called' fire are just products (sc. exhalations) of earth and water; the four strata which make up the sublunar world 7

See also De Caelo 11,7.

and form its matter, are not four equivalent elements which may transform into one another, but everything arises from earth and water only.8 According to Gilbert's interpretation, air and fire exist in the atmosphere as elements on their own, and όαμίς is an intermediate stage between water and air: it consists of very small particles of water, yet as it is moist and hot, it is also kindred to air, similarly, the hot, dry άναθυμίασις is an intermediate stage between earth and fire.9 We shall see below that this is also the interpretation of most Greek and Arabic commentators. Furthermore, the 'fire' and the air, as far as it lies above the mountains, are dragged along with the circular motion of the heaven. This circular motion of the heaven, esp. that of the sun, also confers heat to the sublunar region, because this motion causes friction, which is accompanied by heat.10 As for air, it appears that one may distinguish in it several layers: the lowest layer is that where the heat caused by the reflection of sunrays is still appreciable; no clouds can be formed there. Then comes a cooler area, where clouds may be formed, and which extends until the level of the mountain tops. One might get the impression from 340b29 ff. ("We must assume that the reason why clouds are not formed in the upper region is that it contains not just air, but rather a sort of fire") that the next layer consists of 'fire': where cloud formation stops, 'fire' begins. But it is not probable that Aristotle, considering the cold that reigns on mountain tops, would think that the stratum of 'fire' starts from the mountain tops. Indeed, in 340b34 he says that the air is in motion, as far as it is not inside the circumference that makes the earth a complete circle i.e. the circumference that touches the mountain tops; this implies that another part of air lies above the mountain tops.11 Clouds are not formed there because that air is in motion. Going higher up, the air must become warmer again because one approaches the stratum of 'fire'. Clouds are not formed there, nor in the stratum of 'fire' itself, again because these layers are in motion. As for the stratum of 'fire', the formation of clouds is also prevented by the heat and the fact that this layer contains no moist exhalation at all. Gilbert indeed distinguishes two layers within the air above the mountain tops: a cold layer, and above that a layer which is warmer because it is near the fire. He arrives at this distinction of two

8

Solmsen 397-398. Gilbert 464 and 466n1. 10 In fact, this is incompatible with the previous statement that fire and air are dragged along by the celestial motion: if they are dragged along there is no friction. See also 344a11: "The region of 'fire' and the greater part of the air below it and continuous with it are moved around the earth by the circular motion of the heaven." 9

layers because of the fact that Aristotle gives two different explanations for the single phenomenon that clouds are not formed above a certain level in the atmosphere.12 In his account of the formation of clouds and precipitation Olympiodorus also divides the air into four layers. First there is the layer in which the air is heated by the sunrays reflected from the earth's surface. The next layer is cold because the reflected sunrays have no effect anymore. Here clouds are formed. Then follows the region where no clouds are formed. The lower part is still cold, the next part is warm because it is near the region of the ύπέκκαυμα.13

2. The Greek

commentators

Philoponus begins his commentaries on these introductory chapters of the Meteorology stating that it is Aristotle's intention to explain all phenomena occurring between earth and heaven by means of two exhalations, a moist and a dry, smoky exhalation. He divides the phenomena into two groups: those which have a real, material existence (ΰπαρξις), such as rain and wind, and those which are a visual effect (εμφασις), such as the halo and rainbow. The latter phenomena are just a (visual) impression (φαντασία).14 This division of phenomena into those existing καθ' ΰπαρξιν or καθ' ύπόσχασιν and those κατ1 εμφασιν is well known among the commentators. It was originated by Posidonius,15 but one should note that Aristotle and Theophrastus already used εμφασις for 'image', 'reflection'. Olympiodorus in his introductory commentary says that the goal of the book is to explain the phenomena in the atmosphere by means of their material cause, viz. the two exhalations, and their effective cause. This effective cause is the heaven: the heaven brings the sublunar elements in a circular motion. This motion of the elements is neither in accordance with their nature—for their natural motion is rectilinear, either up or down—nor against their nature (παρά φύσιν), but ύπέρ φύσιν.16 Olympiodorus also divides the phenomena into those which are just a visual impression (φαντασία) and those which have a real existence (ΰπαρξις).17 12 13 14 15 16 17

Gilbert 477 ff. Olympiodorus, in Meteor. 82,17-25. Philoponus, in Meteor. 1,24-2,7 and 6,19-20. See Gilbert 587-588 and Steinmetz 1964 198-204. Olympiodorus, in Meteor. 1,18-2,33. ibid. 10,25-6.

Concerning the exhalations and the structure of the atmosphere Alexander and Philoponus remark: Clouds are not formed in the upper atmosphere. An indication for this is that the tops of the highest mountains rise above the clouds. In that region there is neither rain, nor wind. For people have found ashes and remains of offerings in the same state after several years. No rain had washed them away, no wind had scattered them; even signs people had drawn remained unaltered. People who have crossed high mountains have reported that clouds gathered below them and that rain, thunder and lightning occurred below them.18 The circular motion of the heaven drags along the highest layers of the atmosphere. These layers become thinner, warmer and become ignited. Therefore that region is hot and dry, a kind of fire.19 Philoponus asks: how can there be snow in the highest mountains, if that region is hot because it is moved along with the heaven? He answers: that region is cold because the heat caused by sunrays reflected from the earth do not reach it, while the heat of the stratum of 'fire' is still far. What about the remnants of ashes, found in the highest mountains after a long time, if the air there is moved by the heaven? This air is thin, and its motion is slow and smooth, different from the motion of winds.20 Philoponus and Alexander say that the lower atmosphere, near the earth, is moist and hot; this is air. It is moist because it contains άτμίς and hot because it contains άναθυμίασις. The atmosphere above this layer is dry and hot. The moist άτμίς does not rise to this higher layer, only the dry, hot άναθυμίασις reaches it. Aristotle has called this region ύπέκκαυμα.21 The όαμίς is moist and hot; it is potentially water; it becomes water when it condenses; the άναθυμίασις is dry and hot; it is potentially fire. There are no clouds in the higher atmosphere because that region is filled with this hot, dry άναθυμίασις.22 Philoponus specifies that άτμίς is something intermediate between water and air: it is something that is 'midway' between water and air; it is air that has become thick (παχυνόμενος) and water that has become thin (λεπτυνόμενος). Vapour is hot and moist; yet as it is intermediate between water and air, it is colder than air and hotter than water.23 18

Alexander, in Alexander, in 20 Philoponus, in 21 Alexander, in 22 Alexander, in 55,12-27. 23 Philoponus, in 19

Meteor. Meteor. Meteor. Meteor. Meteor.

16,12-15; Philoponus, in Meteor. 26,32-27,9. 13,14-23; Philoponus, in Meteor. 31,28-38. 3231-33,18. 1430-15,7; Philoponus, in Meteor. 3530-36,7. 15,8-22 and 19,32-2045; Philoponus, in Meteor. 36,11-22 and

Meteor. 28,36-29,1 and 62,17-24.

Olympiodorus in his commentary on Aristotle's statement that the elements may change into one another (339a36 ff.) says that the elements do not directly change into one another, but via certain intermediate bodies, called 'unfinished elements' (ήμιγενη οιοιχεΓα). These 'unfinished elements' are the two exhalations: vapour is intermediate between water and air, dry exhalation is intermediate between earth and fire. The same interpretation of the exhalations appears from his commentary on the beginning of 1,4 where Aristotle introduces the exhalations: the moist exhalation becomes air and the dry one becomes a kind of fire (ύπέκκαυμα) and these bodies fill the atmosphere.24 See above p. 34n4 for Olympiodorus' view on the properties of vapour. The sun transfers heat to the earth through its friction with the underlying layers of the sublunar world. Alexander raises some problems ensuing from this view, and gives solutions without disagreeing with Aristotle's view. Firstly, he asks how it is possible for the sun to cause heat in the sublunar world by friction, when the sphere of the sun is not in contact with the upper layer of the atmosphere; for the sphere of the moon is between them, and that is something not capable of being affected (απαθής), so that it cannot become hot or cold. The answer is that the sphere of the moon, without being affected itself, passes on the affection caused by the sun to the sublunar region. This is comparable to what happens when one uses a glass filled with water as a burning glass. The sun is able to burn bodies after its rays have passed through the burning glass, but the glass and the water in it do not become hot themselves. Something that may cause an affection does not affect everything, only what is susceptible to that affection. Fishermen know there is an electric ray (ναρκή) in their nets because they feel an affection in their hands dragging the nets; but the net itself is not affected. Another answer could be that the heaven is capable of being affected in some respects. Indeed, Aristotle has shown that the heaven is ungenerated and indestructible, that it is not subject to increase and diminution, nor to alteration (change in quality).25 Yet motion is an affection too, and heaven is subject to it. Also, if receiving light is an affection, the moon is subject to it, for the moon receives light from the sun. Thus, it is not impossible that the motion of the sun could affect the adjacent moon sphere, not in such a way that it would become hot, or inflamed—that would be an alteration, which is impossible in the heaven—but that the affection is passed on 24 25

Olympiodorus, in Meteor. 16,15-22 and 40,19-22. De Caelo 270a14 ff. and 1,12.

through it to the sublunar region, where it causes heating and inflammation.26 Alexander also asks why the heating of the earth by the sun is not equally strong in the shadow and in places exposed to the sunrays. For if heating occurs by the sun's motion, the places that are exposed to the sunrays and that are in the shadow do not differ as far as their position under the sun's motion is concerned. The answer is that the sun's motion heats the upper layer of the atmosphere, which in turn heats the layer adjacent to and in contact with it, etc. What is in the shadow is not in contact with the air that would heat it, as it is screened off from it by the object that gives the shadow. Therefore that area is not heated.27 Philoponus presents an extensive critical commentary on the doctrines of Aristotle and Alexander concerning how the sun heats the earth, as follows: The sun is a solid body, says Aristotle, otherwise it would not cause heat by friction. Solid means hard, not yielding to pressure: earth is solid, but water, air and fire are not solid. If a body is solid it must adopt the primary qualities hot / cold and dry / moist, according to De Generatione et Corruptione28 Thus, by stating that the sun is solid, Aristotle implies that it adopts these qualities, and is composed of the four elements. Furthermore, the other stars (including the planets and the moon) will be solid too, and if the sun causes heat by its motion, the other stars will cause heat too. For instance, the sphere of the moon is so much closer, and adjacent to the sublunar region, whereas between the sun and the sublunar region there are three other spheres: those of Mercurius, Venus and the moon. If the sun were to transfer heat to the atmosphere by its motion, it will first have to heat these intermediate spheres. However, these spheres are not capable of being affected, according to Aristotle, so that they cannot be heated. Thus, either the motions of all stars cause heat—this is not true, for it is clear that it is the sun that heats most—or the sun does not heat by its motion, but by some quality. The sun moves faster than the moon, but it is much farther, its distance is 20 times the distance to the moon. The closeness of the moon will compensate its slower velocity. If the full moon does not heat the earth as much as the sun, it means that their motion is not the cause of heating. It must be the light that causes the heating, just as 26 27 28

Alexander, in Meteor. 18,8-19,13. ibid. 19,13-19. See De Generatione et Corruptione

329a32 ff.

fire does. The moon receives its light from the sun and reflects it to the earth, but not pure and un weakened, and therefore the heat caused by the light of the moon is much weaker. Thus, the sun heats the earth not because of its velocity, but because of a quality.29 Aristotle mentions another cause of heat transmission to the earth: parts of the stratum of 'fire' (ύπέκκαυμα) are scattered downward by the heavenly motion. There is no reason why this would not also occur at night, or why the poles would not also be heated in this way. This does not occur, so this way of heating does not exist.30 Alexander adopts Aristotle's doctrine that the sun causes heat in the sublunar world by its motion (friction); he raises the problem how this is possible when the sphere of the sun is not in contact with the upper layer of the atmosphere. He tries to save Aristotle's theory by saying that it is not contrary to Aristotle's doctrine if the heavenly sphere is capable of certain kinds of affection, and transmission of heat could be such an affection. But it is impossible for the sun to transfer heat to the sublunar sphere through an intermediate sphere without this sphere becoming hot also. Alexander also asks why the sun does not transfer heat to places in the shadow. His answer is not satisfactory, because all parts of the atmosphere, whether they are in the shade or not, are connected with one another and have an equal position in relation to the place where the sun is moving—except the (subterranean) parts that are enclosed by earth above it. Air in the shadow of a wall or a rock is in contact with the air that is shone upon and will have the same share in the heat caused by the motion of the sun. It follows that it cannot be the motion of the sun, but a quality that causes heat, like fire causes heat. A body that screens off the sun prevents this quality from penetrating into the shaded area, so that this area is heated only (indirectly) by the surrounding air. Thus, heat is a phenomenon accompanying light, as one observes with fire. If someone were to ask why the sun, if it has a fiery nature, heats more at noon than when it is at the horizon, the reason is clear: when it is at the horizon, only part of the rays reaches us, the other part disappearing under the earth, whereas at noon all sunrays reach us. Furthermore, there is mist around the earth and when the sun is at the horizon, its rays reach us through this mist, which weakens the light. This is also the reason why its light seems red to us. The mist is solved when the sun rises higher. These facts cannot be explained if the sun

29 30

Philoponus, in Meteor. 41,24-43,25. ibid. 45,24-35.

were to heat by its motion. Then its heating would be equal at all times. Also, the sun would heat equally all regions of the earth. But we see that the region of the equator, that is closer to the sun, is very hot, whereas it is cold at the poles, that are further from it. All this proves that the sun has a fiery nature.31 Olympiodorus' interpretation of Aristotle's text on how the sun transfers heat to the earth is as follows: There are two effects: firstly, the motion of the sphere of the sun causes heat; however, this implies that the heating would be equal at daytime and during the night. Therefore, there is a second effect; not only the sphere of the sun, but also the sun itself is a cause of heat, by means of the heat of the sunrays that is reflected from the earth.32 It is not the sunrays themselves that are reflected, for how can something incorporeal be reflected? What happens is that air is heated by the sunrays. The heated air moves downward, is reflected against the earth and then moves upward, just as a ball thrown on the ground. This effect explains why the heating is more during the day than during the night. It also explains why it is hotter at noon and in summer than at sunrise (or sunset) and in winter. At noon and in summer the rays fall on the earth more vertically, at sunrise and in winter more obliquely, and reflection occurs in the same way. The air is better enclosed and better heated when the rays are more vertical; if the rays are more oblique, the air is instead 'poured out' (dispersed), and less heated. Olympiodorus also criticizes Alexander's solution of the problem how the sun can transfer heat to the sublunar region through the sphere of the moon. He says that the moon does not need to transfer heat anyway because the heat arises in the sublunar sphere, not in the heaven.33 Summing up the contributions of the Greek commentators, we saw that, according to Olympiodorus, vapour and smoke are intermediate stages between elements. Philoponus says the same about vapour only. As for the question whether vapour is moist and hot or moist and cold, the Greek commentators adopt the reading: moist and hot; Olympiodorus notices that in another place vapour is called moist and cold and gives his own explanation of the difference. As for the heating of the earth by the sun, Alexander follows Aristotle's view. Philoponus says that the sun is fiery, and this solves the problems ensuing from Aristotle's doctrine. Olympiodorus adds 31

Philoponus, in Meteor. 47,27-53,27. This effect is discussed by Aristotle in 341a33 ff., in Olympiodorus' interpretation, see Olympiodorus, in Meteor. 35,15-21. 33 Olympiodorus, in Meteor. 32,7-33,16. 32

heating by sunrays (via reflection) as a second cause of heating besides the motion, and in this way he can also solve the problems.

3. Ibn al-Bitrīq and Hunayn ibn Ishāq Ibn al-Bitriq's version of the introductory chapters, especially of Aristotle's view on what fills the space between the earth and the sphere of the moon, differs from Aristotle in several respects. Ibn al-Bitrlq does not give the impression that air is a combination of moist and dry exhalation and that the next stratum ('fire') is filled with the dry exhalation only. Rather, the space between earth and moon is filled with air and fire, as usual; the exhalations are a kind of matter intermediate between the elements and when they rise from the earth and water they are mixed with air and fire. He discusses the question as follows: The space from the moon to the water and earth is occupied by fire and air. The fire is purest where it is close to the sphere of the moon. The lower layers of fire, where it borders air, are less pure; there it is mixed with air that penetrates that region. Again, the upper air, adjacent to fire, is purest, the lower air, close to the earth is least pure, as it is mixed with smoky exhalation (wahaj) and dust (gubār) that make the air turbid.34 This is Ibn al-Bitriq's version of 340b6-10; we have seen above (p. 36n5) that it is not clear to which region this paragraph refers, to the heaven or the sublunar region. Ibn al-Bitriq interprets it to refer to the sublunar region, different from the interpretation of the Greek commentators. In his version of 340b24-32 Ibn al-Bitriq says that dry, hot or fiery exhalation (dukān; wahaj) rises from earth and vapour (moist exhalation) (bukār) from water. These exhalations are received by the air immediately surrounding the earth and make that air hotter and moister. The layer of air above that region is hot and dry, not moist, as the moist exhalation does not rise so far. This upper air is contiguous with the fire. The difference between fiery exhalation (wahaj) and vapour (bukār) is that the former is hot and dry, as it is closely related to fire, whereas air immediately surrounding the earth is moist and hot, as it is closely related to water. Because only fiery exhalation rises into the upper air, there are no clouds in that region.35 Thus, Ibn al-Bitriq says that earth and water are immediately surrounded by air that is hot 34 35

Ibn al-Bitriq, Meteor. 1741-18,8. ibid. 193-20,4.

and moist, then comes air that is hot and dry (and no clouds can be formed there), then comes fire. The passage corresponding to 341b6-23 does not completely agree with the picture sketched in the previous paragraph. Ibn al-Bitriq says that when the sun heats the earth, various exhalations (bukārāt) arise: hot, dry exhalation, hot, moist exhalation, and cold, moist exhalation. The hot, dry exhalation rises the highest; below that comes the hot, moist exhalation, and lowest is the cold, moist exhalation. The hot, dry exhalation may ignite the upper air by its heat. The hot, moist exhalation below it will become air, and the cold, moist exhalation may condense and turn into water that falls as rain. The hot, dry exhalation rises until the boundary of the fire; it becomes very hot and may inflame because of its closeness to the fire and the motion of the heavenly sphere.36 Thus, Ibn al-Bitriq distinguishes three exhalations that rise into the air. Ibn Rušd in his Short and Middle Commentary also says that three kinds of exhalation are dissolved (see below pp. 62n84 and 63). The exhalations are discussed again, in the account of precipitation, in a way again different from those above, as follows: The sun dissolves from the earth a moist exhalation and a hot, dry exhalation. When they rise, they separate into different kinds. The hot, dry exhalation rises until it approaches the region of fire; this kind is called fiery exhalation {wahaj). The other kind becomes air, if nothing occurs to it. If it is enclosed, or becomes cold, this air becomes water.37 When the exhalations are brought up again, in the treatises on wind (359b28) and thunder (369a12), Ibn al-Bitriq follows Aristotle in distinguishing two exhalations: a moist and a dry one. It is not specified whether the moist exhalation is hot or cold.38 Ibn al-Bitriq does not mention the motion of the air above the mountains that is caused by the motion of the celestial spheres as another cause of the absence of clouds in the upper atmosphere (340b32 ff., see above p. 37). This is noted by Ibn Tibbon; he then gives Alexander's commentary on this subject, who does mention this cause; furthermore, he says that also Ibn Rušd gave this cause.39 The picture of the exhalations and the structure of the atmosphere, such as it emerges from the account of Ibn al-Bitriq, is rather 35

Ibn al-Bitriq, Meteor. 19,5-20,4. ibid. 30,12-31,9. 37 ibid. 36,2-9. 38 ibid. 63,5 and 81,6. 39 Ibn Tibbon, Otot ha-Shamayim 1,181-213. Ibn Rušd in his inserts a phrase that mentions this other cause, see below p. 63. 36

Middle

Commentary

ambiguous. Anyway, he maintains a layer of fire above the smoky exhalation that forms the upper layer of air. As for the way the earth is heated by the sun, Aristotle's doctrine is followed. Hunayn ibn Ishāq says in his compendium of Aristotle's Meteorology in the place corresponding to 341b6 ff.: When the sun passes over moist places (of the earth), it raises various exhalations (bukārāt muktalifa) between hot, dry and cold, moist exhalation. The hot, dry exhalation rises very high, until it is close to the sphere of the moon. There it is heated and becomes ignited by the motion of that sphere.40 Hunayn does not specify the 'various exhalations'. Possibly he adopts Ibn al-Bitriq's three exhalations. He agrees with Aristotle that the inflammable hot, dry exhalation is adjacent to the moon; there is no special stratum of fire. When precipitation from moist exhalation is discussed, Hunayn says that when moist exhalation is dissolved and rises upwards, heat is mixed in; this makes parts of it thin (lattafa) so that the exhalation becomes air. The parts that remain thick (galīz) become dew and rain when they cool down.41 In other places, when wind and thunder are discussed, Hunayn follows Aristotle, and mentions two exhalations: the dry and the moist exhalation; the moist exhalation is not further specified as being hot or cold.42

4.

Pseudo-Olympiodorus

The Arabic version of Pseudo-Olympiodorus' Commentary on the Meteorology has an extensive introduction in which the doctrine of the exhalations is expounded and the meteorological phenomena are enumerated and classified according to the exhalation from which they arise.43 Such a survey of the phenomena does not occur in Olympiodorus, nor does the way the exhalations are discussed show any similarity to his commentary. Pseudo-Olympiodorus' account basically agrees with Aristotle's doctrines, but they are presented in a strongly systematizing way, so that not everything can be traced back to Aristotle or Olympiodorus, as will become clear from the account below. For instance, the moist and dry exhalations are further divided into a thick and a thin version. From a study of Pseudo-Olympiodorus' treatise as a whole it will appear that this work is not just an excerpt from 40 41 42 43

Hunayn, Jawāmî 298-302. ibid. 51-52 and 68-70. ibid. 139-140 and 239. Pseudo-Olympiodorus, Taf sir 833-88,2.

Olympiodorus' commentary, but rather a systematic account of Aristotle's Meteorology, in which Olympiodorus' commentary has been used extensively, but not exclusively. What follows is an account of the introductory chapter. After a survey of Aristotle's works on natural philosophy the goal of the Meteorology is characterized as the account of the phenomena that occur due to the elements between heaven and earth, and in the earth. The remote efficient cause of these phenomena is the motion of the eternal bodies, more specifically, that of the sun. The direct efficient cause is the heat and coldness caused in the sublunar region by the sun. The four elements are the remote material cause; the direct material cause is formed by the exhalations dissolved from earth and water that are either enclosed within the earth, or rise from it. There are two exhalations, a moist, watery exhalation (bukār ratb) and a dry, fiery exhalation (bukār dukānī). The moist exhalation is heavy and sinks, the dry one is light and rises. However, as long as the exhalations are together each one of them is moved along with the other to its natural place; therefore their motion is oblique. If the exhalations are separate, the smoky exhalation rises to the highest place, and the moist one remains below; the moist one only rises when it is moved along with the dry one. When they are together, they are not mixed; it only means that their parts are close together. Thus, they may become separated, without being corrupted.44 When they have arrived in their specific places, the smoky exhalation changes into fire, the moist one into water. This occurs if neither of them is dominating the other. If the smoky one is dominating, it forces the moist one upwards. If the moist one is dominating, it keeps the dry one below by enclosing it from all sides. The phenomena that arise from the exhalations come about either by itself (bi-dātihī)—when the exhalation is in the place that is specific for it—or accidentally ('alā jihat al-'arad)—when the exhalation is in a place that is not specific for it and where it is kept by force. In general, the specific place for the smoky exhalation is above, for the moist exhalation below. The exhalations do not always exist in the same state, but they can become thinner (latīf) or thicker (galīz). The thin smoky exhalation will move to a place above the thick one, and similarly for the moist exhalation. From the thin, smoky exhalation the following phenomena arise: shooting stars, comets, and suchlike. Their place is above the tops of the high mountains. The thick, smoky

44

See Aristotle, De Generatione by juxtaposition of parts.

et Corruptione.

1,10 about mixing and combination

exhalation gives rise to the saltness of the sea. The thin, moist exhalation generates rain and snow, the thick one dew and hoarfrost. The moist exhalation is an auxiliary cause for the generation of smoky exhalation, for when the moist exhalation changes into water that falls on the earth, it makes the earth suitable for the dissolution of smoky exhalation; one may compare this to the fact that moist wood gives off much smoke when it is burned. All these phenomena occur in accordance with the position of the sun. If the sun is high, for instance in Cancer, exhalations are dissolved and raised by the heat. If the sun is low, for instance in Capricorn, the exhalations move down because of the coldness. If in certain years more (or less) exhalation rises than comes down, this will always be compensated in the following years. The phenomena that occur within the earth on account of the dry exhalation are earthquakes and winds that occur as a consequence of this motion. On account of the moist exhalation in the earth several things may arise. If the exhalation becomes solid by coldness, then the things are generated that are meltable by fire: the metals such as gold, silver and copper. If it becomes solid by heat, then the things arise that are soluble by cold moisture: the purgative drugs, such as borax, salt. If the exhalation changes into water, then springs, rivers and floods arise; it becomes a spring or a river if it flows by itself; it becomes a flood if it is moved by the blowing of wind. If the exhalation becomes water that is subsequently alterated, then hot springs containing hot water with a salty or bitter taste arise; the taste is like that of sulfur or pitch. The phenomena that arise above the earth may be divided as follows: They are due to one of the exhalations or to both exhalations together and they either have a real existence (sahîh qâ'im) or they are a delusive image (kādib mutakayyal). Those due to the smoky exhalation that are real arise either without this exhalation changing into fire, such as winds and the saltness of the sea, or with such a change. Of the latter group some exist for a short period and are easily dissolved, such as the shooting stars, burning flames, torches and 'goats'.45 Others exist for a long time, such as the comets and the Milky Way. The delusive phenomena due to smoky exhalation are the blood- red colours, chasms and trenches. The phenomena due to the moist exhalation that are real arise either in the upper air, far from the earth, such as clouds, rain, snow and hail, or in the air close to the earth, such as dew, hoarfrost and mist. Clouds arise when the exhalation condenses and does not change into water; 45

'Goats' are a kind of meteorites, see above p. 18n24.

rain arises when it changes into water; when the water is divided into small particles, it becomes drizzle; when it is divided into large parts, it becomes a downpour (qitqit). If the exhalation freezes, snow arises; if it first changes into water, and then freezes, it becomes hail. Dew and hoarfrost are in the lower air what rain and snow are in the upper air. Hail has no equivalent in the lower air, because exhalation that is close to the earth cannot change into water and then freeze, as the water will reach the earth before it can freeze. Mist arises when exhalation close to the earth condenses, without changing into water: it corresponds in the lower air to what clouds are in the upper air. The delusive phenomena due to moist exhalation are the halo, rainbow, mock suns and rods. The phenomena due to both exhalations together are thunder, lightning, whirlwinds izawābi') and hurricanes (rīh sahābiyya). They arise from smoky exhalation, but moist exhalation is an auxiliary cause, insofar as it induces the motion or the sound that goes together with them.

5. Al-Kindi A1-Kindī in his treatise "On the cause why the upper atmosphere is cold while the atmosphere near the earth is warm"46 gives an answer to the question how it is possible that exhalation can condense in the atmosphere and that watery vapour is cooled, when we know that air is hot and moist by nature, that it is also heated by the motion of the celestial sphere and that the heat at the surface of the earth is also caused by that motion. Furthermore, he answers the questions how one can know that exhalation does not rise higher than 16 stades above the earth and what the smallest distance is above the earth where exhalation can condense into cloud. Al-Kindi first presents some general methodological remarks on how to proceed if one wants to find an answer to these and other questions. He says, among other things, that one should start with knowledge of the principles, and one of the principles concerning these questions is that from the four primary qualities (hot, cold, moist and dry) four elements are formed (earth, water, air and fire). Heat and dryness are the 'greater' qualities, the former is active, the latter is passive; cold and moistness are, respect46 Al-Kindi, Rasâ'il II 90-100. Apart from the question mentioned in the title, and another, related question, al-Kindï in this treatise also discusses the question whether number is infinite and if so, why what is numbered is always finite. This does not belong to the scope of this book.

ively, the active and passive 'smaller' qualities. The active 'greater' quality (heat) causes an upward motion, the active 'smaller' quality (cold) a downward motion. They are adopted, respectively, by the elements that by nature move upward (fire and air) and downward (earth and water). Furthermore, dryness is adopted by earth and fire, moistness by water and air. Fire rises quicker than air and earth descends quicker than water; thus, dryness is the cause of speed and moistness of slowness. Fire, air and water are flowing, whereas earth is confined. Earth by its own nature is extremely cold; no cultivation or procreation can occur in it. This is also what we experience: if we travel north, we finally arrive in a place where no habitation and cultivation is possible due to the coldness. This occurs at 63 degrees latitude north of the equator.47 If we travel south, we arrive in places that become hotter as we approach the equator. The heat we experience at the surface of the earth comes from the celestial bodies, especially the sun; the sun heats a place when that place is approached by it during its motion. Then the sun dissolves exhalations from the surface of the earth and the water. The earthy exhalation is called smoke (dukān), as it is hot and dry. It consists of earthy particles that have accepted heat and thus tend to be fiery and move upward. The exhalation from water is called vapour (bukār). The watery particles have accepted heat, tend to be fiery and move upward. However, their upward motion is slower than that of the dry exhalation because what is dry moves faster, as was said above. Both these exhalations occur as a mixture in the atmosphere above the earth, until the level where the heat of the reflected sunrays is not perceptible anymore. The atmosphere is thinner in places where the sun shines. This is clear from the fact that it is hotter in such a place than in a place in the shadow. On the other hand, the earth and the surrounding air are heated by the motion of the sun, and there is no difference between a place in the sun and in the shadow in relation to this motion. Therefore, when it is warmer in a place in the sun, this must be due to the sunrays that make the air thinner.48 When the air is thinner, things move more easily in them; thus, the exhalation quickly rises in the air insofar as the reflected sunrays exert their influence. If they arrive in a place where these rays do not exist anymore and the atmosphere is denser, they do not rise further; they cool, are densified and turn into

47

This is the northern border of the inhabitable world, according to Ptolemaeus, see below p. 195. 48 This problem was also discussed by Alexander and Philoponus, see above pp. 42 and 43, but there is no clear connection between their answers and al-Kindi's answer.

their cold nature (i.e. they become earth and water).49 This does not occur at one definite distance from the earth, as it depends on the extent to which the sunrays are reflected; this in turn depends on the height of the sun in that place. If the exhalation condenses, its earthy part pushes the air downward; this is how wind arises. The watery part becomes rain and other kinds of precipitation. Air has been called hot, but it is not invariably hot. Only fire is invariably hot. Similarly, earth is invariably cold, water is not. Fire and earth are hot resp. cold in the absolute sense. Water is cold by nature and if it is compared to air, but it is hot accidentally and if it is compared to earth. Similarly, air is hot by nature and if it is compared to water, but cold if it is compared to fire. If earthy parts become fiery when they are heated by the sun's motion, they become hotter than air. Then air is cold in relation to this exhalation. This is what we experience: if we pour water that is just a little heated over some part of our body, it feels warm when we do it in a cold place. When we do it in a hot bath, the same water feels cold. Thus, air that is close to the earth is hot, due to the reflection of the sunrays; it is colder when it is further from the earth. Water that is close to the earth receives heat from the earth when it is heated by the sun's motion. When it rises in the atmosphere, it cools down and becomes cooler than the surrounding air. When it has arrived in a place where there is no influence of the earth's heat anymore, it only receives heat from the sun's motion. This answers the questions posed at the beginning. Then Al-Kindi discusses another question, which is related to the previous one. It was explained that the height at which clouds are formed is not one definite height, but that it depends on the height of the sun in that particular place. It follows that when there is a cloudless sky as far as the horizon, we cannot know how far this fair weather extends beyond the horizon: it may be cloudy in another place. Also, if we see a cloud somewhere at the horizon, we cannot know how far this cloud is and above which city it extends, because we do not know if the cloud is high or low. Also, local conditions determine up to what height the exhalation rises. For instance, mountains prevent exhalation that has risen from being dragged along with the celestial motion; exhalation which is formed in an open desert spreads and moves about; if it rises, it is dragged along with the celestial motion and does not condense. Therefore we find that wind, precipitation, thunder and

49

Ibn al-Bitriq says that the moist exhalation is densified and 'returns to its nature', i£. becomes water (Ibn al-Bitriq, Meteor. 20,12).

thunderbolts arise in the atmosphere between mountains or in depths.50 Above the highest mountains these phenomena do not occur because there the exhalation moves together with the celestial motion. The height of the highest mountains above which such phenomena are not formed was reported to be 16 stades by Marinus and Ptolemaeus.51 We see that al-Kindi's account of the elements, the exhalations, the heating of the earth by the sun's motion, the upper air being dragged along with the celestial motion and the cause for clouds not being formed below a certain level nor above a certain (other) level is very much Aristotelian; some details are added by al-Kindi. The exhalations are formed besides the already existing air. They are different from air and they differ from earth and water in that they are accidentally hot. Therefore they are able to rise. Al-Kindi's explanation of wind is different from that of Aristotle; see below p. 176 for a further discussion. There are some indications that al-Kindi used Ibn al-Bitriq's version of the Meteorology.

6. Ikyvān as-Safā' and al-Qazwini The Ikwān as-Safā' in Risāla al-āt_ār al-'ulwiyya and al-Qazwini in Ajâ'ib al-maklūqāt show many similarities—also textual—in their discussion of atmospheric phenomena. Sersen has studied their account of clouds and the different kinds of precipitation.52 He concludes that despite the similarities between the two works, certain differences indicate that al-Qazwini's work is not just copied from the Ikwān as-Safā', but that they used a common source. Here we review their account of the structure of the atmosphere. See Chapter 3 for some remarks on their discussion of precipitation. The Ikwān as-Safā' as well as a1-Qazwīnī divide the atmosphere into three layers: the lowest is the sphere of the wind (/curat an-nasīm), then follows the sphere of extreme cold (kurat az-zamharīr) and finally the sphere of the fire (which they call ether: kurat al-atjr). It is within the first sphere that clouds are formed and precipitation occurs. They say that this sphere reaches until 16.000 cubits (dirā') above the earth and that this is more or less the height of the highest mountains. The exhalations do not rise above this height and no clouds are formed 50 Ibn al-Bitriq says that moist exhalation is compressed especially between high mountains (Ibn al-Bitriq, Meteor. 20J1), see also below p. 108. 51 It is not known where this figure comes from. The same figure is mentioned by the Ifcwān as-Safā' and al-Qazwïnî; it is not in Ptolemaeus; see Sersen 175-180. 52 Sersen' 160-196.

there due to the coldness, say the Ikwān as-Safā'. It is heat that makes the exhalations rise and that heat comes from the sunrays that are reflected from the earth. These rays do not reach the area above the mountains.53

7. Ibn Suwār ibn al-Kammār The Treatise on Meteorological Phenomena by Ibn Suwār ibn alKammār only treats the halo, rainbow, mock suns and rods; we shall survey his discussion of these subjects in Chapter 9 below. He gives some introductory remarks concerning the exhalations, saying that when two elements change into each other this occurs via some intermediate body different from both; this third body is an exhalation. Between earth and fire there is smoky exhalation, and between water and air there is watery exhalation54 This is clearly similar to Olympiodorus' interpretation of the exhalations (see above p. 41).

8. Ibn Sīnā Ibn Sīnā treats meteorological phenomena in the fifth section of the TabViyyāt (the part of the Kitāb aš-Šifā' on natural philosophy). This section is described as dealing with the causes of being of the inanimate things, such as minerals and meteorological phenomena. The section is divided into two treatises. The first one deals with phenomena that occur in and on the earth—geological phenomena, as we would call them—the second one with phenomena that occur above the earth. The exhalations play a role in both treatises, but they are introduced in the second one, after the discussion of precipitation. Ibn Sīnā says that all meteorological phenomena are due to vapour (bukār) and smoke (dukān) (i.e. moist and dry exhalation), that are dissolved from the earth by the celestial heat. Vapour is moist and heavy, and rises slowly; smoke is dry and light and rises quickly. Vapour is moist and hot, smoke is dry and hot. Mostly they rise together as a mixture; they are called by the name of the exhalation that is dominating. Vapour rises until a level not far away, together with smoke; then 53

It is remarkable that al-Kindi gives 16 stades as the height of the highest mountains, above which exhalation does not rise. However, he gives as the reason for this that above this level everything is dragged along with the celestial motion; see above p. 53. 54 Ibn Suwār, Treatise on Meteorological Phenomena, see below p. 318.

smoke separates itself from the vapour and rises further until it borders fire. Ibn Sīnā relates that, standing on a high mountain, he has observed smoke separating itself from vapour. The vapour formed clouds gathering below him, and he saw smoke being separated from it and rising quickly above the surface of the clouds. It was black and had a smell of burning. He says that vapour is the matter of rain, snow, etc., and phenomena that appear in it are the halo, the rainbow, mock suns and rods. Smoke is the matter of winds, lightnings, meteors and comets.55 In his discussion of the formation of clouds, Ibn Sīnā says that they are a condensation of vapourous matter, and vapour is something between water and air.56 The fourth section of the TabViyyāt of the Šifā', which is the one preceding the discussion of the meteorological phenomena, is entitled "On action and passion" (or: "On affecting and being affected" - Fī l-afāl wa-l-infi'ālāt). See below p. 311 for a discussion of how this section fits among other sections of the TabViyyāt. It is mostly devoted to the sea and its saltness and to the subjects from Book IV of the Meteorology. The first chapter deals with the question how the elements are ordered in layers and is relevant for the subject under discussion in this chapter. Ibn Sīnā says that the elements do mostly not exist in their pure forms, because the heat of the celestial bodies makes them vapourous and smoky, and also a little fiery and airy. The heat also dissolves watery vapour and earthy smoke. Therefore most water and air occur in mixed form. Fire in the upper atmosphere is the purest element, for the exhalations will not rise as far as that, and if they do, the fire will transform them quickly. Similarly, the deep interior of the earth will be the purest earth, for that area is not influenced by the celestial bodies. The earth may be divided into three layers: the interior of pure earth, a layer in which water is mixed with it and forms clay, and a layer that used to be water, but was uncovered and dried by the sun; that is the layer of the land and the mountains. What was not uncovered is sea and that is the element water. Ibn Sīnā elaborates on the question why the element water cannot be anything but the sea, arguing that it cannot be some quantity of water under the earth.57 Air is also divided into layers: a vapourous layer, a layer of pure air, and a smoky layer. This is the order, because smoky exhalation is hotter than vapourous exhalation and thus rises higher. The vapourous exhalation is in fact rarefied water and water is cold by nature; 55 56 57

Ibn Sīnā, as-Sifa, Tab. 5 39,1-18. ibid. 35,4-6. Ibn Sīnā, as-Sifa, Tab. 4 202,5-204,2.

therefore, when the heat that dissolved the vapour has disappeared, the vapourous air is colder than the rest of the air. However, what is adjacent to the earth is warm due to the earth that is heated by the sun. This layer is surrounded by a cold layer of vapourous air, then comes pure air and finally smoky air. This smoky air is a kind of mixture between air, fire and earth. It is adjacent to the layer of fire.58 The section of Ibn Sînâ's Kitāb an-Najāt in which the meteorological phenomena are treated starts with an introduction, similar in structure to that of Aristotle: it discusses the elements, how these elements are ordered between earth and heaven, how the sun heats the earth, and which exhalations are dissolved by that heating, as follows:59 The elements do not exist in their pure forms, but are always mixed. Fire that exists in its natural place is the purest element. If other elements come to its place they are changed into fire because of its great power. Earth comes after fire as far as purity is concerned; if surrounding matter is mixed with it, earth does not change it. The elements are ordered in layers. Earth and water (land and sea) are surrounded by vapourous air (hawā' bukārī). This consists of two layers: the lower one, adjacent to the earth is heated by the sunrays that heat the earth, which in turn heats what surrounds it; the next layer lacks this heating; therefore it is cold high in the mountains, and that is where clouds may gather. Above these two layers is the layer of almost pure air. Then comes the layer of smoky air (hawā' dukānī). This is most similar to fire and thus belongs above vapour and air. Above that is the layer of fire. The phenomena in the sublunar world occur under the influence of the celestial bodies. They are not hot or cold themselves, but cause heat and coldness in the sublunar world by forces that pour forth from them. Thus, it is not the sun itself that is hot; the power to heat resides in the sunrays. One may observe this from the burning power of the rays that are reflected from a (burning) mirror. Only the spot that is hit by the rays becomes hot; what is above it remains cold. Thus, what is struck by sunrays is heated, and that in turn heats the air. This heating power of the heavenly sphere dissolves vapour (moist exhalation) from the watery bodies and smoke (dry exhalation) from the earthy bodies; mostly a mixture of both is dissolved, and it is called by the name of what is dominating. Vapour is hot, moist and close to the nature of air; smoke is hot, dry and close to the nature of fire. Vapour will not rise further than the stratum of air. If it comes in that

58 59

Ibn Sīnā, aì-Šifa, Tab. 4 2043-13. Ibn Sīnā, an-Mzyāf ' 152,12-154,5.

second layer of air, where the heating influence of the reflected sunrays is not felt anymore, it becomes cold, and condenses. Smoke rises to a higher level than vapour, until it reaches the border of fire. This occurs when it has become free of all admixtures of particles of earth and water. There it may be ignited. Other things happen when particles of earth and water remain in the smoke. Ibn Sīnā subsequently gives an enumeration and short description of the phenomena that arise from ignition of the pure smoke, followed by those connected with vapour (clouds, precipitation, halo, rainbow). Then he describes the formation of winds, sources, earthquakes, the cause of the saltness of the sea, and how thunder, thunderbolts, whirlwinds, minerals and metals come to be. This makes up the rest of the section of the Kitāb an-Najāt on meteorological phenomena. We shall not refer to it anymore, unless points of view are brought forward that are different from those in the Kitāb aš-Šifā'. We conclude that, according to Ibn Sīnā, the exhalations are bodies that come to be and exist besides the already existing air and fire; this is similar to the interpretations of Olympiodorus, Ibn al-Bitriq, Ibn Suwār, al-Kindî and Ibn Rušd. Ibn Sînâ's structure of the atmosphere differs from that of Aristotle: above the stratum where moist exhalation cools and condenses there is a layer of pure air; then follows a layer of air into which smoky exhalation has risen; finally there is the stratum of fire. The sunrays heat the earth, although the sun itself is not hot. The subject is discussed in the Šifā' in the chapter dealing with the inhabitable regions of the earth, see below p. 197. Ibn Sīnā does not mention the motion of the sun's sphere as a cause of heating.

9. School of Ibn Sīnā The sections of Bahmanyār on the structure of the atmosphere and the dissolution of exhalations are mostly a summary of Ibn Sînâ's Šifā' and Najāt.60 Abū 1-Barakāt mentions the exhalations at the beginning of his chapter on clouds and precipitation. His expressions and phrasing are different from those of Ibn Sīnā, but the content is the same.61 Fakr ad-Din's section on the exhalations is a rendering of Ibn Sînâ's Kitāb

60

Bahmanyār, at-Tahsīl 700,6-701,11 and 710,6-7113 is similar in text to Ibn Sïnâ, aš-Šifâ', Tab. 4 202,5-204,13 and an-Najāt 153,7-15, respectively. 61 Abū 1-Barakát, al-Mu'tabar II 213,7-11.

aš-Šifā'.62 He also has a section that corresponds to Ibn Sînâ's chapter on how the elements are ordered in layers. It is a paraphrase of Ibn Sînâ's chapter from the fourth section of the Tabî'iyyât of the Šifā', with many phrases taken from his text.63

10. Ibn Bājja In his Commentary on the Meteorology Ibn Bājja intends to follow the course of Aristotle's book and to give additions and digressions on the subjects treated by Aristotle. Apparently his intention is only to complete Aristotle's account,64 not explain it. It looks, indeed, as if he assumes that the reader already knows and understands Aristotle's text. In fact, it is neither a systematic account of nor a commentary on the Meteorology, but rather a collection of flashes of thought and ideas occasioned by reading Aristotle's book. An edition and translation of the work is given below. Here we give a survey of the most important features. Ibn Bājja begins his treatise with a survey of the elements, their natural motions and their natural place (as Aristotle did in his chapter 1,2). The elements combine to form composed bodies. Composition occurs either by means of mixing (mizāj) or juxtaposition (tajāwur).65 Motion and place of such composed bodies are discussed. Then he states that heat reaches the earth from the celestial bodies, especially the sun, and that this heat acts upon the elements; from them different kinds of bodies arise (the exhalations), all moving upward. They are the cause of various things in and on the earth, such as minerals, earthquakes, certain kinds of plants; they also rise above the earth into the air. The air has two layers: close to the earth, until the level of the mountain tops, is air that is thick and does not move as a whole. Above the mountain tops the air is moved along in a circle by the celestial motion, and thereby becomes thinner (latufa). When exhalations arrive there they are also made thin.66 The sun heats the earth in two ways: by its daily motion, and by the

62

Fakr ad-Dīn, al-MabāhiL II 172,2-9 is similar in text to Ibn Sīnā, aš-Šifā',

Tab. 5

Fakr ad-Din, al-Mabàhil II 140,7-141,19 paraphrases Ibn Sīnā, aš-Šifâ', 202,5-204,13. 64 Ibn Bājja, Commentary on the Meteorology, see below p. 398,21-22. 65 A similar distinction is made by Ibn Rušd (see below pp. 61-62). 66 Ibn Bājja, Commentary on the Meteorology, see below pp. 384-396.

Tab. 4

39,1-8. 63

reflection of its rays.67 That the reflected rays have a heating power is clear from burning mirrors.68 Then Ibn Bājja gives an account of the subjects to be discussed. He divides the air into four layers. The highest is the layer where comets and similar phenomena occur; the second is where clouds and rain are formed; the third is for dew and hoarfrost; the fourth is where plants and animals live. He says that Aristotle will successively discuss the phenomena in these layers. The discussion of the phenomena in the earth will be next. They are caused by the same exhalations as the phenomena in the air. A general account of the phenomena in the earth is in Book IV, and the account of what is specific for the minerals will be in the Book of Minerals. The Meteorology in fact contains the account of the exhalations and what is related to them. They are the most simple composite bodies, and one is informed in this book about how they arise from the elements and again return to them. Heat dissolves the exhalations from the earth and water and they generally rise upward as a mixture. The hotter part rises to the highest place and becomes fire. Sometimes exhalation that arrives in that region is dissolved and burned by a strong motion; then it becomes a visible phenomenon.69 After this introductory section a digression follows on the way in which one searches for the cause of things. The discussion is related to certain subjects from the Analytica Posteriory and contains some references to it. Terms such as proof of the cause and absolute proof70 are mentioned.71 Then Ibn Bājja starts the discussion of the individual meteorological phenomena, beginning with those that occur in the upper part of the atmosphere, the layer adjacent to the celestial motion. The Milky Way is treated first, similar to Ibn al-Bitrlq and different from Aristotle. A survey of his account will be given below in Chapter 2 below. Before turning to the other phenomena in the upper atmosphere, he again summarizes the doctrine of the exhalations: the heat dissolves two exhalations from the earth; one is hot and dry, the other is hot and moist. Both occur in a thin (raqiq) and thick ( k a t j f ) version. The dry, thin exhalation is a kind of fire (wahaj), the dry, thick one is smoke (dukān) and the moist exhalations are both called vapour

67

p. 62). jro 69 70 71

The same doctrine is in Olympiodorus (see above p. 44) and Ibn Rušd (see below Ibn Bājja, Commentary on the Meteorology, ibid., see below pp. 406-410. See Lettinck 75. Ibn Bäjja, Commentary on the Meteorology,

see below p. 404.

see below pp. 412-424.

(bukār).12 They rise together as a mixture until different levels. A part rises in the air until the level of the tops of the mountains. Another part rises in the air above the mountains; that air has a circular motion, which makes the exhalation thinner. It becomes partly (moist) air, partly (dry) fire.73 Then Ibn Bājja turns to the other phenomena in the upper atmosphere (besides the Milky Way): comets, mock suns, 'goats', rods and torches. Their common feature is that they arise by ignition of the inflammable matter in the upper atmosphere. According to Aristotle, the mock suns and rods do not belong to this domain, but are visual effects of reflection in the clouds that are formed in a lower layer of the air. Ibn Bäjja's discussion of comets starts with the refutation of other opinions, those of the Pythagoreans and Hippocrates, like Aristotle's discussion, and then breaks off without being complete.74 One might argue that it is not Ibn Bäjja's intention to give a complete discussion, as he only wants to add to what is already known from Aristotle. However, the whole discussion of the meteorological phenomena above the earth ends here. There is nothing about the other phenomena in the upper atmosphere that were mentioned, nor about rain, wind, the sea, thunder, earthquakes, etc. The next subjects he starts to treat are those from Book IV. At the beginning of his account of the fourth Book he refers to his preceding discussions on rain, the sea, thunder and earthquakes.75 Also, he refers in a previous passage to his account of "the six bodies and accidents that arise from the air that has as its farthest limit the tops of the mountains".76 Apparently his commentary was once more complete than what is now extant and the largest part of Ibn Bäjja's commentary on the Meteorology is missing from the manuscripts. As for his account of Book IV, Ibn Bājja repeats that the subject of this Book is a general account of the things that arise from the two exhalations within the earth, and that their specific properties will be the subject of the Book of Minerals. Then he treats, like Aristotle, but with many digressions, the influence of heat and coldness on the elements. The account breaks off when he starts to discuss moistness

72

Also Pseudo-Olympiodorus distinguishes a thin and a thick exhalations, see above pp. 48-49. 73 Ibn Bājja, Commentary on the Meteorology, see below p. 436. 74 ibid., see below pp. 440-448. 75 ibid., see below p. 450,9-10.

version

of

the

7 ft

ibid., see below p. 436,19-20. He probably means: rain, snow, dew, hoarfrost, hail and the rainbow. The halo, rods and mock suns are thought to occur in the upper atmosphere, see below p. 424,14-15.

and dryness.77 Thus, only a few fragments are extant of Ibn Bäjja's commentary on the Meteorology. From Ibn Bäjja's discussion of the Milky Way it appears that, according to him, the highest stratum of the atmosphere is the sphere of fire. Below that is air, into which the exhalations rise; adjacent to the fire is a layer with smoky exhalation and below that is the purer air; then follows a layer with vapourous exhalation.78

11. Ibn Ru'sd Ibn Rusd's introductory paragraphs of his Short Commentary are similar to those of Aristotle's book. First he describes the subjects of the previous books on natural philosophy and then states that the subject of the Meteorology is to investigate what is connected with the elements air, water and earth, such as shooting stars, rain and earthquakes. They are the least complicated phenomena of the composed bodies (al-murakkabāt, opposed to the simple bodies, i.e. the elements), for no mixing is involved. They are caused by the two exhalations (bukārāri), the hot, dry, smoky (dukānī) exhalation and the cold, moist exhalation. These are treated in Books I-III. The subject of discussion in Book IV is the homoiomerous bodies (al-ajsām al-mutašābiha al-ajzā') and their general properties. It is an addition to and completion of what has been said about the composed bodies in De Generatione et Corruptione.79 In the books following the Meteorology each kind of particular body will be treated, to start with the least complicated ones, the metals (al-ma'ādin).80 After that the investigation will be about plants and animals. Ibn Rušd describes in detail the various subjects to be investigated in connection with animals, and concludes by stating that not all the books he has enumerated are extant from Aristotle.81 Then follows a recapitulation of what is known about the elements from previous books. For instance, from De Generatione and Corruptione we know that the elements do not exist in their pure form, but that each element contains admixtures of the others, by means of

77

Ibn Bājja, Commentary on the Meteorology, see below pp. 452-478. ibid., see below p. 432,12-14 and below p. 87. 79 Ibn Rušd follows Alexander's opinion, see below p. 310. 80 Note that Ibn al-Bitriq mentions a treatise on metals in the introductory chapter on the order of Aristotle's books on natural philosophy, see below p. 311. 81 Ibn Rušd, Short Commentary 2,2-5,7. 78

juxtaposition (tajāwur) and mixing {iktilāt)P This is especially true for the earth; we may observe that it contains water, air and fire. Fire in its proper place is the purest element; the other elements are not easily mixed with it, as they are not light. The elements change into one another when one of their primary qualities changes. Such a change is caused by the sun while it moves in the ecliptic.83 The sun dissolves two kinds of exhalations, a smoky exhalation that is hot and dry, and another exhalation that is either hot and moist, or cold and moist. The smoky exhalation rises high; its nature is close to the nature of fire; the hot, moist exhalation rises until below the smoky one, and still lower is the cold, moist exhalation. The smoky exhalation is easily ignited because of its closeness to the celestial motion.84 When the exhalations are mentioned again, in the discussion of rain, Ibn Rušd distinguishes two exhalations, one hot and dry, the other hot and moist.85 In the discussion of winds and earthquakes, he mentions two exhalations, a moist and a smoky one, without further specification of the moist exhalation.86 As for the way the earth is heated by the sun, Ibn Rušd follows Olympiodorus (see above p. 44): the heating occurs by the sun's motion that heats the upper atmosphere, but also by the sunrays that are reflected against the earth's surface. When the sun rises high above us, this last effect is the most important, as the rays fall almost vertically on the earth.87 From Ibn Rusd's discussion of the Milky Way in his Short Commentary it appears that, according to him, the sphere of the fire is the highest stratum of the atmosphere; then follows air with smoky exhalation; below that is purer air, finally there is a layer of air with vapourous exhalation. Here he has adopted Ibn Bäjja's account. We conclude that Ibn Rušd has used Ibn al-Bitriq's version of the Meteorology and Ibn Bäjja's treatise on meteorology. Ibn Rusd's Middle Commentary consists of a more or less faithful rendering of Ibn al-Bitriq's text, with explanatory remarks of Ibn Rušd inserted between the phrases of this text. Also, longer sections are added by Ibn Rušd. The additional text is partly derived from Alex82

The same remark is in Ibn Bājja, see above p. 58. It derives from Aristotle, De Generatione et Corruptione 1,10 328a6 ff.: elements may combine either by juxtaposition (σύνθεσις) or by mixing (μίξις). 83 Ibn Rušd, Short Commentary 5,18-6,11. 84 ibid. 6,19-7,1, 8,10-12 and 9,8-15. In fact, Ibn Rušd posits three exhalations, like Ibn al-Bitriq. The passage is similar to Ibn al-Bitriq, Meteor. 30,12-31,9. 8Í I b n Rušd, Short Commnentary 20,1-3. 86 ibid. 3 5 3 - 4 and 52,2-3. 87 ibid. 7J.-8.

ander's commentary. We shall see that Ibn Rušd also knew and used the works of Pseudo-Olympiodorus, Ibn Sīnā, Ibn al-Haytam and Ibn Bäjja. Ibn Rusd's version of the section corresponding to 340b24-32, in which the exhalations are mentioned, is somewhat different from that of Ibn al-Bitriq. Ibn Rušd says that the difference between both exhalations is that the fiery exhalation (wahaj) is hot and dry, being matter for fire, whereas the vapour is hot and moist, being matter for water.88 In the section corresponding to 341b6-23 Ibn al-Bitriq mentions three exhalations rising from the earth. Ibn Rušd also says that three exhalations are dissolved from the earth and more or less follows Ibn al-Bitriq's text.89 In the introduction to the account of precipitation the exhalations are mentioned again. Ibn Rušd follows Ibn al-Bitriq, adding some explanatory phrases, but omitting the statement that moist exhalation becomes air when nothing further occurs to it. His account runs as follows: The hot, dry exhalation rises above the region of the clouds, i.e. above the region that is not reached anymore by the sunrays that are reflected from the earth. This exhalation is made thinner by the motion of the celestial bodies. The moist exhalation does not rise above the region of the clouds, but remains there. Ibn Rušd then continues with what happens when this exhalation cools.90 The account of winds starts with the statement that the sun dissolves two exhalations from the earth: a hot, moist one and hot, dry one. In the account of thunder the exhalations are just called dry and moist.91 Ibn al-Bitriq does not mention the motion of the air above the mountains that is caused by the motion of the celestial spheres as a another cause of the absence of clouds in the upper atmosphere (340b32 ff., see above p. 37). Ibn Rušd, in his rendering of Ibn al-Bitriq's text, inserts a phrase that mentions this other cause. He must have taken this from Alexander.92 Ibn Rušd adds a long section in which he deals with two questions: (1) Alexander states that the fire that is adjacent to the sphere of the moon is not real, burning fire, but is called 'fire' homonymously. Real fire is an ebullition of the hot and dry, just as ice is the solidification of the cold and moist, as Aristotle said in De Generatione et Corrup88 89 90 91 92

Ibn Rušd, Middle Commentary 25,1-4. ibid. 35,10-36,5. ibid. 63,10-17. ibid. 97,3-5 and 132,6. ibid. 25,15-16; Alexander, in Meteor. 15,30-16,8.

tione,93 Ibn Rušd amplifies on this, saying that this means that real, burning fire is an excess of the fiery element, just as ice is an excess of the watery element, i.e. the qualities hot / dry and cold / moist are present in them in an extreme degree, stronger than in the elements fire and water. An argument for Alexander's claim is that if that region were filled with real fire, it would burn the air and the other elements. Another argument is that we do not see fire giving light in that area, except when hot, dry particles are ignited and give rise to comets and shooting stars. It has been said that fire becomes luminous by admixture with earthy particles.94 Then Ibn Rušd says that in the pure, simple elements the qualities have the stronger power, for composed substances cannot have qualities in a stronger degree than simple substances; when simple substances are mixed, the power of their qualities becomes weaker. Ice is not the element water, but contains an admixture of earth, and similarly burning, luminous fire contains an admixture. Thus, it is strange how Alexander can say that in burning fire the qualities are present in an extreme degree, unless he means that burning fire is not in the proper place of fire because it is composed, not a pure element. When Aristotle in De Generatione et Corruptione described ice and fire he conceived them as the (composed) substances that are nearest to the simple elements in respect of their primary qualities.95 Ibn Sīnā has said that earth, water and air, such as they exist in their proper places are not pure elements, but are composed of a mixture, whereas fire, such as it exists in its proper place, is simple and pure (see above p. 55). If this were true, fire would destroy the other (composed) substances, since what is simple is more powerful than what is composed. Also, if one considers the matter from the point of view of balance between the elements, it is not possible for three of them to mingle and change into one another, whereas one element would remain pure. If the elements remained pure and did not change into one another, there would be no generation and corruption. However, what occurs is that the total quantity of each element remains the same, but part of an element will change into an equal (i.e. equal in power) part of another element.96 Thus, fire also must be a composed substance. Ibn Rušd concludes the discussion of this subject by saying

93

Alexander, in Meteor. 14,19-28; this statement is in Aristotle (340b23), not in Ibn al-Bitriq. The reference to De Generatione et Corruptione is 330b25-9. 9 * Ibn Rušd, Middle Commentary 27,10-29,7. The statement expressed in the last phrase is found in Ibn Sīnā, see below p. 81. 95 Ibn Rušd, Middle Commentary 29,8-30,18. 96 In modern terms: there is a dynamic, not a static balance between the elements.

that just like ice becomes white due to an admixture of earthy parts in the water, (burning) fire is luminous due to some admixture. This admixture seems to be absent in the fire of the upper atmosphere.97 Ibn Rušd does not give a further explanation about what kind of admixture is present in burning fire and absent in the fire of the upper atmosphere. In the Short Commentary he says that the fire in the upper atmosphere is the purest element (see above p. 62). (2) Commentators have stated that the sun heats the earth, not only by the motion of the celestial sphere, as Aristotle says, but also by its light.98 Indeed, if one considers that a burning mirror or glass may ignite a piece of cotton, one might conclude that light by itself is able to cause burning, especially if many rays of light come together in one point. One of the principles in this discipline is that if something actually comes to be, then this is caused by something that is actually of the same kind. Thus, heat only arises by something hot, and fire is generated by a fiery body. Light is not a body; if it has heating power by itself, this must be due not to the light itself, but to the body that emits it; this body should be hot. In fact, this body (the sun) is not hot. We have to conclude that heat does not arise because of the properties of light itself, but accidentally. The heat arises because of the motion (of the celestial sphere), as Aristotle asserts, and emission of light occurs together with it accidentally. Therefore people thought that light was the cause of the heat, but this is not the case. This heat, caused by the motion, behaves like the rays of light: it is reflected if the rays are reflected and in burning mirrors it moves from many places to one place. Motion also causes heat accidentally—for motion is not something hot; it makes the place suitable to receive heat. For instance, fire is struck from a flint due to the heat that is in the air by means of the motion of the flint. Similarly, the celestial bodies cause heat by their motion. If motion were to cause heat by itself, then rest (absence of motion) would cause cold, and that is impossible.99

97 98 99

Ibn Rušd, Middle Commentary 31,1-32,4. This is stated by Olympiodorus, see above p. 44. Ibn Rušd, Middle Commentary 32,15-34,20.

CHAPTER TWO

PHENOMENA IN THE UPPER ATMOSPHERE

L Aristotle In chapter 1,4 Aristotle starts the treatment of the individual meteorological phenomena with the discussion of the phenomena that occur in the upper atmosphere, adjacent to the sphere of the moon, i.e. the region containing inflammable hot, dry material (so-called fire or ύπέκκαυμα). The hot, dry exhalation which the sun dissolves from the earth rises to this region. The following phenomena occur in this region: burning flames, shooting stars, torches, 'goats'1 (these are discussed in chapter 1,4), comets (1,6 and 7) and the Milky Way (1,8). They are all due to the same process: inflammation of the dry, hot exhalation. Sometimes shooting stars arise in a lower stratum, that is, in air; they are discussed in connection with the phenomena of the upper atmosphere because they are caused by the same process: inflammation of the dry, hot exhalation. Phenomena such as chasms, trenches and red colours are effects of light from the burning upper atmosphere being refracted or reflected in the denser stratum of air (chapter 1,5). According to Aristotle's theory of exhalations the upper atmosphere is filled with hot, dry exhalation which rises from the earth under the sun's influence. This is an inflammable material, which becomes ignited (έκκαίεσθαι) by the motion of the heaven, wherever this material is most suitable for ignition (341b18-23).2 The effect is different according to position and quantity of the material: if it extends 1

'Goats' are a kind of meteorites, see above p. 18n24. When Aristotle later, in 342b1, refers to this place, he says that the material ignites after having been clustered or densified (συνίστασθαΐ). This also appears from the discussion of the comets and the Milky Way, which are formed in the same way (πύκνωσις 344al6; συγκρίσις 344b9, 346a23; συνίστασθαι 344bll). Thus, σύστασις 341b23 refers to such clusters. On the other hand, Aristotle also says, in 340b13, that the upper atmosphere is ignited by the celestial motion when this motion dissolves (rarefies) the material. From 345b34 it appears that dissolution and clustering go together: when the material is dissolved by motion, clusters (σύστασίς) are separated off, from which comets are formed. Philoponus sees the contradiction between ignition of clustered material and ignition after rarefaction. He solves the problem by stating that both effects play a role: the material to be ignited must neither be too dense, nor too rare. Sponge and cork are not easily ignited because they are too rare, nor are ebony and ivory because they are too dense. See Philoponus, in Meteor. 58,4-32. Λ

4

lengthwise and breadthwise, the result is a burning flame (καιομέμη φλόζ), similar to what we see when a field of stubble is burnt; if it extends lengthwise only, we see 'goats', torches or shooting stars. It is called a 'goat' (αι'Ο when it throws off sparks, a torch (δαλός) when there are no sparks; if the material is scattered into small parts, then we observe shooting stars (διάττοντες άστερές) (341b24-35). The hot, dry exhalation that on its way to the upper atmosphere is still lingering in the stratum of air below, may be the material source of shooting stars too. This region is filled with a mixture of vapourous, moist exhalation, and hot, dry exhalation. When the air becomes cold, the moist exhalation condenses (συνίστασθαι), while the hot element is ejected, ignited and then moves as a thing thrown (ριπτεΐσθαι, ρΐψις); this effect is observed as shooting stars, being shot downward, like fruit stones projected by one's fingers. They move downward because the condensation (πύκνωσις) by which they are ejected has a downward inclination. Thus, there are two kinds of shooting stars. One kind is formed in the upper region of 'fire', and is produced by ignition of the dry exhalation, caused by the celestial motion. The other kind is formed in the region of air, and is produced by ejection of dry exhalation, caused by cooling and subsequent condensation (συνιέναι 342a19, συγκρίνειν 342a29) of the moist exhalation, in which the dry one is mixed. The motion of shooting stars is mostly oblique because it is a combination of a forced downward motion and a natural upward motion (341b36-342a30). The Greek commentaries give a more detailed explanation of the shooting stars of the first kind: the ignition of the inflammable material starts at one end; the burning material ignites the material adjacent to it, and is subsequently extinguished. Then the next adjacent material is ignited and so on. This gives the impression of a moving star, whereas in fact different parts of inflammable material are ignited and extinguished successively. This is the same process as when a lamp that has gone out, but still gives off smoke, is ignited by the flame of a burning lamp above it (Aristotle 342a3 ff.). As for shooting stars of the second kind, it is one and the same quantity of hot exhalation that is ejected and then shoots away.3 At night one may observe phenomena such as chasms (χάσματα), trenches (βόθυνοι) and blood-red colours (αίματώδη χρώματα).3 They occur because the upper atmosphere is ignited when there is a suitable 3

Alexander, in Meteor. 21,27-33 and 22,17-18; Olympiodorus, in Meteor. 38,28-31; Philoponus, in Meteor. 61,8-39 and 65,7-13. 4 This refers to the aurora borealis, according to some (c/. Lee), to effects of cloud coloration, according to others (Webster, Strohm).

condensation (clustering; συνίστασθαι) of inflammable material, just as in the case of shooting stars. The light of such an ignition may shine through (διαφαίνομαι) in the thicker medium of air, or be reflected (άνακλάομαι) against air. This causes various colours because the colour of the light of the inflammation is mixed with that of the medium, just as rising and setting stars appear red if their light is seen through smoke.5 If light breaks out from a dark background one gets the impression that the dark places are deeper, so that one sees a chasm. If the clustering (of inflammable matter) goes further, torches are formed (342a34-b21). Aristotle does not give an explanation of trenches. Webster assumes with Thurot a corruption of the text and adds some words that explain trenches. Another phenomenon caused by inflammation of clusters of dry exhalation in the upper atmosphere (ύπέκκαυμα) are comets (κομήτης). The ύπέκκαυμα is moved along by the celestial motion, and this causes inflammation of suitable material. The process is the same as that for shooting stars, except that the inflammable material of a comet is denser, so that the inflammation remains stationary; in the case of shooting stars the material is less dense, so that the fire runs along it and gives the impression of something quickly moving. Comets take different forms depending on the form of the exhalation (344a9-25).6 Sometimes a suitable cluster of inflammable material is formed not independently, but under the influence of the motion of a star or planet. Then a comet is formed which follows that star in its motion. The star seems to have a fringe; however, that fringe is not attached to the star itself, but exists in the sublunar ύπέκκαυμα (344a34-b8). The appearance of comets foretells wind and drought. This supports the supposition that their material cause is the dry exhalation, for winds are dry exhalation too, as we shall see later. Aristotle mentions several examples of occurrences in which wind and the appearance of comets have coincided (344b18-345a5). Comets are rare. They appear outside the tropics more than within them, because most of the dry exhalation collects in the area of the Milky Way (see next paragraph); also, dry exhalation that collects in the tropics is easily dissolved again because of the motion of the sun and planets (345a5-10). 5

More about colours, reflection and refraction in Chapter 9, below pp. 244 ff. Aristotle distinguishes 'hair star' (κομήτης) and 'beard star' (πωγωνίας). Greek authors distinguish several more forms, see Gilbert 656 and Gundel, "Kometen", in: Pauly, Wissowa, Real-Encyclopädie XI/I Sp. 1143 f f ; they are also adopted by Arabic authors. 6

The Milky Way (γάλα) is the last phenomenon discussed by Aristotle of the phenomena that occur by ignition of the dry exhalation in the upper atmosphere. He again relates how comets are formed that seem to be attached to a star or planet: the motion of such a star ignites a suitable cluster (σύσχασις 345b34) of dry exhalation, and this follows the course of the star. The same process occurs for the heaven as a whole and the motion of all stars, especially in the region where the stars are large, numerous and close together. Ignition of dry exhalation by the motion of this celestial region is the cause of the Milky Way. It extends outside the tropic circles, because within them the clustered material of exhalation is dissolved by the motion of the sun and planets. (This also prevents comets to be formed in the tropics, see previous paragraph). That region is also full of big and bright constellations, and of scattered stars; thus, the inflammable material is continually collecting (συγκρίσις) there. The light of the Milky Way is stronger in that part where it is double, and in that part the stars are more numerous and denser. This proves that the cause of the Milky Way is the motion of the stars, such as has been described above. If a comet gives the impression of a fringe belonging to a single star, the Milky Way could be said to be the fringe of a whole circular zone in the heaven; the process is the same in both cases (345b31-346b6). Aristotle discusses and refutes the opinion of others on comets and the Milky Way in chapter 1,6 and in the first part of 1,8. (1) Opinions on comets, (a) Anaxagoras and Democritus say that comets are a conjunction of a star with a planet, or a planet with a planet; when they seem to be very close, they give the impression of a comet. Democritus claims, as a proof for his view, that stars were seen to appear when comets dissolve. Aristotle refutes his opinion, pointing to the fact that, according to this view, this should always occur, whereas Aristotle has observed a conjunction of Jupiter with one of the stars in the Twins, without a comet resulting. Furthermore, the stars appear as indivisible points—although in fact they are not—therefore, their conjunction cannot bring about an appearance of a magnitude larger than a point. (bl) According to some Pythagoreans a comet is one of the planets, a sixth planet. It will be a planet that stays close to the sun, like Mercurius; thus, it does not often become visible. This view is refuted by stating that all planets move within the zodiac circle, whereas many comets have been observed outside it. Moreover, more comets than one have been observed simultaneously. Also, comets have appeared while all planets were visible, or while some were visible and others invisible

because of the neighbourhood of the sun.7 (b2) Hippocrates of Chios and his pupil Aeschylus also consider a comet to be a planet. They say that the tail is not part of the comet itself, but a reflection of our visual rays towards the sun, against moisture which has collected in the atmosphere under the influence of this planet.8 It appears at longer intervals than other stars because it is slow in falling behind the sun (i.e. for a long time it stays too close to the sun to be visible), and also because it acquires a tail only when it is visible north of the tropics, with the sun at the summer solstice. Between the tropics it does not get a tail because the sun dries up the moisture there, and when it is south of the tropics only a small part of its course is above the horizon, and then our vision cannot be reflected to the sun; when the planet is north of the tropics and the sun is in the winter solstice, there can be no reflection to the sun either. If this were true, then a comet should sometimes appear without a tail, as they claim it does not have a tail in every place—then it would appear as just a planet. But no planet has been observed besides the five, which often appear simultaneously above the horizon. Moreover, comets have been observed in other regions than the north, and also when the sun was in the winter solstice. An argument against both views, that a comet is a conjunction of two heavenly bodies and that it is a planet, is that some fixed stars also get a tail (see above p. 68). A further argument against both views is that comets disappear fading away above the horizon, without leaving behind one or more stars. (2) Opinions on the Milky Way. (a) Some of the Pythagoreans say that it is the path of one of the stars that fell from heaven at the time of Phaeton's fall, others say that the sun used to move in the region that is now the Milky Way, but later changed its path; the passage of these bodies has left the region scorched. If this were true, the zodiac would be affected similarly, even more so, because the sun and the planets move in it. (b) Anaxagoras and Democritus say that it is the light of certain stars. When the sun is moving under the earth its light does not shine on some stars because they are in the earth's shadow. Therefore the light of these stars is visible, whereas the light of the stars outside the 7 Olympiodorus says that if someone were to object that there is no reason why a comet, as a sixth planet, could not have a motion outside the zodiac, different from the other planets, the answer would be that in that case the comet would not appear at intervals, but would always be visible, see in Meteor. 533-9. 8 That is, the tail is the sun such as we see it reflected in the collected moisture. Aristotle uses the language of visual rays that depart from our eye and travel towards the object seen. See below p. 245 for this theory of vision.

shadow is invisible, because it is overpowered by the rays of the sun. We see the light of the stars that are in the earth's shadow as the Milky Way. This view is impossible because if the sun changes its position in relation to the earth, the earth's shadow will change its position too; thus, the Milky Way would change its position in relation to the constellations of the fixed stars. Moreover, the earth is very small in relation to the sun and to the distance of the fixed stars; therefore the shadow of the earth will not reach the fixed stars, and the sun will shine equally on all stars. (3) A third view considers it to be the reflection of our vision to the sun, just as the tail of a comet is such a reflection, according to their opinion. It is not clear from Aristotle's text what is supposed to act as a mirror for this reflection. It could be the stars that are in the region of the Milky Way, or some exhalation in the atmosphere that gathers under the influence of these stars, and has the same motion. If we take the analogy with the formation of the tail of a comet to be decisive, then it must be the exhalation. Alexander says that the mirror is the exhalation, but in another place he says that it is the stars themselves.9 Olympiodorus says that the reflection is against the stars, not against vapour.10 Anyway, this view is impossible because the sun changes its position in relation to the stars, and therefore the image of the sun in the mirror will change its position too; however, we see that the Milky Way has a fixed position among the stars.

2. The Greek

commentators

We already saw how the Greek commentators explained the two ways in which a shooting star can be formed in greater detail than Aristotle (see above p. 67). Olympiodorus says that one might ask how it is possible that something hot could arise from cooling, such as occurs when the second kind of shooting stars are formed. He answers that indeed coldness may cause something to heat up, and heating may cause something to cool down. This process is known as άντιπερίσχασις;11 it will be discussed in the next Chapter, see below pp. 97-98. The Greek commentators also give a further explanation of the formation of colours in the sky, and the chasms and trenches. These phenomena are a visual effect (φαντασία, εμφασις), they are 'false' 9 Alexander, in Meteor. 38,28-32 and 40,3-4; 105,25-106,13. 10 Olympiodorus, in Meteor. 68,32-3 and 73,2-5. 11 ibid, in Meteor. 38,10-16.

see

also

Philoponus,

in

Meteor.

(ψευδής) and have no real existence (ΰπαρξις), in contrast to phenomena such as the burning flames, 'goats' and torches discussed before, that are 'true' (αληθής).12 The colours in the sky arise because light from the inflamed 'fire' is seen either refracted through or reflected against the thicker air. The first case arises when the inflammation is above a cloud that is not too thick, the second when it is obliquely above a thick cloud.13 A chasm arises when the light of the inflammation is covered by dark surface, sc. a thick, non-transparent cloud below it. Then one sees light enclosing a dark surface. The dark surface seems to be further away, and appears as a depth or chasm. Light surfaces seem to be closer to us and dark ones further away, because vision is moved by light stronger than by darkness. Painters use this effect: they paint dark the things they want to appear as a hollow, like a well, a cistern, a pit, a cave, etc. Things that they want to appear as protruding, such as a girl's breast, an outstretched hand, or the legs of a horse are painted light, surrounded by dark.14 There is no explanation of the difference between chasm and trench in Aristotle's text. Thus, the commentators have devised various possibilities. Alexander says that maybe there is no difference, or a trench is deeper than a chasm.15 Philoponus says that a chasm is deeper than a trench because for a chasm the cloud is denser and blacker.16 According to Olympiodorus, a chasm appears for a short time, a trench for a longer time.17 As for the comets, the Greek commentators follow Aristotle's theory. Olympiodorus divides the comets according to the manner of their formation in a way different from Aristotle. He first mentions the comets that arise independently from a dense cluster of exhalation, just as Aristotle. However, he interprets 344a29 ff. as a second way of formation, as follows: If a shooting star on its way finds a dense cluster

12 Alexander, in Meteor. 23,21-29; Philoponus, in Meteor. 67,28-37 and 693-9; Olympiodorus, in Meteor. 43,25-6. 13 Alexander, in Meteor. 23,29-24,10; Philoponus, in Meteor. 69,21-36; Olympiodorus, in Meteor. 44,8-15. The^ terms used for the two ways light is influenced by the cloud are: διάκλασις and ανάκλασις (Olympiodorus) or διάφασις and ανάκλασις (Alexander, Philoponus and Aristotle). 14 Alexander, in Meteor. 25,8-13; Philoponus, in Meteor. 7236-73,23; Olympiodorus, in Meteor. 4433-45,10; the examples of painted objects are those mentioned by Philoponus. Olympiodorus mentions breasts as something protruding, and eyes as something hollow. Alexander does not have the example of the painters. The source of this example is Ptolemaeus, Opt. II, 124 and 127. 15 Alexander, in Meteor. 25,19-22. 16 Philoponus, in Meteor. 683-9 and 73,23. 17 Olympiodorus, in Meteor. 4437-45,1.

of exhalation, the ignition stays there, and the shooting star becomes a comet. Besides this division he distinguishes, with Aristotle, between comets that happen to be formed vertically under a star or planet, so that it seems as if this star has fringe, and comets that do not happen to be formed vertically under a star or planet. These comets seem to be formed independently.18 With regard to the Milky Way Alexander follows Aristotle. However, Philoponus and Olympiodorus do not follow his doctrine that the Milky Way is an effect that arises in the upper atmosphere. Philoponus criticizes Aristotle as follows: If the Milky Way were an effect in the atmosphere, how would it be possible for it to always appear in the same way? It does not increase nor decrease, it never changes its form or position, whereas the formation of exhalation is not constant to such an extent. The sun in its yearly motion draws up more or less exhalation according to the seasons, but the Milky Way does not change with the seasons. The Milky Way is more constant than any sublunar phenomenon. Only what is in the celestial world is as constant as the Milky Way, therefore it must belong to the celestial world. Furthermore, if it were a sublunar phenomenon, it would cover some stars and weaken their light, just as a cloud may cover the sun. When the latter occurs, people under the cloud will not see the sun, but in other places it will remain visible. Similarly, the Milky Way would cover certain stars for an observer, but not for another observer in another place. In other words, the Milky Way would be subject to a parallax, and that is not what one observes. Also, the planets would be covered and get a different colour when they arrive in the place where Milky Way and zodiac intersect, i.e. in Sagittarius and Gemini. Aristotle's theory is not much better than the myth that Herakles wanted to suck Hera's milk. The sucking hurt Hera's breast so much that she suddenly drew away her nipple, so that the milk spouted away and formed the Milky Way. Nor is it better than the myth that the Milky Way consists of the souls of the deceased that proceed from the earth to the heaven. The milk will not flow constantly and uninterrupted, and the number of souls will not always be constant, so that this cannot explain the stability of the Milky Way. Similarly, no sublunar phenomenon can explain this stability either. For everything under the moon is subject to generation and corruption, alteration, growth and diminution, and these are not observed in the Milky Way. Therefore it must exist in the celestial world.

18

Olympiodorus, in Meteor. 59,31-60,14.

If Aristotle's view were correct, then how could the stars, being so far away, have such an influence on the sublunar world that a continuous flow of dry exhalation is drawn up during summer as well as winter? And how could the stars at the same time also draw up exhalation intermittently—namely, for the formation of comets of the second type? According to Aristotle, the Milky Way and such comets are caused by the same motion.19 Olympiodorus enumerates four arguments originating from Ammonius against Aristotle's view. (1) If the Milky Way were a sublunar effect, it would be subject to change. For instance, its light would be stronger in summer because then more exhalation is dissolved. (2) The Milky Way is seen in the same way by observers in different places of the earth (i.e. it has no parallax). (3) The moon has a parallax, as Ptolemaeus has shown in the Syntaxis,20 and the Milky Way has no parallax; therefore the moon must be under the Milky Way. (4) If the Milky Way were under the planets, they would get a different colour when they arrive in Sagittarius and Gemini, where the zodiac and Milky Way intersect. Another argument is that if the Milky Way were something in the atmosphere, it would not arise where it intersects the zodiac, for the exhalation formed under the zodiac is dissolved by the sun, as Aristotle has said.21

3. Ibn al-Bitriq and Hunayn ibn Ishāq Ibn al-Bitriq's version differs from Aristotle in the following respects. First, the order in which the subjects are treated is different. Ibn al-Bitriq's order is: the Milky Way (Aristotle's chapter 1,8), the comets (1,6 and 7), burning flames, shooting stars, etc. (1,4) and colour effects in the sky (1,5). We shall follow Aristotle's order, and first consider the chapter on burning flames etc. Ibn al-Bitriq enumerates the following phenomena arising from ignition of a suitable cluster of dry exhalation in the region of 'fire'. If the available quantity of inflammable material is long and broad, we get a burning flame Üamüd an-nār>, if it is thin, we see it as having a length only; if it is small in length and breadth, we see something as a lamp (sirāj), round as a star; if its parts are continuous with one another,22 and it stretches lengthwise, we see a

19 20 21 22

Philoponus, in Meteor. 11334-11635. Ptolemaeus, Almagest V, 17-19. Olympiodorus, in Meteor. 75,24-763· According to Aristotle, the parts are scattered in the case of a shooting star.

shooting star (šihāb).23 Thus, Ibn al-Bitriq does not mention torches and'goats', unless he refers to these with the second item. The round, star-like object called 'lamp' is not mentioned by Aristotle. Like Aristotle he distinguishes two kinds of shooting stars. He says that the second kind, that arises in the region of air, has a muddy colour.24 The red colours in the sky at night arise when parts of air condense due to cooling during the night. Light from the region of 'fire' shining on this condensed air25 is reflected (raja'a) against it towards the air under it, just as light that is reflected against water towards a wall. Then we see various colours in the sky.26 Ibn al-Bitriq only mentions reflection of light, not refraction through the condensed air. We see a chasm (jawba) or a trench (wādin) when light breaks out from a dark background.27 Ibn al-Bitriq's version of Aristotle's discussion of the comets (kawkab dū d-du'āba, pl. kawākib dawāt ad-dawā'ib) is incomplete. This applies to the refutation of the view of others and even more to the account of Aristotle's view. The refutation of Hippocrates' view that the tail is due to reflection of sunlight against moist air shows some differences with Aristotle. The following arguments are given. If the tail of a comet were to arise by reflection against moisture in the air, then it would sometimes also appear as a star without a tail. Also, all stars in the same region could acquire such a tail, so that we would see a number of comets together, not only one (not in Aristotle). Furthermore, comets would always appear, not seldom. Furthermore, not more than five comets have been observed; if the tail would arise by reflection in air, the number of comets would be more than five. This last argument differs from Aristotle due to a wrong translation of Aristotle's phrase (343a31) "Not more than five planets have been observed." Ibn Rušd has read this argument in Ibn al-Bitriq and comments in his Middle Commentary that it is unclear.28 The account of Aristotle's view on comets is restricted to three lines: a comet arises from ignited air that surrounds the star; the fire is joined with the light of the star, so that it gets an elongated form.29 Apparently this refers to comets of the second kind. Ibn Tibbon does 23

Ibn al-Bitriq, Meteor. 31,9-32,4. ibid. 32,4-7. 25 That is, clouds, as the Greek commentators say; but al-Bitriq mention the word 'cloud'. 26 Ibn al-Bitriq, Meteor. 33,9-34,5. 27 ibid. 34,9-11. 28 ibid. 29,2-8; for Ibn Rušd see ibid. 29n2 and below p. 93. 29 ibid. 30,4-6. 24

neither

Aristotle

nor

Ibn

not mention the incompleteness of the account of the comets. Ibn Rusd gives a complete account in his Middle Commentary. Maybe his copy of Ibn al-Bitriq's text was more complete than Ibn Tibbon's copy and the extant copies. He may also have taken his more complete text from Alexander. Ibn al-Bitriq's account of the Milky Way (majarra) starts with the statement that it is a phenomenon belonging to the celestial world; the phenomena of the atmosphere will be treated after this.30 This might be the reason why he has changed Aristotle's order of treating the various subjects: what is in the celestial world must be discussed before what is in the sublunar region. First, the opinions of others are discussed. As for the opinion of those who say that the Milky Way is an effect of a previous path of the sun, Ibn al-Bitrlq adds to Aristotle's objection another one: the heavenly region where the sun moves cannot be affected at all.31 Ibn al-Bitriq's account of the Milky Way is as follows: "The air close to the celestial sphere is inflamed and fiery.32 In the celestial sphere in the place where the Milky Way is seen there are many small and big shining stars close to each other. When their light shines from behind this inflamed, fiery place one sees an elongated patch of light in that place. These are fixed stars which almost touch one another. They receive the light from the sun, and their light forms a continuous whole. Therefore one sees the Milky Way in one and the same place in the heaven, not moving away from it."33 The difference between Aristotle's view on the Milky Way and the view expressed in Ibn al-Bitriq's version of the Meteorology may be formulated as follows: According to both views the stars and the sublunar 'fire' cooperate to cause the phenomenon of the Milky Way, but their share in its realization is differently assessed. According to Aristotle, the source of its light is located in the inflamed 'fire' and this inflammation is due to the stars above it. According to Ibn al-Bitriq's version, it is the light of the stars that is in fact the light of the Milky Way, and the 'fire' causes the light of the individual stars to become one continuous patch of light. How this occurs is not clear from Ibn al-Bitriq's version. He also says that the stars are so close together that their light gives the impression of a continuous patch, and then one 30

Ibn al-Bitrlq, Meteor. 233-4. ibid. 24,1-3. 32 This phrase is from Ibn Rušds Middle Commentary 54,9 and has been inserted by Petraitis as a better translation of the Greek in 345b32 instead of "The pure fire close to the celestial sphere is inflamed and luminous" from the manuscripts of Ibn al-Bitriq's text. 33 Ibn al-Bitriq, Meteor. 25,11-263. 31

could ask what the 'fire' still adds to this effect. We shall see that Ibn Bājja takes this version of Ibn al-Bitrlq as the basis for his view (see below p. 88). Hunayn's account of the phenomena in the upper atmosphere is very similar to that of Ibn al-Bitrlq; some details of his version, however, show differences, such as the use of synonyms for certain words. Also, Hunayn is in general shorter than Ibn al-Bitriq, but on the other hand he has passages that are neither in Ibn al-Bitriq, nor in Aristotle. Hunayn treats the phenomena of the upper atmosphere in the last three paragraphs of his Compendium, in this order: burning flames, shooting stars, etc.; red colours in the sky; the Milky Way. Comets are mentioned once, but not explained; this reflects the incompleteness of Ibn al-Bitriq's version on this subject. The discussion of other opinions is entirely omitted. We mention here that Hunayn discussed comets from an astrometeorological point of view in his treatise Risāla fī dawāt ad-dawā'ib wa-mā dukira fīhā min al-'agā'ib. This treatise has not yet been edited. A list of the manuscripts and a description of the contents has been given by Sezgin.34 In the paragraph on burning flames etc., Hunayn enumerates the following phenomena, like Ibn al-Bitriq, but partly different from Aristotle: burning flames, that arise when the inflamed exhalation is long and broad; something like a lamp or star when the exhalation is small; shooting stars when the parts of the exhalation are continuous.35 He has a further explanation of shooting stars that is also given by the Greek commentators, but not by Aristotle nor by Ibn al-Bitriq, namely that the ignition of the exhalation starts in a certain place, then ignites an adjacent part of the exhalation, and so on; this gives the appearance of a falling star (kawkab munqadd), for as soon as the next part is ignited the previous part is extinguished (see above p. 67).36 The paragraph on red colours in the sky is also similar to the corresponding passage of Ibn al-Bitrlq. Hunayn says that these colours arise because light is reflected towards the air against parts of air that have condensed, just as light that is reflected against water towards a wall. There is a remarkable difference too: According to Ibn al-Bitrlq, this colouring of the sky occurs at clear nights and it is caused by light from the ignited exhalation. The latter is not explicitly expressed, but follows from the context. Hunayn says that the colouring occurs on clear days, and that it is caused by the light from the sun or stars. 34

Sezgin, GAS VII, 327-328. Hunayn follows Ibn al-Bitriq here, and differs from Aristotle who says that the parts are scattered. 36 Hunayn, Jawāmi' 302-5, 312-5. 35

The account of the Milky Way is very short and corresponds to Ibn al-Bitriq's text.37

4.

Pseudo-Olympiodorus

The Arabic version of Pseudo-Olympiodorus distinguishes the two kinds of shooting stars (kawkab munqadd) as those arising by themselves (independently - bidātihā) and those arising accidentally (bi-l-'arad). 38 The first kind arises if in the upper atmosphere smoky exhalation forms a cluster that extends lengthwise and is scattered and that cluster is then ignited by the celestial motion. The ignition goes from one part to a next one; when the next part is ignited, the previous one is extinguished; this gives the impression of a shooting star. The shooting stars that arise accidentally are formed when smoky exhalation is in the lower atmosphere, enclosed within moist exhalation. When this exhalation suddenly condenses, the smoky exhalation escapes and is projected as olive stones shot away by one's fingers. The burning flames (lahīb), torches (misbāh) and 'goats' ('anz) are explained, in the same way as Aristotle, as due to the same process as shooting stars; the form of the cluster of ignited exhalation is different in each case. A burning flame is comparable to a field of burning alfa. Goats arise when the ignition gets tongues; they appear as the hairs of a goat. These phenomena may also occur by themselves or accidentally.39 Blood-red colours, chasms and trenches are delusive effects. The origin of these effects is the light from burning flames, torches, or goats. Blood-red colours arise in two ways. The first way occurs when a thin, white cloud is situated opposite and vertically below one of the mentioned phenomena. Then our visual rays are refracted (mun'akis mutafarriq)40 through this cloud towards the luminous object and we see a mixture of both colours, sc. that of the cloud and that of the luminous object. The second way occurs if a thick, dark cloud is 37 Hunayn, Jawämi' 324-7 is almost identical to Ibn al-Bitriq, Meteor. 25,11-26,1. Note that the first phrase of this passage in Hunayn is the one inserted by Petraitis from Ibn Rusd's Middle Commentary, instead of the phrase from the manuscripts of Ibn al-Bitriq. 38 I.e. arising from exhalation that is in its specific place and arising from exhalation that is not in its specific place; see above p. 48. 39 Pseudo-Olympiodorus, Tafsīr 953-96,7. 40 See Pseudo-Olympiodorus, Tafsir 145,16 ff.: There are two kinds of deflection (inkisàr) of light: refraction (dispersion) (inkisàr wa-taitït) and reflection (in'ikäs or inkisār wa-rujū'). Also 160,2-4: Deflection (inkisàr) occurs either by refraction (dispersion) (in'ikäs wa-tabaddud) or by reflection (in'ikäs wa-rujū'). Here the first kind of inkisär is called in'ikäs wa-tafarruq.

situated far from and diagonally opposite one of the mentioned phenomena. Then our visual rays are reflected against the cloud towards the luminous object, and we see a mixture of the colours of the cloud and the luminous object, that is, blood-red.41 Chasms (hāwiya) occur when a cloud that is not very black is situated in the middle of light from one of the mentioned phenomena. Because of its darkness, the cloud seems to be further away than the light, so that something like a shallow ditch (ukdūd) appears in the sky. The blacker the cloud, the deeper and further away the ditch seems; a deep ditch is called a trench ( t a j w ī f ) . The same effect is used by painters. Parts of the body that are hollow are painted black, such as the pupils of the eyes. Protruding parts, such as breasts, are painted white.42 Comets (kawkab dû d-du'āba or kawkab dū d-danab) arise from thick smoky exhalation that gathers in the upper air, in the stratum above the tops of the mountains, that is moved along with the celestial motion. A comet either is formed independently, or its formation is dependent on a shooting star. An independent comet may arise under a planet or a fixed star, so that it seems to have a fringe; then they move together, and rise and set together. Or the comet does not arise under a star or planet; then it moves independently from it. A comet that is formed due to a shooting star arises when the shooting star finds dense material. This burns longer, so that the shooting star becomes a comet.43 A shooting star is like the burning of wool or straw: this material is light and is quickly burned up. A comet is like the burning of oakwood that burns a long time because of its density. The comets may have different forms: round (this is called 'comet'), elongated or triangular ('beard star'), rectangular with equal length and breadth,44 rectangular with greater length than breadth,45 or five-sided ('arrow-shaped').46 The account of the Milky Way begins with the statement that there are two opinions concerning this phenomenon. One of them is that it is an effect due to dense smoky exhalation, the other is that it always exists in the celestial world. Arguments for the first opinion are mentioned—they are Aristotle's arguments—followed by arguments for

41

Pseudo-Olympiodorus, Tafsir 96,9-18. ibid. 96,20-97,6; the example of the painters is from Olympiodorus, see above p. 72. 43 This division of comets according to their way of formation is somewhat different from Aristotle. It derives from Olympiodorus; see Olympiodorus, in Meteor. 5931-60,14, above p. 72-73. 44 There is a blank in the manuscript instead of the name of this form. 45 See previous note. 46 Pseudo-Olympiodorus, Tafsir 97,8-98,4. 42

the other opinion. This is similar to Olympiodorus' commentary.47 The arguments for the second opinion are as follows: (1) Phenomena in the air are subject to change; the Milky Way is not subject to change, therefore it cannot belong to the phenomena in the air. (2) Phenomena in the air are not seen in the same way from different regions of the earth; the Milky Way appears as one and the same body from all regions. (3) Phenomena due to exhalation in the upper air appear more or stronger in summer than in winter because in summer more dry exhalation rises upwards; the Milky Way is always the same. (4) If it were a phenomenon in the air, it would not appear, or appear more weakly, in the place where it intersects the zodiac, sc. in Gemini and Sagittarius; however, this does not occur. (5) If it were something in the air, it would be under the moon. But it exists above the moon, because an eclipse of the moon appears differently for observers in different places of the earth, whereas the Milky Way always appears the same for every observer. In other words, the moon has a parallax, the Milky Way does not have a parallax, therefore it must be above the moon. (6) If the planets are in the constellations of the zodiac where the Milky Way and zodiac intersect, their light is not hidden by the Milky Way; thus, the Milky Way must be above the planets.48 These arguments have all been adduced in almost the same way by Olympiodorus. From a comparison with Olympiodorus' commentary it appears that many passages are clearly derived from Olympiodorus. Other passages indicate that the treatise of Pseudo-Olympiodorus is not a mere summary or paraphrase of Olympiodorus.

5. Ibn Sīnā Ibn Sīnā in his Kitāb aš-Šifā' tells us that the shooting stars (šihāb ar-rujum) arise from light smoky exhalation that is quickly dissolved. When this exhalation arrives at the fiery area of the atmosphere and is ignited, the ignition runs through it; it is ignited and then dissolved; this gives the impression of a star that is projected. Sometimes the ignition lasts longer, and throws off sparks; this occurs when the inflammable material is denser—then we get phenomena such as a comet. Shooting stars may also be formed when coldness reaches smoky exhalation. If this exhalation is enclosed by strong coldness, it gets hot and is ignited; it is also possible that the hot exhalation is squeezed out by the

47 48

See Olympiodorus, in Meteor. 74,17-76,5, see above p. 74. Pseudo-Olympiodorus, Tafsir 98,6-99,19.

coldness, ejected downward, and is ignited by its motion.49 Before continuing his treatment of the phenomena in the upper atmosphere, Ibn Sīnā presents a digression on fire, and how fire is extinguished. A burning fire exists due to two essential principles: that which causes its ignition, sc. heat—that is the pure fire—and the material that is ignited, sc. smoke. Light emitted by a burning fire is not an essential property of pure fire, but it arises when the pure fire is in contact with smoke. When fire is burning, it does not remain one and the same fire, with the same smoky material, but it is always renewing itself. It is a continuous process of extinction and renewal, for fire moves upward as soon as it comes to be; then coldness descends upon it, and it is extinguished because it has become weak, as it has become remote from its source. Light is extinguished when one or the other of the two principles of fire perishes. Thus, it is extinguished when its principle of ignition (heat) is changed by coldness or moistness; therefore fire is extinguished by cold air or water. It may also be extinguished because its material principle gives out. When all earthy matter in the smoke has been changed into fiery matter, the smoke has disappeared, so that the fire does not emit light anymore: it has become transparent. What is left is pure fire that does not emit light and is not observable, so that people say the fire has been extinguished.50 Shooting stars, comets and similar phenomena are not extinguished because of the first reason, for coldness and moistness do not arrive in the area where they exist. Thus, they are extinguished because the smoky material gives out; they become transparent and invisible. If the smoky material is light, the ignition is quickly extinguished. If it is dense, heavy and extended, the ignition lasts a long time, as a comet (du'âba, danab) or as a star, such as the star that appeared in 397 A.H. and remained three months. First it was dark and green, then it emitted sparks and became more white; finally it was extinguished. Comets may have the form of a beard, a horned animal, and other forms. This occurs because parts of the dense material gradually become lighter and then are thrown off as sparks or horns. Among these phenomena are also the 'goats', which emit sparks that look like hair. All these phenomena are subject to the circular motion of the air that is moving along with the celestial motion. The reason why these phenomena rarely appear is that suitable matter seldom rises to that area. Often it is scattered on its way and loses its density.51 49 50 51

Ibn Sīnā, as-Šifa, ibid. 71,15-72,17. ibid. 72,18-74,5.

Tab. 5 71,4-14.

If the smoky material is too thick or too moist, it is not ignited, but smoulders and glows. Then one sees immense patches of red colour. When the light of the sun is reflected against clouds, red colours also arise, such as we see in clouds in the morning and evening. Sometimes the clouds pile up and become darker; then they appear as chasms (hūwa), ditches (ukdūd), and dark holes (manfad). If they are broad and not very dense they are called hollow (wahda); if they are denser they are called depth (gawr) and chasm (hūwà). If they are narrower the effects are stronger because black gives the impression of remoteness and dark holes. If black and white are together in one plane, the white seems to be closer than the black.52 Ibn Sînâ's explanation of most of the phenomena in the upper atmosphere is Aristotelian. However, he does not exactly follow Aristotle's distinction of the different phenomena. Goats are mentioned, not burning flames and torches. A certain kind of ignition of the smoky exhalation, similar to a comet, is called 'star'. The red color in the sky is glowing smoky exhalation; he does not say that it is light shining through a thicker medium. He distinguishes a wide variety of apparently hollow patches in the sky. Ibn Sīnā does not mention the Milky Way. When he gives an account of Ptolemaeus' Almagest in the part of the Kitāb aš-Šifā' dealing with astronomy, he omits Ptolemaeus' description of the Milky Way in terms of its position in relation to the fixed stars. From the fact that Ibn Sīnā does not mention the Milky Way among the phenomena of the upper atmosphere we may conclude that he considers it, with Ibn al-Bitriq, Pseudo-Olympiodorus and Ibn alHaytam,53 to be a phenomenon in the celestial world.

6. School of Ibn Sīnā Bahmanyâr's account of phenomena due to ignition of dry exhalation is an extract of Ibn Sînâ's Kitāb aš-Šifā'. He says that if the ignited exhalation is light, it becomes a shooting star; if it is dense, it burns for a longer time and becomes a comet. The comets move with the same motion as the heaven because the upper atmosphere is moved along

52

Ibn Sīnā, aš-Šifa, Tab. 5 74,5-15. Ibn al-Haytam shows that the Milky Way is something in the celestial world, not in the atmosphere, from the absence of parallax, i.e. if the Milky Way is observed in different places of the earth, it has the same position in relation to the fixed stars. This argument already occurs in Olympiodorus and Pseudo-Olympiodorus. See Wiedemann 1906 for a translation of Ibn al-Hayiam's treatise on this subject. 53

with this motion. Fire may be extinguished in two ways: either it is changed into air by cold, and thus becomes transparent; or the burning material gives out and the fire becomes pure fire, which is transparent too. The latter way is responsible for the vanishing of comets. If the ignited material is very dense, then it smoulders and appears as a red or black colour in the sky and as ditches, for if black and white are together in one plane, black seems further away than white.54 The account of Abū 1-Barakāt is more extensive than that of Ibn Sīnā, and has some particular features. It runs as follows: Phenomena such as shooting stars and comets occur when smoky exhalation has risen close to the sphere of fire and is ignited. The effect is similar to what occurs when rising smoke is ignited by a fire above it; then the flame returns downward to the source of the smoke. If a lamp has gone out, but still gives off smoke and another, burning lamp is placed above the extinguished one, then the smoke will catch fire and the flame will go down along the smoke to the extinguished lamp and ignite it. If the amount of inflammable material is small and rare of constitution, it is quickly ignited and extinguished; then a shooting star will be formed. If the material is dense, it will burn for a longer time and it will have the form of a star with a tail attached to it (comet). Some people say that a comet is one of the known stars that seems to have a tail when exhalation gathers under it. If that were true, we would also sometimes see a tail without a star, because it is not necessary that such a tail is formed under a star. Then one argues that this is necessary, because the star draws the exhalation to itself. This is a weak argument, because we see the star of a comet as something new, not as one of the known stars, and when the tail disappears, the star disappears with it. Therefore it is a phenomenon in the atmosphere, not in the celestial world. The jirab,55 torches and mock suns belong to the same kind of phenomena.56 Abū 1-Barakāt continues by saying that these phenomena retain their forms for a few hours or many days. This means that there must be some specific cause that keeps up the phenomenon, namely a celestial force that adheres to the smoky material and ignites it. He says that he has seen forms that resembled a waterspout or lightning, and they kept their twisted forms until they vanished. Also, appearances in the form of rods, round suns, 'goats' and torches keep their forms.57

54

Bahmanyār, at-Tahsīl 714,11-715,6. jirab, sg. ajrab: spots that look like scab. Maybe he means reddish light effects in the sky. Cf. Lane under ajrab. 56 Abū 1-Barakāt, al-Mutabar II 222,11-223,12. 57 ibid. al-Mu'tabar II 223,13-21. 55

Furthermore, he describes the appearance of a big, weak star, with a short, broad tail, that occurred in his time; that star moved with the daily motion of the heaven, but it also had a motion from north to south, from the constellation of Cassiopeia (sūra dāt al-kursī) until the southern horizon, that was performed in eleven days, and then it disappeared. If one wants to explain this by ignition of light exhalation, it is difficult to see how this ignition can be sustained for such a long time, unless one assumes that there is a supply of inflammable material, such as a lamp kept burning by a supply of oil. But how can this supply move with the special motion of this star, how does the ignition keep its form, and why does it not spread lengthwise and breadthwise? One cannot explain this phenomenon by ignition of thick exhalation either, for that exhalation would descend to the earth due to its weight, like the iron and bronze bodies that come down with a thunderbolt.58 Also, in both cases, how should one explain the motion of that star? Apart from the daily motion it also has a specific motion of its own. That motion is not natural—for it is neither towards nor from the centre of the universe—so it must be volitional, and volition comes from a soul, namely the spiritual and angelic soul that appears with its light in the sky of the world and is observed by people as an extraordinary event.59 Abū 1-Barakāt further describes an effect he has seen at night during a stormy wind: he saw large pillars of light that extended from the earth into the sky, more than twenty or thirty of them. Everybody who saw this phenomenon agreed that it was not a visual effect. It was not hot, it was not something burning, and it did not disappear quickly such as lightning. If someone stood in this light, it was as if he stood in moonlight. Abū 1-Barakāt relates that later the wind came back carrying dust and then he again saw such pillars of light. It was like the light of a lamp or torch when it falls on earthy dust particles. He verified this himself by using a lamp: a pillar of light was seen when he put on the lamp and disappeared when he extinguished it. Possibly the light effect that was seen when no lamps were on was due to the light of the stars. Seamen have seen similar effects in stormy weather around the mast of their ships.60 The red colour that is sometimes seen extending from the horizon until the middle of the sky and that remains for several nights belongs 58 Ibn Sīnā describes in the Kitāb aš-Šifa the kind of bodies that descend to the earth together with a thunderbolt; they are formed from the material of a thunderbolt, sc. dense smoky exhalation that contains earthy particles; see below p. 237n31. 59 Abū 1-Barakât, al-Mu'tabar II 223,23-224,23. 60 ibid. 224,24-225,21.

to the same kind of phenomena. All of them are due to celestial forces that act on smoky exhalation.61 Abū 1-Barakāt discusses the halo around the sun and the moon and the rainbow in the same chapter because he considers them to belong to the same kind of phenomena, sc. light effects in the sky that are governed by celestial forces. He mentions the opinion that they are an imaginary effect due to reflection of the sun or moon against a cloud, like an image we see in a mirror, and admits that, indeed, the halo and rainbow are caused by the sun and the moon. But the reason why the red and green colour in the rainbow form certain well-determined circles is unclear. He mentions several features of the halo and rainbow that are well known from Aristotle and Ibn Sīnā, e.g. that the different colours of the rainbow arise due to the fact that the light falls on parts of the cloud with different density and that the difference in colour between a halo and a rainbow arises because the clouds in which they appear have a different density and a different distance to the observer. He concludes by saying that the halo and rainbow are formed due to a celestial force—just like phenomena such as comets, rods, etc.—that acts on a cloud.62 Thus, Abū 1-Barakāt considers shooting stars, comets and such phenomena together with mock suns, rods, halo and rainbow and certain 'extraordinary phenomena' to be phenomena that are formed and retained by celestial forces. These forces work in the smoky, dry exhalation that has risen in the upper atmosphere, where this exhalation is ignited, or in clouds, where the halo and rainbow are formed from the light of the sun and the moon. Abū 1-Barakāt discusses the Milky Way in the section in which he treats the subjects from Aristotle's De Caelo. He says that it consists of star-like bodies that are too small to be seen separately. The opinion that it is a phenomenon in the upper part of the atmosphere cannot be correct, because then it would be subject to parallax: observers in different places on earth would see the stars in relation to this phenomenon in different places; that is not what is observed.63 Fakr ad-Din in certain respects follows Ibn Sīnā in more detail than Bahmanyār. He says that the dry exhalation in the upper atmosphere may be ignited and then appear as shooting stars. It may also burn for a longer time and appear in the form of a comet, a star, a beard or a horned animal. Sometimes the appearance remains several months. If the exhalation is thick and dense, one sees immense patches of red and 61 62 63

Abü 1-Barakāt, al-Mu'tabar ibid. 226,7-23. ibid. 141,1-10.

II 225,22-226,6.

black. If the ignition lasts a long time one sees it moving with the motion of the heaven. If the exhalation is thick and gets ignited, one sees a red colour, like something smouldering; if it is still denser, it appears black as coal, or as if it were a hole. It is also possible that smoky exhalation gets hot in cold air due to recoil (ta'aqub άνιιπερίσχασις) 64 and then is thrown out and ignited. In the next paragraph Fakr ad-Din deals with fire and the ways of its being extinguished; it is a version of the corresponding paragraph of Ibn Sīnā.65 The Milky Way is discussed by Fakr ad-Din, as it was by Abū 1-Barakāt, in a chapter in which subjects from De Caelo are treated. His text is almost the same as that of Abū 1-Barakāt.66

7. Ibn Bājja We have seen above that Ibn Bäjja's discussion of the comets is mainly restricted to an account and refutation of other opinions, and that he does not discuss shooting stars, torches, etc., at all. This may be due to the incompleteness of the manuscripts. In fact, the only subject from the Meteorology that is discussed in a complete and orderly way, is the Milky Way. Ibn Bäjja's account of the Milky Way starts, like that of Aristotle, with an enumeration and refutation of other opinions. In the main, Aristotle's account is followed. Ibn Bājja mentions the mythical view that the Milky Way is the milk of the suckling of some stars that flows from the nipples of a breast (not mentioned by Aristotle). He refutes the view that the Milky Way is an effect of a previous path of the sun, saying that the region where the sun moves cannot undergo any influence at all.67 This remark is not in Aristotle, but occurs in Ibn al-Bitriq's version of the Meteorology (see above p. 76). Then Ibn Bājja turns to Aristotle's theory and discusses it as follows: He says that, according to the commentators, Aristotle's theory is that a collection of many stars that are close together causes smoky exhalation to collect in the upper atmosphere under them, and this exhalation is ignited, analogous to the formation of comets. Against this view the objection can be raised that this would imply that the Milky Way 64

Ibn Sīnā does not use this term in this connection. He uses it, with Aristotle, in the explanation of hail, see below p. 112; the concept of άνΐίπερίστασίς is explained on pp. 97-98. 65 Fakr ad-Din, al-MabāhiL II 189,2-190,3. 66 Fakr ad-Din, al-Mabähii II 99,13-17 is similar in text to Abū 1-Barakāt, al-MuÛtabar II 1414-8. 67 Ibn Bàjja, Commentary on the Meteorology, see below pp. 424-428.

would be seen in different positions in relation to the stars in northern and southern countries, due to parallax. Another objection would be that it would be subject to generation, decay and change, such as all sublunar bodies. It would not always appear with the same strength, because the amount of exhalation is not the same always and everywhere. For instance, there is more dry exhalation in Yemen and Ethiopia, less in Roman countries, so that it would appear stronger in the former countries.68 Then Ibn Bājja says that he has found another opinion in his copy of the text (sc. Ibn al-Bitriq's version, as we shall see), and the abovementioned objections do not apply to that opinion. There it is stated that "the pure fire close to the celestial sphere is inflamed and luminous; in the celestial sphere in the place where the Milky Way is seen there are many small stars close to each other, and big stars which are far apart. When their light shines from behind this inflamed place, one sees an elongated patch of light in that place."69 Ibn Bājja gives his own theory as follows: Earth and water are surrounded by air, and air is surrounded by fire. We see the stars through these elements. From optics we know that rays of light in a medium are refracted (inatafa), so that they are bended from the thinner to the thicker medium. Therefore we see things differently (larger, smaller, closer, with different colour), depending on the different media through which we see them. For instance, the sun is seen redder at its rising and setting, and yellower at times of great heat. The light from all stars is subject to refraction when it passes through the layers of pure fire, smoky exhalation and air. When these stars are many and close together, the effect of this refraction is that one sees a continuous patch of light, in contrast to what one sees in the region where stars are separate and far from each other. There the stars appear as circular. Thus, the Milky Way is the light of many stars, closely packed together, that is refracted through the layers of sublunar matter in the atmosphere. The refraction causes its particular form. Whoever says that the Milky Way is a sublunar phenomenon is partly right and partly wrong, just as whoever says it is a phenomenon in the celestial region. The truth is that both regions have a share in its formation.70 Thus, Ibn Bājja argues that the Milky Way cannot belong to the sublunar region only, because it is a constant phenomenon that does not change. It cannot be a purely heavenly phenomenon either because of 68

Ibn Bājja, Commentary on the Meteorology, see below pp. 428-430. ibid., see below p. 430. The last phrase is a quotation from Meteor. 25,11-26,2. 70 ibid., see below pp. 430-434. 69

Ibn

al-Bitriq,

its shape: only what is purely circular belongs to the heaven; the shape of the Milky Way is more irregular.71 Ibn Bäjja's view is in fact the view expressed in Ibn al-Bitriq's version of the Meteorology, the light of the Milky Way originates in the (stars of) the heaven, different from Aristotle's view that it is the light of the inflamed sublunar 'fire'. The share of the 'fire' in the formation of the Milky Way (not yet specified by Ibn al-Bitriq) is that the light is refracted through it, so that we do not see the light of individual stars, but a continuous patch of light. Ibn Bājja is aware of the objections against Aristotle's theory, but still wants to 'save' it. Although his theory is fundamentally different from that of Aristotle, he tries to keep a certain aspect of Aristotle's doctrine, when he gives the sublunar region a certain role in the formation of the Milky Way.72 That Ibn Bājja has used Ibn al-Bitriq's version appears from the quotations from Ibn al-Bitriq and from the fact that he discusses the Milky Way before the shooting stars, like Ibn al-Bitriq, but unlike Aristotle. The arguments against Aristotle's doctrine of the Milky Way follow Olympiodorus and Pseudo-Olympiodorus.

8. Ibn Ru'sd Ibn Rušd in his Short Commentary first discusses the group of five phenomena: shooting stars (kawkab munqadd or šihāb), burning flames (lahīb), torches (misbāh), 'goats' ('anz) and comets (kawkab dp. d-du'aba or kawkab dū-danab). They all have the same material and effective cause; only their form is different, dependent on the quantity and density of the material. If the material is extended lengthwise and scattered, the ignition goes from one part to the next one; this gives the appearance of a star that is shot away.73 Maybe the ignition occurs by means of the fire jumping from one part to the next one, or maybe by the celestial motion. In the first case we must say that the fire moves to where it can find suitable inflammable matter—for the fire does not move this way by nature, at least if the exhalation is extended downwards or to the left or right.74

71

Ibn Bājja, Commentary on the Meteorology, see below pp. 434-436. The same could be said of other subjects concerning which Ibn Bājja differs from Aristotle, e.g. the definition of place and the theory of motion of a body in a medium. See Lettinck 302-304 and 548-549. 73 This phrase (Ibn Rušd, Short Commentary 10,5-6) is similar in formulation to Pseudo-Olympiodorus, Tafsir 95,5-6. 74 Ibn Rušd, Short Commentary 10,2-16. 72

A shooting star may also be formed in another way. If the inflammable smoke is enclosed by cold moist air, it is projected as an arrow. This occurs because of the contrariety between the hot exhalation and the cold air that encloses it. These shooting stars appear with a muddy colour, as if they are extinguished by a coldness that penetrates them.75 Burning flames arise if the cluster of smoke extends lengthwise and breadthwise, and is wholly ignited, like burning reed or alfa.76 Torches arise if the ignited smoke extends lengthwise. 'Goats' arise if the ignition has fiery tongues, which appear as goat's hair.77 Comets arise if the burning exhalation remains stable for some time, either because the cluster of exhalation is dense or because the burned exhalation is replaced by exhalation that arrives from below. They may arise under a star or planet—then they move together with it—or they do not arise under a star—then they move with the motion of the universe. The comets may have different shapes: round, rectangular with equal length and breadth, rectangular with greater length than breadth, or five-sided. Often a comet dissolves into a shooting star, or it arises from it, if it finds suitable material.78 The next group of phenomena to be treated are light effects in the sky. Ibn Rušd says that some of them are only a visual effect (ru'ya), such as the blood-red colours, chasms (ukdūd), trenches (hufra), halo, rainbow and Milky Way. They arise because things get different appearances due to the different media through which they are seen, such as different distance, size and colour. The blood-red colours that are visible at night are caused by light shining on a thick, black cloud, for light that hits a dense, transparant body is dispersed in it. Then a colour arises that is between the white of the light and the black of the cloud. Chasms and trenches arise when a thick, black cloud is situated under a luminous object, so that the light cannot penetrate in all of its parts. The black parts seem to be further away than the light ones, and appear as a trench. The effect is similar to what painters do: they make protruding parts of the body, such as the breasts, white, and hollow parts black.79 75

Ibn Rušd, Short Commentary 113-18; the muddy colour of the second kind of shooting star is not in Aristotle, but is mentioned by Ibn al-Bitriq, see above p. 75. 76 The comparison with alfa is also in Pseudo-Olympiodorus (see above p. 78); the formulation of the whole passage (Ibn Rušd, Short Commentary 11,18-12,1) is similar to Pseudo-Olympiodorus, Tafsir 95,17-19. 77 Again, similar formulation by Ibn Rušd and Pseudo-Olympiodorus, compare Ibn Rušd, Short Commentary 12,2-3 and Pseudo-Olympiodorus, Tafsir 96,5-6. 78 Ibn Rušd, Short Commentary 12,4-133; the passage again contains several similarities with the corresponding passage of Pseudo-Olympiodorus, see above p. 79. 79 Ibn Rušd, Short Commentary 13,5-15,2; the example of painters is also in Pseudo-Olympiodorus and in Olympiodorus, see above pp. 79 and 72.

Finally Ibn Rušd turns to the discussion of the Milky Way. He says that it is doubtful whether it is only a visual effect or ignited exhalation, comparable to comets. He gives an account of the latter view, saying it is the view taken by Alexander. In fact it is Aristotle's view. Ibn Rušd argues against this view as follows: If the Milky Way were to arise because a great number of stars that are close together draws smoky exhalation into the upper atmosphere under them, and this exhalation is ignited, then it is not clear how this could occur continuously without interruption. Also, the stars would display a different position in relation to the Milky Way when they are observed from different places of the earth, for if we draw lines from observers in different places on the earth to the same star, these lines will intersect the region of ignited exhalation, i.e. the Milky Way, at different points. For instance, we see Aquila in our country at the edge of the Milky Way, on the eastern side; if we go to a place that has a smaller longitude, we will see Aquila in another place; this has not been observed. In fact, all phenomena under the moon are seen differently in different places. Also, if the Milky Way were ignited exhalation, one would expect that it would appear stronger in summer because more exhalation is formed at that time, but it remains the same at all times. Furthermore, the exhalation would have to be ignited continuously over a very large place; then one expects that all air will have changed into fire. One also expects that in the areas on earth under the Milky Way there is little rain and a great heat. Therefore the Milky Way cannot be ignited exhalation. On the other hand, the Milky Way cannot belong to the heavens either, for everything in the heaven must be circular, and the Milky Way has an elongated form. What remains is that it must be a phenomenon due to that cluster of stars, occurring in the surface of that ignited body through which the light of these stars appears. The light is deflected (in'akasa) 80 at the surface of fire or the surface of the light, smoky matter that is between fire and air. When it is deflected, the light of the different stars is mixed, and an elongated form arises. This is similar to what we would see if there were several moons near

80

The word in'akasa is often used to refer to deflection of light in general; there are two ways of deflection: refraction (iríitāf) and reflection (in'ikäs in a more specific sense). See Ibn Bājja, Commentary on the Meteorology, see below p. 432,11 and also above p. 78n40 and below pp. 267 and 273. Ibn Rušd here follows Ibn Bājja, as appears from the correspondence of their accounts, and Ibn Bājja says that the light is refracted; therefore Ibn Rušd here uses in'akasa and means: refraction. On the other hand, in what follows he compares the formation of the Milky Way with that of the halo, which is formed by reflection, according to him and others. Therefore we use 'deflection' as a general designation.

one another, and each moon would have a halo. The result would be an elongated patch of light. Having shown these two aspects of the Milky Way, its explanation is complete.81 Ibn Rušd concludes his discussion of the Milky Way by saying that the view that he has explained above is Aristotle's view, such as he has found it in the copy he had at his disposal. This must have been Ibn al-Bitriq's version of the Meteorology, for indeed, as we have seen above, he locates the source of the light of the Milky Way in the stars, and makes the exhalation in the upper atmosphere responsible for its appearance as a continuous area of light (see above p. 76). However, Ibn al-Bitriq did not specify how this effect occurs. That it occurs by dispersion in the medium of that exhalation is Ibn Bäjja's view, which is adopted here by Ibn Rušd. It appears that Ibn Rušd has used Alexander's commentary: he mentions Alexander's view on the Milky Way and moreover, he discusses 'goats' and torches that are not mentioned by Ibn al-Bitriq. Several passages in Ibn Rušd's text correspond to passages in Pseudo-Olympiodorus. The muddy colour of the second kind of shooting star is not in Aristotle, nor in Pseudo-Olympiodorus, but occurs in Ibn al-Bitriq. Besides the arguments against Aristotle's view of the Milky Way that are also in Olympiodorus, Pseudo-Olympiodorus and Ibn Bājja, Ibn Rušd has some others as well. We conclude that Ibn Rušd in the Short Commentary used Ibn al-Bitriq's version of the Meteorology, Alexander's commentary and that of Pseudo-Olympiodorus. His view of the Milky Way is Ibn Bäjja's view. The way how Aristotle's view is criticized and how the problems are solved by stating that the Milky Way has aspects from both the celestial and the sublunar world is similar in Ibn Bäjja and Ibn Rušd. There are also similarities in formulation. We give a survey of corresponding phrases below. Ibn Rušd, Short Commentary 13,8-10: kull al-mubsarāt ya'ridu lahā bi-ktilāf al-jism al-mutawassit alladī yurā bihī iktilāf manzar min al-qurb wa-l-bu'd wa-l-'izam wa-l-sagar wa-l-lawn wa-1-kafā' wa-1-zuhür. Ibn Bäjja, Commentary on the Meteorology, see below 23,11-13: al-mar'īyāt yalhaquhā bi-hasab iktilāf basa'it al-ajsām allatī tazharu fīhā ahwāl muktalifa min 'izam wa-sagar wa-qurb wa-bu'd wa-kat_rat zuhūr wa-qillatihī wa-ktilāf lawn. I.R. 16,1: hal ukida fīhi say' intawā fīhi kidb am lā. I.B. 24,21-2: qāla haqqan gayr annahū tawā fīhi kidban. 81

Ibn Rušd, Short Commentary

15,12-18,19.

LR. 17,8: we see Aquila (an-nasr at-ta'ir) at the edge ( f ī l-hāfa). I.B. 22,4-5: they see the luminous stars which are under the tail of Aquila {at-ta'ir) which is in the middle (of the Milky Way) at its northern edge ('alā hāfatihī aš-šamāliyya). LR. 18,6-8: fa-qad baqiya an yaküna dālika 'ārid ya'ridu li-tilka l-kawäkib al-mundamma al-mutaqāriba fī sat h al-jism al-multahib allatī tazharu tilka l-kawākib bi-tawassutihī. I.B 23,15-16: wa-hādā kulluhū innamā huwa 'arid, 'arada li-kayālihā l-hāsil fī basīt dālika l-jism. I.R. 18,8-10: wā-dālika annahā li-taqārubihā ya'ridu lahā an tanakisu adwā'uhā fī sath an-nār aw al-jism ad-dukānī al-latīf alladī huwa ka-t-tukūm bayna n-nār wa-l-hawā'. I.B. 23,18-21: fa-kull kawkab fa-yalzamuhù in'itäf du' wa-huwa darb min durūb al-in'ikās wa-yalhaquhū wa-n-nār allatī hiya al-ustuqus sāfiya wa-ba'da hādā bukār ka-mā qālahū Aristū multahib wa-huwa ka-t-tukūm bayna n-nār wa-l-hawā'. In his Middle Commentary Ibn Rušd follows Aristotle's order of the subjects, not that of Ibn al-Bitriq. Like Ibn al-Bitriq, Ibn Rušd mentions as phenomena that arise from ignition of a suitable gathering of dry exhalation in the region of 'fire': the burning flame ('amūd an-nār\ the lamp (sirāj), and the shooting star (šihāb). Concerning the latter phenomenon he adds that this is the kind of shooting star that seems to move, but in fact does not move; he gives Alexander's explanation of how exactly this comes about (see above p. 67). Ibn Rušd mentions the second way in which shooting stars are formed, viz. when hot exhalation surrounded by cold air is ejected due to the contrast between the hot exhalation and the surrounding cold. He further follows Ibn al-Bitrlq, with some additional phrases of his own.82 In the section on colour effects and chasms in the sky Ibn Rušd combines Ibn al-Bitriq's text with text from the Short Commentary. He first states that we know from optics that if a luminous body is seen through air, this may occur either directly or by reflection or by refraction. In the last two cases different colours may arise because the light of the luminous body is mixed with the darkness of the dense, dark body through which it passes. The resulting colour depends on the density of the medium. Often a purple colour is seen when evening 82

Ibn Rušd, Middle Commentary

36,6-40,10.

falls; this results from light from the sun being mixed with the darkness of the night. Note that, according to Ibn Rušd, it is the light from the sun, not that from the fire in the upper atmosphere, as Aristotle and Ibn al-Bitrlq have it, that causes these light effects. The section on depths that seem to appear in the sky follows the Short Commentary, not that of Ibn al-Bitriq.83 Ibn Rušd reviews the opinions on comets of the older writers, along the lines of Ibn al-Bitriq's text: the opinion that a comet is a conjunction of planets (Democritus, Anaxagoras), or one of the planets (the Italians), and that its tail arises due to reflection from moist air (Hippocrates). Then he presents the following arguments against them. If a comet were one of the planets, it would only appear in the zodiac. If its tail were due to reflection against moistness, then it would also sometimes appear without a tail, like the moon does not always have a halo and the sun does not always have mock suns. Moreover, other planets would also get a tail due to reflection. Also, no other planet has been seen besides the five known planets. Also, if the tail were due to reflection, we would see more kinds of comets, whereas we have only found five kinds. Ibn Rušd remarks that he found this last argument in the translation he got, but that it is not clear.84 These arguments follow Ibn al-Bitriq (the third one is in Ibn al-Bitriq only, not in Aristotle). The last one is clearly due to a mistranslation in Ibn al-Bitriq (see above p. 75) and it is not surprising that Ibn Rušd found it difficult to understand. He read 'five comets' in Ibn al-Bitriq, turned this into 'five kinds of comets' and made this refer to the different shapes of comets that arise due to different shapes of the ignited exhalation. PseudoOlympiodorus indeed mentions five kinds of comets (cf. above pp. 68n6 and 79). The account of Aristotle's opinion on comets, which is almost completely absent in Ibn al-Bitriq's version, is given by Ibn Rušd as follows: A comet is ignited exhalation. Either the star and its tail are both ignited air—if the ignition does not occur in the neighbourhood of a star—or the comet consists of a star and a tail, with the latter being exhalation that is ignited near that star so that it seems connected with it. The tail is not really connected with the star, but the lights of both phenomena are connected.85 Ibn Rušd also gives Aristotle's statements on the relation of comets and dryness of the atmosphere and the occurrence of winds. Despite the incompleteness of Ibn al-Bitriq's text 83 84 85

Ibn Rušd, Middle Commentary ibid. 44,16-46,10. ibid. 47,12-483.

40,14-42,17.

on comets, Ibn Rušd gives a complete account. Maybe his copy of Ibn al-Bitriq's text was more complete than the extant copies (and that of Ibn Tibbon, who gives the incomplete version). He may also have taken his more complete text from Alexander. The discussion of the opinion of older writers on the Milky Way more or less follows Ibn al-Bitrlq. The account of Ibn Rusd's own opinion starts with Ibn al-Bitriq's phrases: "The air close to the celestial sphere is inflamed and fiery. In the celestial sphere in the place where the Milky Way is seen there are many small and big shining stars close to each other." What follows is not in Ibn al-Bitriq anymore. Ibn Rušd says that two conclusions are possible from this statement. (1) The Milky Way is a visual effect (ru'ya) and arises from reflection (in'ikäs) of the light of these stars against the inflamed air that is present in that place (of the Milky Way). Due to the multitude of stars the air in that place is heated more and becomes thinner than in other places of the upper atmosphere. If one assumes that this causes light to be reflected, then there place light is reflected more and that place looks lighter than other places. (2) The Milky Way is inflamed fiery air, as comets are. This is Alexander's opinion and he says that it is Aristotle's doctrine. However, in our copy of Aristotle's text {i.e. Ibn al-Bitriq's version) we find instead the first view. If the Milky Way were inflamed air, then the stars that are seen in the Milky Way would have a parallax. For instance, a star at the eastern edge of the Milky Way is seen by people in the west at the other end. This is not what has been observed. Ibn Rušd relates that he has seen Aquila in Cordoba and Morocco in the same place in relation to the Milky Way. Also, if the Milky Way were fire, it would appear stronger in dry years and weaker in moist years. These facts rule out the possibility that the Milky Way is inflamed air.86 Ibn Rušd elaborates on the first possibility, that the Milky Way is a visual effect formed by reflection of light. In the Milky Way one sees the stars themselves directly and one sees their light in the space between them. This can only occur by reflection; it is comparable to the moon that is seen directly, together with its reflected light that is seen as a halo around it. However, there is a difficulty with this view: reflection always occurs from a body that is dense, in which light does not penetrate. Thus, reflection cannot occur from the stratum of fire, because fire is thinner than air; therefore what occurs when a halo is formed around the moon, cannot occur here.87 On the other hand, such a phenomenon, in which the luminous object is seen twice, namely 86 87

Ibn Rušd, Middle Commentary 54,7-56,9. Ibn Rušd criticizes his own view from the Short Commentary,

see above pp. 90-91.

directly and via small mirrors that reflect colour only, not shape, can only be formed by reflection.88 Ibn Rušd does not give a final solution for this dilemma. He suggests three possibilities. (1) The reflection occurs because vision is too weak to grasp the stars as they are, since they are so small and remote. Just as reflection may occur against a dense medium, it may occur when vision is weak.89 This reflection forms a luminous circle around each star. Because the stars are close to each other, these circles overlap and give the impression of an elongated patch of light. (2) The area of the heaven in which these numerous stars are present receives their light and becomes luminous by itself, that is, each point in that area emits light and is directly seen by us without reflection or refraction. In the heavenly sphere reflection and refraction do not occur because its material is homogenous. That something may become luminous by itself due to something else is quite possible: the moon is luminous and receives its light from the sun; this does not occur by reflection. Ibn al-Haytam has explained this in a special treatise.90 Maybe the part of the heaven where the Milky Way appears is denser than other parts, because the stars there are close to each other, and this density contributes to the formation of the phenomenon, such as the density of the moon disperses the sunlight. It is not impossible for heavenly matter to differ in density. This appears from the dark spots on the surface of the moon. This is not a visual effect.91 (3) It is also possible that in that part of the heaven there are many stars which are so small and close to each other that they are not observed as separate bodies; their light is mixed together and this is visible as the Milky Way. It is well known that if something is small and far away, only its light is observed, not its shape.92 Thus, Ibn Rušd in his Middle Commentary first expounds what according to him is Aristotle's view on the Milky Way; it is the view 88

Ibn Rušd, Middle Commentary 57,7-58,15. See below pp. 245, 288 and 295 for a further explanation. 90 Ibn al-HayLam, Maqàla fî daw' al-qamar, in: Majmū' ar-Rasä'il, Hyderabad 1938; German translation: K. Kohl, "Über das Licht des Mondes", in: Sitzungsberichten der Physikalisch-Medizinischen Sozietät zu Erlangen 58-59 (1926-1927) 305-398. Ibn alHaylam explains that the moon, after having received light from the sun, behaves as if it has become luminous itself, that is, all points of the moon emit light into all directions; this means that the moon does not act as a mirror. 91 The Latin version adds: "as has been explained by Ibn al-Hayiam". This refers to his treatise Risäla fï mä'iyyat al-aiar aliud ι fi wajh al-qamar, ed. A.I. Sabra in: Journal for the History of Arabic Science vol. 1 no. 1 (1977) 166-80; German translation: C. Schoy, Die Abhandlung des Schaichs Ibn 'Ali al-Hasan ibn al-Haytam über die Natur der Spuren (Flecken), die man auf der Oberfläche des Mondes sieht, Hannover 1925. Ibn al-Hayiam explains that the dark spots are places where the moon is denser. 92 Ibn Rušd, Middle Commentary 58,16-61,17. 89

also set forth in the Short Commentary and in Ibn Bäjja's treatise. He criticizes this view and gives three other solutions. He concludes by saying that knowledge of the Milky Way is defective because it is not evident what kind of thing it is.

CHAPTER THREE

PHENOMENA IN THE LOWER ATMOSPHERE DUE TO MOISTURE

1. Aristotle Aristotle discusses in chapters 1,9-12 the phenomena arising from moist exhalation in the lower atmosphere above the earth. Their effective cause is the sun in its yearly motion. Water is evaporated by the sun's heat, and rises upwards as moist exhalation. In a higher region it cools down because the heat that made it rise is dispersed or extinguished due to the coldness of that region.1 Then it condenses again into cloud (νέφος) and subsequently falls down as water. It is called drizzle (ψακάς) when it falls in small drops, rain (ύεχός) when the drops are larger. Mist (ομίχλη) is what is left after a cloud has condensed into water; therefore mist is a sign of fair weather. This cycle of rising and falling moisture is parallel to the sun's approaching and receding motion in the ecliptic (346b16-347a12). Moist exhalation that does not rise far during the day because the amount of heat that raises it is not large, falls down again when it is cooled at night. It becomes dew (δρόσος) when it condenses into water, hoarfrost (πάχνη) when it freezes before it can condense. Dew occurs in mild weather, hoarfrost when it is cold. Both phenomena only occur in clear weather—otherwise no exhalation rises—and when there is no wind—otherwise no condensation occurs. There is no hoarfrost on mountains because vapour rises from low places and the heat cannot carry it to a great height, and also because such vapour is dissolved by the flow of air at that level (347al3-36). Dew is formed when south winds blow because they bring mild weather that can produce exhalation. In Pontus the situation is the opposite: there dew is formed when north winds blow. The south wind does not make the weather mild enough to produce exhalation. The cold north wind surrounds and concentrates (άνιιπεριίστημι) the heat, and in this way causes exhalation to rise. Aristotle here uses άντιπερίστασις as an explaining principle for a certain phenomenon and several other effects will be explained by it too. It means that opposite 1

That region is cold because the sunrays reflected from the earth have no effect anymore, as is explained by Alexander (in Meteor. 44,25-26) and Philoponus (in Meteor. 122,21-24).

qualities like coldness and heat exercise a mutual reaction on each other. If coldness, for instance, is dominating, it drives heat together and concentrates it, so that the heat becomes stronger than it would have been without surrounding coldness; or the coldness drives the heat away, as if it recoiled or were repelled. Vice versa, heat may have a similar effect on coldness. This explains why wells (φρέαρ) give off vapour when it is cold and why caves are cold in hot weather and warm in cold weather. It also explains why rain falls in summer: coldness is concentrated by the surrounding heat and causes a cloud to condense rapidly. Then the rainfall is heavy and with large drops (347a36-347b11 and 348b2-16). Snow (χιών) is formed when a cloud freezes before it can condense into rain; its formation is analogous to that of hoarfrost on the earth. Similarly, the formation of rain from a cloud corresponds to that of dew on the earth. The difference between rain and snow on the one hand and dew and hoarfrost on the other hand is that the former arise from cloud, i.e. a large amount of vapour that has collected in the sky for a long time over a large area, and the latter arise from a small amount of vapour that collects over a small area for a day only. The cloud and the vapour from which these things arise still contain much of the heat that caused the exhalation to rise, therefore cold weather or a cold country is necessary for snow and hoarfrost to be formed (347bl2-28). Hail (χάλαζα) is also formed from clouds, but there is nothing that corresponds to it on the earth. The reason will become clear when the formation of hail is explained. We are faced with the following difficulties in connection with the formation of hail. (1) Hail is ice (κρύσταλλος), yet it rarely occurs in winter, mostly in spring and autumn when it is not very cold. Also, it occurs more in milder areas than in cold ones. (2) How can water freeze in a cloud? When exhalation has become water it cannot stay in the atmosphere for a long time. In the case of rain we can maintain that first small particles of water are formed that are able to remain floating in the air, and then come together to form larger drops that fall. This cannot occur for hail, because solid bodies cannot coalesce like liquid ones. Anaxagoras' opinion is that a cloud producing hail has risen to a higher, and consequently colder level in the atmosphere than a cloud that produces rain; the water is frozen at that level. This especially occurs in warm areas and in summer, because the greater heat is able to carry the cloud further up. Against this the following objections can be risen. (1) Hail rarely falls in high places. (2) Hail often falls from clouds that are close to the earth. Then the stones are large and have an irregular shape; this

is an indication that indeed their fall has taken a short time only, and that they were formed close to the earth. Stones that fall a long time, from a higher level, must be small and round, as they are worn away during their fall. Aristotle's explanation of hail uses άντιπερίσχασις as follows: We already saw above (p. 98) that a cloud in a warm environment produces heavy rain because of the άντιπερίστασις, i.e. because the cold is concentrated by the surrounding heat. If this occurs to a still larger extent, the cold freezes the water before it has had time to fall on the earth, and in this way hail is formed. The freezing may also occur for rain drops during their fall. The effect is stronger when it occurs closer to the earth, where it is warmer—just the opposite of Anaxagoras' theory. Then we get larger raindrops and hailstones. For the same reason the rain in Arabia and Ethiopia is especially heavy in summer: because of the great heat the clouds are cooled quickly. Water freezes more quickly when it is heated before being put in a cool place.2 In summer there is less hail because less moisture is dissolved (347b28-349a9). The question posed at the beginning of the treatise on hail, sc. why nothing is formed close to the earth that corresponds to hail, is not explicitly answered by Aristotle, but the answer is clear: if vapour that does not rise very far has to condense and then freeze, this would take more time than the time in which the condensed vapour reaches the earth as dew.

2. The Greek

commentators

Alexander says that mist is what is left of a cloud after it condensed into water; mist is also formed when vapour condenses cloud, but to a lesser extent than the condensation that produces a cloud.3 The cycle of rising exhalation and falling rain is a yearly cycle

has into rain that

Aristotle gives two examples. When people want to cool water quickly they first put it in the sun. Fishermen who are fishing on the ice in Pontus put hot water on their rods; then it freezes more quickly. They use the ice to stabilize the rods. It seems that the supposed effect that previously heated water cools more quickly is intended as an example of άνΤΙΠερίσίΟίσΐζ, but this is not how ά ν τ ί π ε ρ ί σ τ ο ί σ ί ς is assumed to work, unless one supposes that the vessel containing the water gets hot by being put in the sun and this surrounding heat cools the water by ΟίντίπερίσίΟίσίς. This is not how the commentators interpret it. See Gilbert 505-506 and the interpretation of the commentators, below p. 104. 3

Alexander, in Meteor.

44,30-45,6.

follows the yearly motion of the sun in the ecliptic. The daily motion of the rising and setting sun also causes a cycle of exhalation and precipitation, sc. exhalation that does not rise very high and falls at night as dew and hoarfrost.4 Alexander discusses the rainfall in Ethiopia in summer. Aristotle explains this by means of άνχιπερίσχασις. Alexander asks how vapour can gather there in summer, when the air is dry and answers that the vapour and clouds are formed elsewhere; then the wind drives them to the mountains in Ethiopia; they are stopped in their course, so that they are compressed and condensed. Then the άντιπερίσιασις causes them to change into rain, as Aristotle says in his book On the rising of the Nile? Alexander here mentions another way of cloud condensation, besides cooling: clouds may also condense when they are driven against mountains. This is Theophrastus' theory of condensation. He recognized compression (πίλησις) as another way of condensation.6 Note that Alexander combines the Aristotelian and Theophrastian explanation of rain in Ethiopia. The question is discussed by Olympiodorus in a similar way. He asks: How can there be rain in summer in Ethiopia and Arabia, when there is no moisture that can be dissolved? The answer is that the moisture gathers in another place; then the north wind carries the clouds until they come upon the Mountains of the Moon or Silver Mountains.7 They condense by compression and change into rain. People say that this is also the cause of the rising of the Nile. Three other causes are mentioned: the snow in the Mountains of the Moon melts in summer, the source of the Nile is in the southern hemisphere, where it is winter when it is summer with us, so that there is enough moisture; the άντιπερίστασις: the heat surrounds what is cold, so that it condenses and produces water.8 Philoponus examines Aristotle's statement that, when water changes into air, vapour is an intermediate stage, whereas cloud is the intermediate stage for the opposite transformation (346b32). Usually 4

Alexander, in Meteor. 4530-46,7. ibid, in Meteor. 53,5-16; Steinmetz has shown that the pseudo-Aristotelian On the rising of the Nile was written by Theophrastus. See Steinmetz 1964 290-1 for a discussion of this passage from Alexander. 6 Theophrastus, Meteorology ch. 7,2-9; Steinmetz 1964 217-221; Olympiodorus, in Meteor. 8030-81,1. 7 Aristotle mentions the Silver Mountains as source of the Nile (Meteor. 350b14). Olympiodorus identifies them with the Mountains of the Moon, located in Ethiopia, according to Eratosthenes. See Steinmetz 1964 282-3 and 290. 8 Olympiodorus, in Meteor. 94,4-17. Apparently Olympiodorus refers to the book On the rising of the Nile, without mentioning it by name. See Steinmetz 1964 289-90 for a discussion of this passage from Olympiodorus. 5

two opposite transformations occur via the same intermediate stage, such as tepid that is intermediate for the change from hot to cold and from cold to hot. In this case, when air changes into water, it first condenses into vapour, and this becomes water at further condensation. Clouds are a gathering of much vapour. Vapour that arises from water does not gather to form clouds, because that vapour immediately moves upwards; it does not stay at one spot to wait for more vapour so that a cloud could be formed.9 Olympiodorus' commentary on this statement is somewhat different. He also says that the intermediate states in a transition between two extremes must be the same. Vapour and cloud are both intermediate stages for the transition from water to air as well as for the opposite transition. He does not specify the difference between vapour and cloud; he will probably agree with Philoponus that cloud is a gathering of much vapour.10 Philoponus describes άντιπερίσιασις as follows: heat and cold recoil from one another because they cannot be together in the same place; each one of them pushes the other away from its place. Therefore caves are cool in summer when the surrounding air is warm, and warm in winter. The reason is not that in fact the temperature is always the same, whereas we feel it as cool in summer because we are warm ourselves and we feel it as warm in winter because then we are cold ourselves. We see that in winter caves and wells exhale vapour; this means that there is heat that evaporates the moistness. Also, in winter our inner body is cold in summer and warm in winter. This appears from the fact that we eat less in summer and our digestion is worse; in winter we eat more, and our digestion is better. Also, in winter vapour often escapes from our mouths and smoke rises from our excrement.11 Then follows another description of the process of άντιπερίοτασις: in winter the air and the material that forms the surface of the earth are denser. The heat that exists in the interior of the earth cannot be dispersed outwards because this is prevented by the density of the surrounding material. Thus it gathers in the interior and heats the air and water under the earth. In summer the pores at the earth's surface are opened by sun's heat, so that the heat is able to disperse upwards and the interior cools down. Something similar occurs in our bodies. In winter our natural heat remains inside because of the density of the

9

Philoponus, in Meteor. 122,31-123,17. Olympiodorus, in Meteor. 84,19-26. 11 Philoponus, in Meteor. 125,10-126,15. 10

surface of our bodies. In summer this heat disperses outwards because the body's surface is thinner.12 About hail Philoponus says that the freezing of water may occur in the cloud itself or during its fall as rain. One may ask: if water freezes in the cloud, why does this water not fall from the cloud as soon as it is formed, so that it does not have the time to freeze there? The answer is that the cloud has a certain size in the vertical direction, and the falling water spends some time in the cloud, during which it freezes. Another question is why the hail does not melt as soon as it has come out of the cloud, as the surrounding air is warm. The answer is that the freezing is so intense, that the hail only melts when it has arrived on the earth. One could further ask why the cooling inside the cloud by άντιπερίστασις does not produce snow. The answer is that for a direct freezing of the vapour a stronger cooling is required than for the formation of hail. This only occurs in winter and in cold places.13 Olympiodorus gives an account of the exhalation that brings about precipitation as follows: The vapour that is dissolved by the sun from the moistness on earth rises together with the dry, hot exhalation. It is the dry exhalation that carries the moist one upwards until a certain height. Then the dry exhalation either continues rising alone, leaving the vapour behind, or it is quenched and destroyed by the cold that reigns at that height. The vapour may stay as it is, or change into rain, snow or hail. If the heat that dissolves the exhalations is less, the vapour is carried upwards, but not very far; there it remains as it is, or changes into dew or hoarfrost. There is nothing close to the earth that corresponds to hail, for the water that is formed from the vapour reaches the earth before it can freeze. One might consider ice that has frozen from water as the kind of thing which corresponds to hail, but they are different, as hail originates from vapour and ice from water.14 Olympiodorus says that vapour changes into water by cooling (ψΰξις), according to Aristotle, but that Theophrastus mentions another cause, sc. compression (πίλησις), besides cooling. This could explain why in Ethiopia there is no cooling and yet there occurs rain. There are high mountains, against which the clouds are pushed. According to Theophrastus, this causes condensation and subsequent rainfall. We see this also happen in cooking pans, when water is formed in the inside of the covers and the same thing occurs in the cupolas of a bath house. There is no cooling in these cases. Olympiodorus objects to Theophrastus' theory of compression by saying that in Thebes there are no 12 13 14

Philoponus, in Meteor. 126,16-127,1. ibid. 127,24-6 and 128,24-129,19. Olympiodorus, in Meteor. 7932-80,29.

mountains, nor cooling, yet rain occurs there. Here the cause of rain must be άντιπερίστασις, that is, the surrounding heat pushes the cold towards the inner parts of the cloud. Then he says that he agrees with Aristotle that cooling is the only cause of rainfall. It is nonsense that compression were the cause of water gathering in covers and cupolas. One should say that much vapour gathers in such areas, so that the heat is 'choked', and that cooling also occurs because the vapour stops to move there.15 Then Olympiodorus inserts a digression on rainfall in Thebes and sailors to India. He says that the rain in Thebes comes down in a heavy downpour, and that its taste is bitter. The reason is that the vapour rises together with the smoky exhalation. When clouds are formed the smoky exhalation is not completely separated; the cloud, and also the rain, contains some admixture of this smoky exhalation. Therefore the rain is bitter. The smoky exhalation also carries earthy particles upwards. They find their way in the rain and make it heavy, so that it falls fast. The rain falls heavily especially in summer; this agrees with the explanation, because in summer much dry exhalation is dissolved, and many earthy particles are mixed with it. That earthy particles are mixed with exhalation also appears from the constitution of the sea. For seawater is heavier than water in lakes. Therefore it can carry larger ships and ships can carry heavier loads on sea than in a lake. Sailors to India who do not understand this, are shipwrecked. For they fully load their ships and are able to sail the sea, but when they arrive at a river or lake, they are wrecked, because these waters do not contain earthy particles and cannot carry these loads. Also, if someone throws an egg into water, it floats on sea water, but sinks in fresh water.16 As for the relative temperatures of the different kinds of precipitation, Olympiodorus states that hoarfrost is colder than dew and snow colder than rain. This follows from their formation: more cold is required for the freezing of vapour than for its condensation to water. Snow is colder than hail, because snow is (warm) vapour that has frozen and hail is (only) frozen water. Similarly, hoarfrost is colder than ice. Hail and ice are a hard material, therefore they seem colder than the soft snow and hoarfrost, just as red-hot iron seems hotter than a

15 Olympiodorus, in Meteor. 8030-81,10; note that in another place Olympiodorus mentions compression as the cause of rainfall in Ethiopia, without expressing his disagreement, see above p. 100. The quotations from Theophrastus are included in Theonhrastus 1992, 19932, vol. 1 378 no. 221B. Olympiodorus, in Meteor. 81,13-823- The account of the constitution of the sea, with the examples of the loaded ships and the eggs, corresponds to Aristotle 359a5 ff.

flame: we do not dare to touch red-hot iron, not for a second, whereas we can strike with our hand along a flame.17 Olympiodorus explains that the freezing of water to form hail does not occur in the cloud, but during the fall of water as rain drops. For as soon as the vapour turns into water in the cloud it starts to fall, and it has no time to freeze there. And hail cannot be formed by the joining of small particles of snow, as rain drops are formed from small particles of water in the cloud, for what is liquid may coalesce, but this is impossible for what is frozen.18 Olympiodorus claims that hail occurs more often in autumn than in spring, because in autumn the air is preheated by the summer and what is preheated cools down more quickly.19 That hail occurs more often in autumn than in spring is not mentioned by Aristotle, whereas on the other hand Aristotle mentions the principle that what is preheated cools more quickly, without giving an application of it in the theory of hail. He gives the impression that this principle is an example of άνχιπερίσχασις, which it is not, see above p. 99n2. Maybe Olympiodorus invented the effect that hail occurs more often in autumn than in spring in order to supply an application for the principle that what is preheated cools more quickly. Philoponus also relates the principle to the formation of hail, saying that preheating of the water in the cloud makes it more suitable for being frozen,20 so that hail rather occurs from clouds in warm air. Later authors will adopt Olympiodorus' addition to Aristotle's theory of hail, see below pp. 107 and 112-113. We see that Alexander and Olympiodorus mention Theophrastus' theory of condensation by compression. Olympiodorus rejects it; Alexander adopts it, but combines it with Aristotelian theory. Philoponus' commentary on the subjects under discussion is incomplete here because of a lacuna in the manuscripts. We know, however, that he was also acquainted with Theophrastus' theory, for in his commentary on De Generatione et Corruptione he mentions compression as the explanation for the rain in Ethiopia and for the drops of water which are formed against the roof of a bath.21

17 18 19 20 21

Olympiodorus, in Meteor. 86,14-87,5. ibid. 8837-89,10. ibid. 9330-5. Philoponus, in Meteor. 129,19-21. See Steinmetz 1964 288-289.

3. Ibn al-Bitriq and Hunayn ibn Ishāq Ibn al-Bitriq's version of these chapters starts with an account of the exhalations (see above p. 46). Then he says that if the lower exhalation is enclosed ('arada lahū. inhisār) or becomes cold, it condenses and returns to the earth as rain. It is called drizzle (nadan) if it falls slowly and in small drops; it is called rain (matar) if it falls fast and in larger drops.22 He says that no dew (nadan) or hoarfrost (jalīd) is formed in the high mountains because (1) the heat will have dissolved the exhalation before it arrives there, and (2) that area is close to the ignited air, so that exhalation that does arrive there will be dissolved by the heat of that air.23 That snow and hoarfrost are formed in different circumstances appears from the fact that snow is soft and hoarfrost hard. Snow is soft because there is still heat in the cloud, which impedes the parts of the cloud from becoming too dense. Hoarfrost is hard because the cold that freezes the exhalation causes its warm parts to concentrate and become dense.24 As to hail, Ibn al-Bitriq remarks that it may be formed in cold seasons (more rarely) as well as in warmer seasons (more often). In the former case the cold is all over the air, not only in the cloud. In the latter case the heat (in the air) opposes (yudāddu) the cold, so that it becomes concentrated (inqabada) in the cloud and freezes the water (άνχιπερίστασις).25 He again describes άνχιπερίσχασις when he says that raindrops are larger on warmer days because the heat causes the cold to recoil (nāfara) in the cloud, so that it becomes concentrated (inqabada) and the parts of the cloud condense.26 Hunayn precedes his account of precipitation from moist exhalation with an exposition of how such exhalation arises (see above p. 47). Then he gives a summary of Aristotle's discussion on rain, dew, hoarfrost and snow, without further special features.27 In the account of hail the άντυιερίσιασις is described as a mutual recoiling (tanāfur) of cold and heat; the same word was used by Ibn al-Bitriq.28

22

Ibn al-Bitriq, Meteor.

36,6-37,5. Ibn al-Bitriq's uses nadan

'dew'. 23 24 25 26 27 28

ibid. 37,16-38,3. ibid. 39,5-9. ibid. 3 9 , 1 3 - 4 0 , 9 . ibid. 40,15-413. Hunayn, Jawami' 44-70. ibid. 74.

also as a translation of

4.

Pseudo-Olympiodorus

According to Pseudo-Olympiodorus' account, the generation of rain has two aspects: firstly, moisture on the surface of the earth is dissolved into exhalation (vapour) by the sunrays; this vapour rises upwards due to the heat it contains; this heat leaves the vapour because it is dispersed or extinguished. Then, secondly, the vapour becomes denser due to coldness, because heat has left it and because it has arrived in a colder region. Clouds are formed in this way and also because contrary winds press the vapour together; when the condensation increases, the cloud produces rain. Rain also occurs in hot countries like Ethiopia, although there is no cold that will condense the vapour. There are, however, mountains in that country; clouds are moved by the wind and then are stopped in their course by those mountains; they are compressed, become denser and change into rain. The effect is comparable to vapour that condenses against the roof in a bath and becomes water and to vapour that gathers in the covers of cooking pans.29 Pseudo-Olympiodorus' description of the first stage of rain formation follows Aristotle exactly. In the second stage a new feature is added. Vapour condenses into cloud and subsequently into rain, not only because it cools off, but also because it is pressed together by wind or when it meets an obstacle. This is Theophrastus' theory of condensation by compression (χιίληοις). Aristotle's explanation of rain in Ethiopia by means of άντιπερίστασις is not adopted.30 Vapour is an intermediate stage when water changes into air; clouds (gaym) are an intermediate stage when air changes into water. Mist (dabāb) is a kind of cloud, but less thick and less cold. Snow (t_alj) is formed in the same way as rain when the cold is stronger; then the vapour freezes before it has become water. Dew (tall) and hoarfrost (jalīd) are analogous to rain and snow. The former two arise close to the earth from vapour that rises daily, the latter arise further away from vapour that rises yearly.31 Hail (barad) is formed when a cloud changes into water and this water subsequently freezes. However, the freezing does not occur in the place where the water is formed from the cloud, because as soon as the water is formed it falls down as rain before it can freeze. And the water is not divided into small parts that freeze and subsequently join 29

Pseudo-Olympiodorus, Tafsir 99,16-100,3. See Olympiodorus, in Meteor. 80,30-81,1; Theophrastus, Steinmetz 1964 217-221. 31 Pseudo-Olympiodorus, Tafsir 100,5-101,8. 30

Meteorology

ch. 7,2-9;

to form hail, just as small parts of water coalesce to form drops of rain. The raindrops freeze while they descend and become hail.32 If the freezing occurs high above the earth, the hail stones are small and round; if it occurs close to the earth, they are large and irregularly shaped. Hail rarely occurs in winter, but mostly in spring and autumn, when the air is warm. Such freezing in warm air is said to occur accidentally (bi-tarīq al-'arad), whereas freezing by cold in winter is said to occur essentially (bi-dātihī). In the former case the heat of the air causes the cold in the vapour to flee (haraba) from it and to become concentrated in the vapour, where it freezes the water that falls as hail. In the latter case snow is formed. In autumn there is more hail than in spring because the matter has been preheated in summer, so that it is more quickly affected by coldness.33 One sees that Pseudo-Olympiodorus' explanation of hail is the same as that of Aristotle, and that he uses άντιπερίστασις, although the same principle was rejected in the explanation of rain in Ethiopia. Some passages are similar to Olympiodorus' commentary, including the one in which it is claimed and explained that hail more often occurs in autumn than in spring.

6. Al-Kindī A1-Kindī deals with rain in his treatise "On the reason why in some places it almost never rains".34 The formation of wind is also discussed. He says that the cause of rain is the motion of the celestial bodies to the north and the south that occurs due to the obliquity of the ecliptic. Of these bodies, the sun exerts the most important influence. It heats the place of the earth that is vertically below it, dissolves from it moisture and fine earth and heats the air adjacent to it, whereas the places far from it remain cold. A body that becomes cold contracts (inqabada) and occupies a smaller space; a body that becomes hot expands (inbasata) and occupies a larger space. If air is heated, it will flow from the hot place where it expands to the cold place where it contracts and this flowing is called wind. Thus, when the sun is in the southern tropic, wind will blow from south to north and when the sun is in the northern tropic, wind will blow from north to south. Therefore

32

Note that Pseudo-Olympiodorus, Tafsir 101,11-17 is similar in text to Olympiodorus, in Meteor. 88,38-89,10. 33 Pseudo-Olympiodorus, Tafsir 101,10-102,9. The last phrase is similar to Olympiodorus, in Meteor. 9330-35. 34 Al-Kindi, Rasâ'il II 70-75; English abstract in Sersen 220-223.

in summer most winds are northerly and in winter southerly. Certain local conditions, however, such as the presence of wadis and streams, swamps, meadows, mountains on which the sun shines, fields, forests and thickets may cause exhalation that flows in various other directions, and different kinds of winds arise varying according to the height and depth of a place. The sun's yearly motion causes a circulation of exhalation along the earth from north to south and vice versa. The ancient Greeks have compared this circulation with Okeanos' that surrounds the earth.35 If the exhalation arrives in a place that is far from where the sun is vertically above, it cools and is enclosed (inhasara); it becomes denser and thicker. Then the watery exhalation turns into rain, and the earthy exhalation turns into earth. This pushes the air by its weight and thrust and turns it into wind, for wind is flowing air. The exhalation is even better enclosed when it flows through depths and between mountains—it is enclosed and prevented from flowing by the cold there36—or when it meets another, opposite flow of exhalation, for instance, one that was formed by one of the above-mentioned local conditions. If the exhalation arrives in a place where it does not become cold and enclosed, it goes on to a place where these causes (cooling and enclosure) exist and no rain is formed in places where they do not exist. This is the case in Egypt, for it has no mountains in the north and most exhalation that flows from south to north from the Ethiopian Sea (Indian Ocean) is arrested by the mountains in Ethiopia and flows in the direction of Iraq. If some exhalation reaches Egypt, it will arrive in hot places, for the air is heated by the Ethiopian Sea in the east and the Sea of Alexandria (Mediterranean Sea) in the north, so that exhalation is not densified there. Because the Nile flows through Egypt, the earth absorbs much moistness and consequently much exhalation rises from the earth because of the heat of the sun. If the atmosphere cools off at night, this exhalation turns into water and becomes a weak flow, since no enclosure has occurred. In this way dew is formed on the earth. Al-Kindi treats mist in the treatise "On the cause of the formation of mist".37 He says that clouds are formed when exhalation rises to cold layers of the atmosphere, as has been explained in the treatise "On the reason why in some places it almost never rains". If wind arises above a cloud, it pushes the cloud downward. If the cloud arrives at the earth, 35

See Aristotle 346b36-347a8. Ibn al-Bitriq says that moist exhalation is enclosed especially mountains (Ibn al-Bitriq, Meteor. 20,11). 37 Al-Kindi, Rasa il II 76-78; English abstract in Sersen 223-224. 36

between

high

and evaporates by the heat close to the earth, it is called mist. It is a sign of fair weather, because the wind by pushing down the cloud has removed it from the upper place where it could condense into rain. A wind may also arise in a cloud, in the part that is closer the earth, due to pressure of the cold from above and from the sides. This wind pushes the main part of the cloud downward to the earth, while the remaining part of the cloud stays where it is. The mist that is formed in this way is not a sign of good weather. If one does not see the sun or the moon, it means that the upper cloud is present. If one does see these celestial bodies through the mist, then the upper cloud has descended. Often one may see, standing on a mountain, a cloud below; that cloud looks like mist. If one descends into the cloud one finds it is just like mist on the earth, except that it is denser and moister, so that it is difficult to breathe in it. This may also occur at the surface of the earth, but usually the heat of the air dissolves such a thick mist. There are other causes that may push clouds to the earth. In the upper part of the cloud the earthy part of the exhalation exceeds the watery part, whereas in the lower part the watery part dominates. If the upper part cools off, the earthy exhalation changes into earth and pushes the lower part down before it can turn into water. Rain, snow and hail are discussed by al-Kindi in "On the causes of snow, hail, lightning, thunderbolts, thunder and zamharīr " 3 8 He says that snow, hail and rain have the same remote cause, namely, enclosure of exhalation in the atmosphere, as has been explained in the treatise "On the reason why in some places it almost never rains": if a cloud arrives in a cold place, its watery parts condense into water. Hail is frozen water. It is formed when cold changes the earthy parts in the exhalation into earth. They push one another in all directions and move about in the cloud, so that a wind arises within the cloud, its strength being dependent on the thickness of the cloud and the intensity of the cold. If the cloud is thick and this wind is strong, and the cold is intense, then the water is frozen and gradually descends from the cloud. The descending hailstones strike against one another so that they break into pieces and when they arrive in warmer regions the sharp edges melt away. When they get to the earth they are nearly round, with worn down edges. The size of the stones depends on the distance they have fallen and on the quantity of material they consist of. If they fall from a great distance and consist of little material, they have melted away before having reached the earth or they are only small when they

38

Al-Kindî, Rasâ'il II 80-85; partial English abstract in Sersen 225-226.

reach the earth. If they fall from a near distance and consist of much material, they reach the earth in large particles. Hail mostly occurs in spring and autumn, when the sun is near the zenith and heats the atmosphere. Exhalation rises because of its own heat and that of the atmosphere, until it reaches a colder area, where the sunrays that are reflected from the earth have no influence anymore. If the weather is warm, a cloud may be pushed downward; then the exterior parts are warm, whereas the inside gets cold. This effect occurs because of the effect that the contrary active qualities recoil (iqtasama) to contrary places (άντιπερίσχασις). Then the watery parts inside the cloud turn into water, freeze and fall down as large hailstones. Their form is sometimes not round, because they are not formed by the cooling effect of wind, but by the recoil of the cold inside and the warm exterior of the cloud. Snow is formed when a cloud condenses by cold, so that raindrops arise; if the atmosphere is cold, then these drops freeze during their fall to the earth and become snow. If their size is sufficiently large, they arrive on the earth as snow, otherwise they melt before having reached the earth and become rain. If there are no clouds and the upper atmosphere becomes very cold due to a cold wind, then the air with the watery exhalation freezes before it is able to condense into clouds. Then it falls down as snow while the sky is clear. These snowflakes have an elongated form because the wind freezes them together. This kind of snow is called zamharīr. Reviewing al-Kindi's treatises that were discussed above in this chapter and in Chapter 1, we see that al-Kindï describes the formation of precipitation and wind by the exhalations in different ways. On the one hand he says that there is a horizontal motion of exhalation or air from north to south and vice versa, which is caused by expansion due to the sun's heat; this horizontal flow is wind and when it arrives in a cold area, it densifies and turns into rain. On the other hand, there is a vertical motion of exhalation: it rises from the earth to cold layers of the atmosphere, where it is densified; the moist exhalation becomes water (rain and other precipitation), the dry exhalation becomes earth; these earthy particles push the air by their weight and in this way wind is formed. The latter description of the formation of rain corresponds to that of Aristotle, the former one does not. The formation of wind in both descriptions is not Aristotelian. Note that, according to Aristotle, clouds are a densified form of both exhalations, but that he uses this only in his explanation of the sea's saltness (see below p. 126) and of thunder (see below p. 225), whereas it is used throughout al-Kindfs discussion of the effects of the exhalations.

Al-Kindi's account of rain being formed when exhalation passes between mountains reminds one of Theophrastus' doctrine that exhalation becomes rain when it is driven against a mountain (πίλησις; see above p. 100), and this made Sersen suggest that al-Kindi was influenced by Theophrastus.39 However, according to al-Kindi, the cause of condensation into rain is always coldness, which may occur everywhere, but especially between mountains, whereas, according to Theophrastus, the condensation occurs by compression against moun- tains, even when the atmosphere is warm. Note that al-Kindi gives two explanations of hail: one of them explains hail falling in autumn and spring from lower clouds by means of άντιπερίστασις and corresponds to Aristotle's explanation. The other is not in Aristotle and deals with hail from clouds in cold strata of the atmosphere. Also, Ibn al-Bitriq distinguished between these two kinds of hail, see above p. 105. For further evidence of an influence of Ibn al-Bitriq, see the chapter on lightning, below p. 234. For a discussion of al-Kindi's theory of wind, see below p. 176.

6. Ikwān as-Safā' and al-Qazwīnī The account of clouds and precipitation given by the Ikwān as-Safā' and al-Qazwini is discussed by Sersen.40 They say that high mountains and cold cause condensation of clouds, which again made Sersen suggest influence of Theophrastus.41 However, the passage concerned resembles al-Kindi's description of what happens when exhalation arrives in a cold area (see above) more than any passage in Theophrastus. Another supposed indication of Theophrastus' influence, namely that both the Ikwān as-Safā' and Theophrastus distinguish between two kinds of snow,42 has been invalidated since Daiber's publication of Theophrastus' publication of Theophrastus' Meteorology, no such distinction was made by Theophrastus.

7. Ibn Sīnā Ibn Sīnā in his Kitāb as-Šifā' describes the formation of clouds, saying that clouds are densified vapour floating on air. This vapour is, as it 39 40 41 42

Sersen 181. ibid. 180-194. ibid. 180-181. ibid. 186-189; see also Steinmetz 1964 191n2.

were, something between water and air. It arises from water when it evaporates from air when it is densified. Clouds are formed from exhalation that comes to be in either one of these ways. Ibn Sīnā reports that he has seen cloud formation and subsequent snow or rain occurring in cold places high in the mountains. He observed, however, that this also occurred in a lower stratum of the atmosphere, where it is not so cold. Then the vapour does not rise far, but yet it is condensed. Such a condensation may be due to wind. It occurs either (1) because the wind prevents the vapour from moving upward, or (2) because the wind pushes the vapour against mountains that prevent it from moving further or (3) because contrary winds compress it, or (4) because the front stops and what follows is pressed against it, without an obstacle such as a mountain being present, or (5) because the wind is extremely cold. Thus, condensation of vapour does not necessarily require cold. In Ethiopia, for instance, it is hot and still there is much rain. The exhalations are carried there by the wind, and they are compressed (indagata) against the mountains. In most cases however, the vapour rises to the colder stratum of the air; there it cools, also because the hot, smoky exhalation that accompanies it is separated from it. It condenses into clouds that change into water that falls in the form of a downpour (wābil) or continuous rain (dīma).43 Dew (tall) arises not from cloud, but from a small amount of vapour that is daily evaporated, slowly rises and cools in the night and then condenses into water. If it freezes before condensing into water it becomes hoarfrost (saqV). If vapour in a cloud freezes before raindrops are formed, snow is formed. If it freezes after having become drops of water, hail is formed. This mostly occurs in autumn and spring, not in winter. When there is an intense cold in winter, snow is formed. Hail is formed in spring and autumn if the cloud gets surrounded by hot air, then the cold suddenly flees (haraba) to its interior and densifies the cloud, as in a process of recoil (αντιπερίστασις - ta'äqub). The vapour condenses into drops that subsequently freeze. Heat makes the material thinner and thus more suitable for freezing, as hot water freezes more quickly when cooled. Therefore there is more hail in autumn because the summer heat has made the material thinner, and what is thin is more easily affected by cold and heat. If the cloud arrives in a warm area the accidental effect of the heat27 is stronger. Another way in which hail is formed is that a cold wind hits upon a warm cloud that is close to the earth. Hail from high clouds is small and round because its 43

Ibn Sīnā, aš-Šifa, Tab. 5 35,4-36,5. That is, the concentration of cold by the surrounding heat (άντίπερίσίασίζ); see Pseudo-Olympiodorus, above p. 107 for this use of the term 'accidental'. 44

edges melt during its fall in the air. Hail from clouds near the earth is large and not round. If no freezing occurs, a dense rain (qitqit) falls from such clouds. For the raindrops coalesce into larger drops, which during their fall come apart into small parts. If one pours water from a high place, it falls apart into drops before it reaches the ground.45 Mist is of the same matter as clouds, but it does not have the same constitution. If it descends from above, especially after rain, it is a sign of clear weather; if it is formed below and then rises, it is a sign of rain.46 We conclude that Ibn Sînâ's own observation plays a rather important part in his account. He takes, like Theophrastus, compression as a second way in which a cloud may condense into precipitation. The rain in Ethiopia is explained in this way by compression, not by άνχιπερίσχαοις. However, hail is explained by άνχιπερίοχαοις in the Aristoxelian way. Several instances of influence by Pseudo-Olympiodorus have been noted. The effect that there is more hail in autumn than in spring, mentioned by Olympiodorus and Pseudo-Olympiodorus (see above pp. 104 and 107), is also discussed by Ibn Sīnā, with a more extensive explanation, sc. that preheated water freezes more quickly because it is thinner and thus more easily influenced by cold.

8. School of Ibn Sīnā Bahmanyār says that vapour which rises into the cold layer of the atmosphere condenses into cloud because of the cold. One may observe a similar effect in a bath: if the door is opened and cold enters, the air becomes foggy. Hot water is affected by coldness more easily than cold water because it is finer, this appears from the fact that if hot and cold water are exposed to cold air, the hot water becomes cold more quickly. Vapour is formed to a greater extent in mountains because they are like an alembic that prevents the vapour from being dispersed. Various things may occur to the condensed vapour (cloud). If there is only a little amount of it, it is dispersed by the sunrays. If it is stronger and not influenced by the sunrays, it densifies, especially if wind contributes to this process. If the cloud densifies, it becomes water that descends. If cold affects it before raindrops have been formed, it becomes snow. If cold affects the cloud suddenly, or after raindrops have been formed, it becomes hail. In spring there is more hail due to

45 46

Ibn Sīnā, aš-Šifa, ibid. 38,6-9.

Tab. 5 36,7-37,17.

the mutual recoil (ta'äqub - άντιπερίσιατσις) of heat and cold. Then the cold inside the cloud becomes stronger, so that it freezes. If the vapour rises only a little, dew and hoarfrost are formed.47 Most of this account is derived from Ibn Sînâ's chapter on clouds and precipitation in his Kitāb aš-Šifā'. The comparison of a mountain with alembic originates from the chapter on the advantages of mountains (see below p. 142). The example of the bath is not found in Ibn Sīnā. Abū 1-Barakāt on clouds and precipitation is more extensive than Ibn Sînâ's Kiîāb aš-Šifā' and goes beyond its contents and that of Aristotle's Meteorology. We especially call attention to his account of the formation of hail. He says that vapour (moist exhalation) rises to the cold stratum of the atmosphere where the influence of the reflected sunrays has stopped and which is still so far from the stratum of fire that it does not feel its heat. This stratum is cold, when earth and water are cold because of the absence of sunrays during the night and their weakness during the day in winter. If earth and water are heated by the sunrays, this heat gradually rises until a certain level, depending on the intensity of this heat. A strong, continuous heat, such as the heat of the summer, will make the cold disappear from all air and the air will become hot. Therefore there will be little or no rain in summer. In winter the cold of the night will also remain during the day, because no heat will make it disappear during the day. If the vapour arrives at that cold stratum, it cools; cold wind from mountains, as well as cold water and snow that is carried with it, may contribute to its cooling. The cooled vapour descends again and meets other, rising vapour, which in turn is cooled by it. In this way thick clouds pile up. The clouds change into drops and become rain. At first a watery drizzle (radād) is formed, consisting of small particles. When they descend, they approach one another and coalesce into larger drops. The higher the place they start to descend, the larger the drops that fall on earth. If the cold is more intense, the drizzle freezes and descends as snow. If snow falls and the cold becomes stronger, no vapour is formed anymore and the atmosphere near the earth becomes as cold as the place from where the snow falls. If snow is falling while there is a wind blowing, the snowflakes collide against one another and adhere to one another. They are subject to two motions: descending and getting attached to one another by the wind. The result is a circular motion and consequently they coalesce into spherical particles, and this is hail. The stronger the wind and the higher the cloud, the more hail is formed. Thus, hail

47

Bahmanyār,

at-Tahsïl

711,4-712,3.

cannot be formed without wind and it is formed in times that are warmer than when snow is formed. This place, where rain, snow and hail are formed, used to be called the sphere of extreme cold (falak az-zamharïr)}s The earth does not have the form of a perfect sphere because of its hills and mountains and depths and valleys. Water descends into these depths and valleys, whereas heights and mountains remain uncovered. Thus, water does not cover the whole earth. Earth and water and the air that surrounds them are heated by the sunrays and when this heat is intense, in summer and in hot countries, no clouds are formed, nor does any rain fall, except in areas near the sea. There clouds are formed from rising vapour that from far away look as if they would give rain or snow, but no rain is formed in them. A cloud may be lower than the top of a mountain, and someone may enter the cloud. Then he sees nothing that is different from what he sees on a day with rain and fog. The cloud is not something that carries rainwater, as the common people think, but the cloud is the rain itself. A cloud that does not rain consists of piled-up vapour that does not become cold enough to turn into rain. Wind carries clouds along with its motion and this motion opposes the (natural) downward motion of the rain in clouds, which is less forceful than the motion caused by the wind. Rain descends when the wind subsides; therefore people say that rain stops the wind. Or, if the parts of the cloud coalesce by its motion and the cold gets stronger, then rain will occur, while wind is blowing.49 Vapours also rise from the sea. They accumulate because of the continuing supply and the motion of the wind. When they cool by the blowing of the wind they rain down on the earth near the sea as well as farther away, in times of cold as well as heat; for hot winds may also cause cooling. Rain clouds are formed more often near the sea, or are carried from there by strong winds. Thus, a south wind brings rain if it comes from near a sea, and similarly for the other winds. Summer rain is generally due to vapour that is brought by winds from the sea and not caused by cooling and condensation of clouds. Countries where it does not rain are hot, far from the sea and low lying. Their soil is either like a salt marsh or sulphurous, reflecting much heat to the atmosphere, and there is not much wind from the sea. Countries where much rain falls are either near the sea, or there is much seawind, or they are near mountains with snow or near large rivers or lakes and

48

Abū 1-Barakāt, al-Mu'tabar II 213,7-215,1. The expression falak in e.g. Ikwān as-Safâ', see above p. 53. 49 Abū 1-BaraÌcāt, al-Mu'tabar II 215,2-24.

az-zamharīr

occurs

their soil is able to retain much moistness.50 Abū 1-Barakāt sums up his account by saying that rain is formed when vapour rises and accumulates, and then cools and condenses. Auxiliary causes are vapour that rises in other places and are brought along with the wind. Thus, vapour may rise in a mountainous area and cause rain there, or be carried from there to another place and cause rain in that place. The same holds for vapour rising from the sea. Vapour that rises in the mountains generally turns into rain at the same spot, because the atmosphere is cold there. Vapour rising from the sea does not turn into rain there, because the atmosphere is warm; it is carried by the wind to other places. If this were not the case, it would always rain on the sea, for the rising of vapour there is continuous.51 The account of Fakr ad-Din is mostly a systematic rendering of the Kitāb as-Šifā'. He says that clouds and precipitation are mostly caused by densification of vapour and sometimes by densification of air. In the former case, if there is only little vapour and if there is some heat in the air, then the vapour is broken up and turned into air. If this does not occur, the vapour rises and either reaches the cold stratum of the atmosphere or it does not reach it. In the former case, if the cold is not strong, the vapour is densified and condensed—this is the formation of clouds—and turned into drops—this is rain; if the cold is strong and affects the vapour before drops have been formed, then snow is formed; if it affects the vapour after that, then one gets hail. In the latter case (i.e. the vapour does not reach the cold stratum), if the amount of vapour is large, then either it condenses into clouds that produce rain, or it does not condense. If it condenses, then this occurs because of one of five causes. Fakr ad-Din enumerates the causes adduced by Ibn Sīnā (see above p. 112). As for the fourth cause, he specifies that the front part of the exhalation may stop moving because it is heavy. The fifth cause is that the air close to the earth is cold. If the amount of vapour that does not rise far is large and does not condense, the result is fog (formulated differently by Ibn Sīnā). If the amount of vapour that does not rise far is small, then dew or hoarfrost is formed. If clouds are formed from air, when the air is cooled and densified, the same kinds of precipitation may be formed.52 Various properties of the different kinds of precipitation are enumerated. (1) Hail is formed in spring and autumn, not in winter or summer. Its formation occurs by άντιπερίσιασις. (2) There is more hail in autumn than in spring because in summer the material is preheated. 50 51 52

Abū 1-Barakāt, al-Mu'tabar II 215,24-216,13. ibid. 216,14-21. Fakr ad-Din, al-MabàhiL II 172,16-174,8.

(3) Hail from high clouds is small and round; hail from clouds close to the earth is large and not round. (4) There is much rain in Ethiopia, although the weather is hot. (5) Raindrops in summer are large and far apart; in winter the opposite is the case. The reason is that in summer the rising vapour contains much earthy parts that are the material of wind. This wind causes the raindrops to coalesce into large drops. In winter the air is at rest and such a process does not occur. (6) Mist is a sign of clear weater when it descends from above, especially after rain; if it rises from below, it is a sign of rain. (7) According to some people snow occurs in all sections of the earth, except the fifth.53 The description and explanation of these properties follow the text in the Šifā', except for (5) and (7).

9. Ibn Rusd Ibn Rusd's account in his Short Commentary of the formation of clouds and precipitation runs as follows: The sun dissolves from the earth two exhalations, a hot, dry exhalation and a hot, moist exhalation. When the hot, moist exhalation rises and arrives in the place where the influence of the sunrays reflected from the earth's surface has stopped, it cools down, condenses and forms clouds. If it condenses further, drops of water are formed that fall down. This phenomenon due to cooling of moist air we see also occurring in baths and in the process of destination (taqtīr), 54 The cyclical process of dissolution and condensation of moist exhalation is parallel to the course of the sun in the ecliptic. The moon, however, also has an influence on this process. When the moon is waning at the end of the month, its light decreases, and this also causes a cooling of the air. Therefore rain occurs often at that time, and the nature of the moon is called cool and moist.55 If the air is hot and moist the cold has more effect, and it suddenly changes into large drops; then a downpour (wabl) arises; otherwise we get a drizzle (rašš or radād). Hot air is affected by coldness more quickly than cool air; this is similar to the phenomenon that warm water cools more quickly. Therefore no rain arises in cold weather and when there is a north wind. In hot weather and dry air there is no rain

53

Fakr ad-Dīn, al-Mabāhii II 174,9-175,14. The 'fifth section of the earth' is the section between the tropics, see below p. 194. 54 Ibn Rušd, Short Commentary 20,1-21,10. 55 ibid. 21,12-19.

because no moist exhalation is dissolved.56 Ibn Rusd's account of snow, dew (nadan) and hoarfrost (jalīd) has no special features. His account of hail is as follows: Hail occurs when water that has condensed in a cloud is frozen by heavy cold before it falls down. It mostly occurs in autumn and spring. Then the surrounding air itself (bil-dāt) is not cold to such an extent that it can cause this freezing. Thus, it must occur accidentally (bil-'arad).57 Then the cold parts in the cloud gather together in its interior, fleeing (haraba) from the surrounding warm air and this causes hail.58 For each one of two contraries is reinforced when the other contrary is present, out of fear of being destroyed otherwise. Also, water is more affected by cold when it is warmer. Thus, when doctors want to cool water quickly, they first heat it. Therefore raindrops are larger from clouds that have become warmer.59 Influences from Pseudo-Olympiodorus are clear. The Middle Commentary is mostly a rendering of Ibn al-Bitriq's text, with additional remarks of Ibn Rušd inserted between the phrases of this text. The order of two sections is reversed by Ibn Rušd: the section on the question that dew and hoarfrost are not formed high in the mountains and the section on the question why dew is formed when a south wind blows except in Pontus, where it is formed when a north wind blows. Also, in the latter passage the north and south winds are exchanged.60 In the account of hail the order of Ibn al-Bitrlq is not always followed either and Ibn Rušd is more elaborate. He says that there are two reasons why hail is formed in warmer times rather than when it is cold, in winter: (1) The surrounding heat causes the cold to concentrate within the cloud (αντιπερίσιασις). The effect of the cold is stronger than in winter, because now the cold is concentrated, whereas in winter the cold is spread all over the air. (2) In warmer times the cloud is warmer and also the water that is formed in it. The effect of cold on this preheated water is stronger than on cold water, i.e. this water freezes more quickly. An indication for this is that if one heats water

Ibn Rušd, Short Commentary 223-14. The formation of a downpour is an effect of άντιπερίστασις, as Aristotle says in 348b10, but the way it is described by Ibn Rušd is not how αντίπερίσίαοίς is supposed to work. The confusion is caused by Aristotle himself, see above p. 99n2. 57

The same distinction of causes is in Pseudo-Olympiodorus, see above p. 109. The same formulation is in Pseudo-Olympiodorus, see above p. 109. Ibn Rušd, Short Commentary 24,6-25,4. Cf. also the Middle Commentary below, where Ibn Rušd discusses more extensively the two reasons (ανίίπερίσίασίς and preheating) why hail is better formed in warm surroundings. 60 Ibn Rušd, Middle Commentary 66,6-67,2. 58

59

and then puts it in a cold place, it accepts the cold more quickly. The reason is that the effect of one contrary (cold) is stronger in the presence of the other contrary (the hot water). This resembles the first reason, but the difference is that here the heat is in the body receiving the cold and in the first case the heat comes from outside. Thus, the cold air is opposed by two contraries: by the surrounding warm air and by the heat of the body that receives the cold.61 Ibn Rušd adds a section in which he gives Aristotle's account and refutation of Anaxagoras' 62 view on the formation of hail (338a15 ff.—not in Ibn alBitriq), taken from Alexander's comment, as he says, and with additional remarks of his own.63

61

Ibn Rušd, Middle Commentary 68,7-69,13. Ibn Rušd has: 'Pythagoras' instead of 'Anaxagoras', maybe due to a mistranslation in Alexander's commentary. 63 Ibn Rušd, Middle Commentary 69,14-71,5. 62

CHAPTER FOUR

RIVERS AND THE SEA

1. Aristotle Aristotle begins Chapter 1,13 with an introduction to his account of wind. He mentions the opinion of some people who say that wind is air which is in motion, so that its nature is the same as the nature of water, which is condensed air. Some concluded from this that all wind is in fact the same, only differing in place of origin. This theory is just as nonsensical as it would be to say that all rivers flow from a single source. Thus, we must investigate what winds are and how they arise; whether they come from a kind of vessel and blow until it is empty or have their own origin (349a17-349b2). Here Aristotle breaks off his account of wind and turns to an account of rivers. In chapter 11,4 a new start is made with the account of wind. Aristotle thinks that the explanation of winds is analogous to that of rivers in many respects. Therefore he wants to discuss the rivers first. Chapters 1,13-11,3 must be considered as a later insertion by Aristotle or a long digression.1 We shall treat the subject of wind in the next chapter. Aristotle first mentions the theory of others that rivers (ποταμοί) flow from a subterranean reservoir—either all from the same reservoir, or each from a different one—that is filled in winter by rain. This could explain why rivers are higher in winter, and why some dry up in summer, whereas others flow the whole year round, for some reservoirs are smaller than others and do not gather the rain that is sufficient for a continuous flow. According to this view, all the water in the rivers is rain water that has collected in winter. Aristotle criticizes this view, pointing out that a reservoir that could contain the whole yearly flow of all rivers would have to be almost as large as the whole earth. He admits that rivers may be fed from reservoirs, but one has to consider another process as well. Here Aristotle turns to his own opinion. He says that it is obvious that if cold condenses vapour into water above the earth, the same will occur below the earth. The water of the rivers is supplied not only by rainfall, but also by condensation

1

Strohm 1984 155-6.

of subterranean vapour; drops that are formed by this condensation become quantities of water at a certain spot in the earth and this is the origin of a river. That this process occurs is clear from the construction of wells: the water that the earth is sweating out, as it were, is collected in pipes and channels. We see that the headstream of rivers flows from mountains and that most springs (κρηνοα.) are also found in high places, for mountains are like a sponge; they receive the rain and also cool the vapour that rises in them; this vapour is subsequently condensed into water (349b2-350a13). Aristotle then gives a geographical survey of mountains and rivers that flow from them. He repeats his view that rivers are not only fed from a supply of existing water, but also from water that continuously gathers at the foot of mountains from subterranean condensation and in this way forms the source (πηγή) of a river. He admits that there are subterranean lakes, but they are not so large that they are able to supply all the water of a river. Subterranean rivers are an indication that hollow spaces exist in the earth are subterranean rivers. When a river has no outlet to the sea because mountains prevent it, its water is swallowed up by the earth and flows underground. For instance, many rivers flow into the Caspian Sea that has no outlet. Their water runs beneath the earth until it comes up near the 'depths of Pontus' (in the Black Sea). This is a spot in the sea that is very deep, where fresh water comes up over a large area (350a13-351a18). In chapter 1,14 Aristotle gives an account of the variation in moistness of certain parts of the earth in connection with rivers and the sea. The moistness of a certain place of the earth varies in accordance with rivers that come to be or dry up; also, the areas that are land and that are sea are not always the same: the border between land and sea is subject to change. The process is cyclical in accordance with the increase and decrease of cold and heat due to the course of the sun. While some places become dry when sources and rivers dry up, other places become wet and there rivers come into existence. Also, sea will be pushed back by rivers in one place, where land is formed by silting; then land will be flooded in another place. When the rivers dry up, the sea will flood the former place and recede from the latter. Such changes are very slow: their period is longer than a man's life or even the time in which a whole people perishes; therefore they cannot be observed, nor recorded. For instance, Egypt is a deposit (πρόοχωσι,ς) of the Nile, and is slowly getting drier. There is no record of the beginning of this process, but we may conclude that this process has occurred from the fact that first the higher places (around Thebes) were inhabited, the lower ones still being too wet. When these lower places

dried up they became suitable for habitation. This is proved by the fact that the mouths of the Nile, except one, are artificial. Places that used to have a good climate become too dry and unsuitable for habitation. For instance, in Greece, Argos used to be too wet for habitation, while Mycene had a good condition. Now the opposite is the case: Mycene has become too dry and Argos is cultivated (351a19-352a17). Some people say that such effects are part of a process of comingto-be of the world as a whole. They have found that places which used to be sea have become land and therefore think that the sea is gradually drying up and is becoming less and that this is part of a process of universal change. They overlooked that there are also places where the sea has flooded the land. Anyway, such effects are too small to be ascribed to a process in the universe. The cause of a flood is rather a 'great winter' and excessive rain occurring at a certain period in a certain place. This makes the place where it occurs very wet for a long time. Certain areas will dry quicker than others: places with small mountains, composed of porous material like stone or clay—such places do not supply much water for rivers, so that these rivers become dry earlier—dry up more than those with large, dense and cold mountains—such places supply much water for rivers (352a17-352b16). Another example indicating that the same places are neither always moist nor dry is that when one of the kings of Egypt wanted to make a canal from the Nile to the Red Sea, he found that the sea was higher than the land; therefore he stopped digging the canal, otherwise the sea water would mix with the water of the river and spoil it. This proves that the land used to be sea. Other examples are the depression in Libya that is lower than the land nearer to the sea—here a river formed dry land by silting and behind it lakes that subsequently dried up—and lake Maeotis (Sea of Azov) that will gradually dry up because of silting by the rivers that originally produced it. The current through the Bosphorus forms a sandbank before the shore; behind it a lake is formed that dries up; then another sandbank is formed, and so on; the channel will become narrower and eventually dry up (352b26-353a14). In chapter 11,1 Aristotle starts the account of the origin of the sea (θάλαχτα) and its saltness (άλμυρότης). 2 First the views of others are mentioned. (1) The ancient 'theologians' thought that the sea has sources (πηγή). (2) Others, more philosophically minded, thought that at first water surrounded the whole earth and then the sun dried part of it. What was not dried became sea. They think that the sea is gradually 2 Cf. Fontaine, "Why is the sea salty?" 1995 for a review of various Greek, Arabic and Hebrew texts on this subject.

drying and will finally be dried up entirely. (3) Finally, some believe that the sea is a kind of sweat (ίδρως) of the earth that arises because of the sun's heat; this would also explain its saltness, for sweat is salty. Another explanation for its saltness is that earthy particles are mixed with the water, such as water that is filtered through ashes becomes salty (353a32-353b16). · Aristotle shows that the sea cannot have sources. (1) Water that exists on earth may be classified as follows: it is either flowing or stagnant. Flowing water comes from sources (πηγή). Stagnant water is either water that has collected and remains stationary, such as pools (τέλμα) and lakes (λίμνη), or water that comes from a source that is artificially made, such as a well (φρέαρ). Thus, water from a source is either flowing, or comes from an artificially made source. The water of the sea belongs to neither class, therefore the sea does not have sources. (2) There are seas that are not connected with others, such as the Red Sea, which is connected with the ocean by a narrow channel only, and the Caspian Sea, while people are living all around them. If they had sources they would have been discovered (353b17-354a5). Still, the sea seems to be flowing, but it is not an indication that it has sources; this flowing may be explained in other ways: (1) The sea seems to flow in narrow straits, but that is a kind of swinging to and fro; in open sea this effect is imperceptible; it becomes observable when the sea is narrowed by land. (2) The whole Mediterranean Sea flows in a direction that depends on the depths of the sea-bed and the number of rivers that flow into it. Many rivers flow into the Sea of Azov and Black Sea, and they are rather undeep seas; the water of the Sea of Azov flows into the Black Sea and this flows into the Aegean Sea. The sea becomes deeper in the order Sea of Azov, Black Sea, Aegean Sea, Sicilian Sea, Sardinian Sea, Tyrrhenic Sea, so there is a flow in this direction. In general there is a flow from north to south because the northern regions are higher. Therefore, the seas in the north are less deep, for their water flows away (354a5-28). Having dismissed the theory that the sea has sources, Aristotle continues his investigation of the origin of the sea and its saltness in 11,2, mentioning the view of his predecessors that the sea is the primary and main body of water, the source of all moistness and water on earth. Just like the elements fire, air and earth have their primary and main body, water must have its main body too. That must be the sea, according to this view, for rivers do not form such a single and stable unity. This view implies that rivers not only flow into the sea, but also out of it; the water of rivers becomes fresh because the salt is filtered out. Aristotle's objection to this view is that it does not explain why

the sea is salty, for if it were the main body of the element water, it should be fresh (354b1-23). Aristotle repeats that the sun's heat dissolves (διακρίνω) vapour from water and raises it to cold regions of the atmosphere, where it condenses and falls again to the earth.3 What is evaporated from the sea is the fresh water because it is light; the heavy, salty water remains. The element water must have its natural place, and that is the place which is occupied by the sea. Therefore rivers flow into that place, for water flows to the deepest places and the deepest places are filled by the sea. That place, however, is not the natural place of the salty water of the sea, because the salty water may be considered to be a sediment (ύπόοταοις) after the fresh water has evaporated. Thus, the sea is not the source of all water, rather a final stage, a residue (περίττωμα). One may compare this to what happens in living things. When liquid food enters the body, the fresh and sweet part of it is extracted by the natural heat of the body, and subsequently changes into flesh and other parts of the body. What remains is bitter and salty. Still the belly is considered the proper place of fresh food, although this quickly disappears and not the proper place of the sediment which remains (354b24-355b20).4 Many rivers flow into the sea, but yet it does not become larger. This means that a vast amount of water evaporates. This is possible because of the large surface of the sea.5 Aristotle concludes the chapter with a refutation of Plato's account of rivers and the sea. He describes this theory as follows: All waters (rivers and seas) are connected by 3 Here a digression occurs concerning the view that the sun is 'fed' by the moisture it draws up from the earth, just as fire burns as long as there is fuel to feed it, and that the cause of the solstices is that outside the tropics the sun does not find sufficient nourishment. Aristotle refutes this view with several objections, the main one being that we see that evaporated water entirely returns to the earth, so that no moistness is left available for the feeding of the celestial bodies. The same argument refutes the above-mentioned view that at first water surrounded the whole earth, and then the sun dried part of it and what was not dried became sea. Olympiodorus presents eight arguments against nourishment of the sun and other celestial bodies; five of them are taken from Aristotle, the other three are arguments of his own, see in Meteor. 135,29-137,11. 4

It seems as if Aristotle claims that the sea is salty because the light, fresh water has evaporated and the heavy, salty water has remained, like a salty residue of food remains when the fresh part has been extracted in the digestion. However, in the next chapter 113 he will say that evaporation is not the cause of the saltness, for indeed it does not explain where the salt in the remaining water comes from. Aristotle's intention here is just to show that the place of the element water is the place of the sea, even if this place is occupied by salty water. The commentators got confused by this question, as we shall see below. 5 Aristotle compares this to water in a cup that may not evaporate in a day, whereas it quickly evaporates when it is spread over a large table.

canals under the earth and their common source is a body of water at the centre of the earth. This water surges and flows back because it oscillates around the centre and thus causes rivers to flow, which in turn form lakes and seas. Several objections are brought forward against this theory (355b20-356b3). Having explained that the sea is not the primary and main body of water, but that its salty water is a sediment or residue after the evaporation of fresh water, Aristotle still has to explain the origin of the saltness. This is done in 11,3, together with a discussion of the question whether the sea was generated or not. He says that it is clear that sea and cosmos are coeval, i.e. both are generated simultaneously, or both are eternal. Aristotle rejects Democritus' view that the sea decreases and will finally dry up. If one adopts this view, one must deny that the moistness that is evaporated by the sun returns to the earth; however, this process occurs as long as the sun follows its course; therefore the sea will not dry up. One must admit that some places are drier than they used to be, but this is not due to a process of change in the world as a whole; such places will become moist again and are subject to a local cyclical process (see 352a17 ff.) (356b4-357a3). If one thinks that the sea has been generated, one cannot explain its saltness. (1) If one claims that the sea is what is left of the moisture on earth after the rest has been evaporated by the sun, then saltness must have been in the water from the beginning. Then one still has to explain how it became salty and also, if salty water could evaporate then, why it does not evaporate now. (2) If one claims that the sea is salty because of an admixture of earth, carried down to it by rivers, then it is strange that rivers are not salty. (3) If one claims, with Empedokles, that the sea is salty because it is the sweat of the earth, then this is not an explanation, as it is not a priori clear why sweat that is produced by our body from sweet drink is salty, no matter whether it occurs by the loss of its sweet parts, or by admixture of something, such as when water is filtered through ashes. Furthermore, how could the heating and drying up of the earth cause the secretion of such a large amount of water, in particular when this can only be a part of the total amount of water in the earth? Also, why do we not find the earth sweating now, when it did sweat before? What in fact happens is the opposite: it absorbs moistness when it gets dry. In this way Aristotle rejects the view that the sea is generated as the sweat of the earth. However, he uses the view as a starting point for his own theory. He remarks that the production of sweat (by our body) seems to be similar to the process which makes the residue that collects in the bladder salty, whereas our liquid food is sweet, viz. in urine and sweat

certain admixtures are carried through the body and secreted from it. These admixtures are the sediment of undigested food. If the salt of the sea arises from a similar process, then it must the admixture of some substance from the earth that causes the saltness of the sea; it remains to be investigated how this admixture is produced (357a4-357b24). That it must be an admixture that makes the sea salty can be gathered from parallel phenomena. The part of liquid food that is not digested, that is, its residue, is bitter and salty (urine, sweat). What is not burnt in a process of combustion is left as ashes, and these are salty. The dry exhalation that is dissolved from the land by the sun also contains such an earthy residue that is salty. When moist and dry exhalation rise together and the moist one condenses to clouds and rain, a certain amount of these earthy particles will be mixed in it and be carried down in the rain.6 This explains the saltness of the sea (358a3-28). This also explains why the rains from the south are brackish. The south winds are hot; they come from dry, hot regions and contain little moist and much dry exhalation. Therefore rain that falls with the south wind contains much of this earthy admixture. Rain that falls in autumn is also brackish, because the heavier rain, that is, the rain that contains earthy particles, falls first. It also explains why the sea is warm, for these earthy particles contain heat, like any residue from what has been exposed to fire, such as ashes and excrement. The saltness of the sea remains constant, although earthy particles are continually added whenever rain falls, for an equal quantity of salt disappears from the sea together with the evaporation of the fresh water. Sea water is water with an admixture; this is indicated by the following phenomena. If vapour from salty water condenses, it forms fresh water, and the same holds when other liquids that have a taste are evaporated, such as wine. If a closed waxen jar is put into the sea, one finds that the water that enters through the wax is fresh; the salt is prevented from entering as it were by a filter. Furthermore, sea water has a higher density; therefore heavily loaded ships that float in sea may sink in rivers, which has caused loss to ignorant sailors. Also, eggs float on salty weater, not on fresh water. There seems to be a lake in Palestine that is so salty that people and animals float on it. There is a spring in Chaonia that gives salty water, when this water is evaporated by heat, salt is left in the form of powder. In Umbria people burn reeds 6

Note that the dry exhalation itself is not salty, but the saltness is in the earthy particles carried by it, as has been pointed out by Webster; Lee and the ancient commentators say that the dry exhalation itself is the residue and is salty.

and rushes; they throw the ashes into water and then boil off the water; what is left is salt. If earth is burned, different tastes arise that are adopted by water that is filtered through it, and so springs and rivers acquire various tastes, like salty, bitter and acid. Most salty rivers must have been hot before, because the salty constituent retains the heat from the combustion of which they are the residue (358a28-359b21).

2. The Greek

commentators

Alexanders's commentary here mostly just follows Aristotle. We only trefer to his commentary on 358b12 ff., see below p. 133. Olympiodorus, in his account of the origin of the rivers, first mentions, following Aristotle, the opinion that they come from a vessel which is supposed to contain the whole yearly supply of water for the rivers in actual form (ενεργεία); Aristotle's arguments against this opinion are adduced. Thus, rivers do not get their whole supply of water from reservoirs. However, this does not mean that there does not exist subterranean water in actual form at all. This is clear from the fact that rivers appear from the earth and are swallowed by it, and that fresh water arises from the sea. If a river does not find an outlet into the sea it will make its way under the earth. If much water has gathered under the earth it will be driven upwards with much wind and appear as a river. In the same way fresh water may come to the surface in the middle of the sea.7 Rivers mostly flow from mountains, for mountains are suitable to receive much moistness as they contain cavities and clefts, like a sponge (σηραγγώδης). They are also suitable to retain it: the water does not leak away because they are stony. Furthermore, they are suitable for the generation of water: rising vapour will change into water because the mountains are cold.8 Olympiodorus distinguishes springs (κρήνη) and rivers (ποταμός); the former arise from a small source, the latter from a large one. In both cases the water comes out at the surface of the earth and flows away. Wells (φρέαρ), on the other hand, contain water that arises deep in the earth; that water is stagnant. The water of lakes is also stagnant, but it is always the same; the water of wells is always replenished from the earth.9

7 8 9

Olympiodorus, in Meteor ibid. 103,24-30. ibid. 106,23-31.

99,12-100,6.

As for land changing into sea and vice versa, Olympiodorus says that this occurs due to the so-called 'great winter' and 'great summer'. A 'great winter' occurs when all planets are in a hibernal sign of the zodiac: Aquarius or Pisces. A 'great summer' occurs when they are all in a summer sign: Leo or Cancer. If each planet causes heat when it rises high, and cold when it remains low, like the sun, then it follows that when all planets rise high this must cause a 'great summer' with extreme heat and dryness. Then sea may become land. The opposite occurs when all planets are low. Another cause for sea becoming land and vice versa are the rivers. Here Olympiodorus follows Aristotle's account of silting.10 An example of sea becoming land is Egypt that became dry because of silting by the Nile. Olympiodorus adduces Aristotle's arguments and mentions as an additional argument the fact that shells are found in Egypt as remnants from the time when the land was sea. However, this is not Olympiodorus' own opinion; he says that Egypt used to be moist and uninhabited, not because it was sea, but because it was marshy. This interpretation agrees with Aristotle's text, he says. That shells are found does not prove that the place used to be sea, for one also finds shells in high mountains far from the sea. They could have been carried there by strong winds from marine areas.11 In another paragraph Aristotle explicitly says that Egypt used to be sea, because the land is lower than the sea. This became clear when one of the kings of Egypt wanted to dig a canal connecting the Red Sea with the Nile. Olympiodorus' commentary on this passage just follows Aristotle. Olympiodorus begins his commentary on Aristotle's account of the sea by saying that Aristotle intends to discuss whether the sea is generated or not. Aristotle does not raise this question in relation to its existence—for his doctrine is that the sea is ungenerated and eternal, together with the whole cosmos—but in relation to its saltness, i.e. he asks whether it has always been salty or it was fresh before and became salty later. Different opinions have existed on this question. The 'theologians' say that the saltness is ungenerated because the sea has salty sources from which its water has collected. The 'philosophers' say that the saltness has been generated later. They have different views on how this occurred. Some of them say that the sea used to be fresh and became salty because the sun, moon and other stars dissolve the lighter, fresh parts of the water and use it as nourishment. What was left is a sediment that is salty. An indication is that if we boil water for a long

10 11

Olympiodorus, in Meteor. 111,28-112,22. ibid. 112,22-113,17 1163-33.

time, it becomes salty. This view is false because, according to this view, the water of rivers and lakes ought to be salty too. Or, if the water of rivers is fresh, the water of the sea would also have become fresh, as rivers continuously flow into it. Or, as the fresh, evaporated water condenses again and falls as rain, the sea should be salty in summer, when there is much evaporation, but fresh in winter, when the condensed vapour returns. Others say that the saltness of the sea arises from sweat of the earth. This cannot be true either, for if the earth were to secrete sweat, that is, some residue of fresh water, and this sweat were to form the sea, then the amount of water in the sea would have to be smaller than the amount of fresh water in the earth, for a residue of something must be less that the useful part. This appears from the fact that our urine and sweat are less in quantity than our useful fluids. Again others say that the cause of the saltness is the earth. This agrees with Aristotle's view in a certain respect, because Aristotle also makes the earth the source of the sea's saltness. However, Aristotle makes the dry exhalation from the earth responsible for it, whereas, according to these people, salty earth from under the sea is mixed into the sea by filtering. Their view is false, for dry exhalation is more easily mixed into something than earth. Moreover, if the sea were salty because of an admixture of earth, it would be turbid, not clear12 Some of the arguments adduced by Olympiodorus as refutation of his predecessors' views are not in Aristotle. He continues with the refutation of the view that the sea has sources. As we have seen (above p. 123), Aristotle's refutation starts from a division into classes of the different kinds of water on earth. This division is given by Olympiodorus as follows: Water is either flowing or stagnant. The former has sources in high mountains, the latter exists in plains. If there is much flowing water, it becomes a river (ποταμός); if there is little, it becomes a spring (κρήνη). Stagnant water either comes from sources, or it has collected from rain. If there is much collected water, it is a lake (λίμνη); if there is little it is a pool (τέλμα). Stagnant water from sources arises either artificially or of its own accord. In the latter case it is a well (φρέαρ) that has formed because an earthquake has made a chasm in which water appeared.13 In the former case a well has been constructed by water explorers. They find water by digging a hole in the earth, putting a vessel in it and leaving it there for twenty-four hours. If they find water in the vessel after that time, they say that 12 13

Olympiodorus, in Meteor. 126,3-127,14. Olympiodorus gives a similar division at in Meteor. 106,23-31, see above p. 127.

there is water not far from the surface of the earth. If they find that only the bottom has become moist, they say that there is water deep in the earth. If they do not find any moisture, they say there is no water at all at that spot.14 After this Olympiodorus follows Aristotle, arguing that the sea does not fall under any of the classes of water that comes from a source.15 Olympiodorus interprets Aristotle's text of chapter 11,2 in a rather special way. According to Olympiodorus, Aristotle in this chapter shows that, as each element has a main body which contains most of its mass and to which all other parts of that element move to join it, the main body of the element water is the sea. As we have seen above (pp. 123-124), Aristotle mentions this view as belonging to others, and disagrees. His view is that the place of the sea is the proper place of the element water, but that the sea itself cannot be the main body of the element water, because of its saltness. Olympiodorus' view can only be maintained if he takes the saltness of the sea not to be an essential property. This is indeed his opinion, as we shall see below. He mentions four arguments to show that the main body of water is the sea; they are also in Aristotle: (1) The sea is the only single coherent and stable mass of water, rivers do not fulfill these requirements. (2) Other bodies of water, namely rivers, move to it in order to join the main body. (3) The place of the sea is the place where all water, fresh and salty, gathers. In fact, the sea is salty because the fresh water is dissolved from it by the sun and other celestial bodies and the salty water is left as a residue. But this does not mean that one ought to consider the place of the sea to be the proper place of salty water only and not of fresh water too. That would be similar to considering the belly of an animal the proper place not of the food, but of the residue only. (4) It is a characteristic property of water that it moves to the deepest places and remains there; indeed, the sea occupies the deepest places.16 Olympiodorus considers some objections that could be raised against this view. If the sea is the main mass of water, it must be 'natural' (κατά φύσιν) water, whereas the sea, being salty, is non-natural (παρά φύσιν) water. The answer is that freshness or saltness does not make water natural or non-natural. Water by nature does not have any qualities like taste; the only natural properties of water are coldness and moistness. Another objection could be that water by nature cools and

14

A similar description of the method of water explorers is given in his commentary in 1,13, where it is adduced as an argument that water gathers in the earth to form sources; see in Meteor. 99,23-28. 15 Olympiodorus, in Meteor. 127,14-128,8. 16 ibid. 133,8-13432.

moistens, whereas sea water heats and dries; therefore it must be non-natural water. The answer is that the sea does not heat and dry by itself; this is due to the smoky exhalation that is combined with it. Fresh water does cool and moisten. That the sea water is fresh by itself is indicated by the fact that if one digs a well near the shore, one finds fresh water. This occurs because the sea water has been filtered on its way from the sea to the well and has separated the smoky exhalation that wás combined with it, not by mixing, but by juxtaposition.17 Thus, Olympiodorus considers the fact that the sea is salty not to be relevant for the question whether it forms the main body of the element water. For Aristotle the saltness was the argument that made him deny that the sea can play this part, although he admits that the place of the sea is the place of the element water. On the other hand, both agree that the saltness originates from an admixture. Olympiodorus stresses that it is an admixture by juxtaposition; this must also have been Aristotle's view, as appears from his example of the waxen jar into which fresh water is filtered from which the salt has been separated (see above p. 126). In his interpretation of Aristotle's enumeration and refutation of other opinions on the origin of the saltness of the sea (11,3 357a5 ff.), Olympiodorus again interprets Aristotle's text in a special way. He says that Aristotle refutes four views. (1) The sea has become salty because the light, fresh parts have been dissolved by the sun, whereas the heavy, salty parts were left behind. (2) Saltness is by nature a property of sea water. (3) The rivers carry salty earth to the sea that is mixed with the fresh water. (4) The sea is like sweat from the earth. Olympiodorus says that Aristotle already mentioned these views before (in 353b5 ff.),18 but did not refute them yet. Now follows their refutation. Aristotle refutes the first view, according to Olympiodorus, by saying that if the sea became salty because the fresh parts were evaporated, it would be salty in summer, when evaporation occurs, and fresh in winter, when the condensed fresh vapour returns to it and when moreover the rivers carry fresh water to it. Furthermore, according to this view rivers, also being subject to evaporation, would have to be salty too. The second view gives no explanation for the saltness. The refutation of the third view follows Aristotle. The fourth view is discussed by Olympiodorus as follows: Sweat, or more generally, any residue in the body of animals may become salty because of two causes: either by 17

Olympiodorus, in Meteor. 134,32-135,15. See above pp. 48 and 61-62 for the difference between mixing and juxtaposition. 18 See Olympiodorus, in Meteor. 126,14 ff., where these views were also refuted, see above pp. 128-129.

addition of some indigested substance—this is why urine is salty—or because the light parts are dissolved from liquid food and the dense parts are left behind—this is why sweat is salty. One has to investigate which of these processes causes a salty residue in the sea. In fact, both processes occur. Light parts are dissolved from the sea water by the sun and the dense, salty parts remain. This can, however, not be the only process, otherwise the objections against the first view would be valid; the process of admixture of certain undigested substances from the earth will occur too.19 What is undigested is clearly not earth itself, but the smoky exhalation that is dissolved from it. This is mixed with the sea water and makes it salty. It does not make it dense, for exhalation is light. The density arises due to evaporation of the lighter parts from the sea. Smoky exhalation may be also mixed with other waters, for instance in the downpours that occasionally occur in summer, and in the rains in Thebes. Such precipitation is salty because smoky exhalation has been mixed with the vapour from which they are formed. It is not heavy because here no evaporation of lighter parts occur.20 The smoky exhalation is not added into the sea by mixing, but by juxtaposition, for the salt may be separated so that the water becomes fresh, such as we find in wells that are dug near the shore (see above p. 131). Finally, Olympiodorus mentions that his ancestor, the great philosopher Ammonius, thought that it is not the earth under the sea that causes its saltness, otherwise the earth around the sea would also be salty, which is not true: that earth is fertile soil.21 Then follows Olympiodorus' interpretation of Aristotle's own view. In fact, this interpretation was almost completely given in the previous paragraph, when Olympiodorus set forth his reaction to the view that the sea is the sweat of the earth. Olympiodorus first states that Aristotle's view is different from that of Theophrastus, who says that the saltness is caused by the earth under the sea.22 If that were the case, the water found in wells that are dug on the sea's shore should also be salty. This is seldom the case. The cause of the sea's saltness is the smoky exhalation from the earth around the sea. This is a kind of undigested substance that is left if the earth is heated. This exhalation

19

Aristotle denies that evaporation of the light, sweet parts of water causes the saltness; Olympiodorus is misled by what Aristotle says about this in 11,2, in connection with the proper place of water, see above p. 124n4. 20 This does not agree with Olympiodorus' description in Meteor. 81,13-823, see above p. 103. There he says that the sea as well as such precipitation is heavy because the smoky exhalation carries earthy particles. 21 Olympiodorus, in Meteor. 150,29-153,11. 22 See Steinmetz 1964 296-8. The quotations from Theophrastus are included in Theophrastus 1992, 19932, vol. 1 394 no. 220.

either remains near the earth and is mixed into the sea, making it salty and heavy,23 or it rises and is mixed with the vapour that causes the downpours in summer and the rains in Thebes, making them salty.24 One might ask why the water of the rivers is not also made salty by admixture of smoky exhalation. The answer is that admixture does not occur because rivers are always streaming. Moreover, the smoky exhalation always moves to the deepest places, and these are occupied by sea. Aristotle adduces six proofs, says Olympiodorus, for his claim that the saltness of the sea is due to admixture of some undigested substance: (1) All other saltness arises as an undigested residue in a process of heating or digestion (ashes, urine, sweat). (2) The sea is warm. (3) If no smoky exhalation were mixed with it, the saltness would not remain constant, as it would increase in summer because of the evaporation and decrease in winter because condensed vapour returns. The exhalation that is carried down with the rain, however, makes the saltness constant. The λέξ,ις gives a different interpretation, more in tune with Aristotle's text: when vapour is dissolved from the sea, a small amount of saltness is carried upward with it. This is compensated by the saltness of the smoky exhalation that is mixed with the condensed vapour that returns. The salty vapour that is dissolved turns into fresh water when it condenses. Alexander in his commentary on 358b12 ff. adds the example of people who boil salty water in a kettle. The vapour gathers at the cover and condenses as fresh drinking water.25 (4) The experiment with the waxen jar: the salt is separated by filtering. Also, sailors boil sea water and gather the vapour in a sponge. The water that is squeezed out from it appears to be fresh. (5) A certain volume of sea water is heavier than the same volume of fresh water. (6) Sea water is denser than fresh water. Olympiodorus gives Aristotle's examples to show this.26 We see that Olympiodorus systematizes Aristotle's account: different opinions and their refutation, discussed by Aristotle in scattered places, are presented together; sometimes arguments are added that are not in Aristotle. The systematic description of Aristotle's account is not always in agreement with Aristotle's own intention and sometimes leads to confusion and discrepancies within Olympiodorus' text.

23 This does not agree with Olympiodorus' commentary in the previous paragraph, where it is said that the smoky exhalation makes the water lighter. See in Meteor. 152,21-34. 24 Note that the evaporation of light, sweet parts of the water, adduced as one of the causes of saltness in the previous paragraph, is not mentioned here anymore. 25 Olympiodorus, in Meteor. 162,5-8 and Alexander, in Meteor. 86,1-87,10. 26 Olympiodorus, in Meteor. 156,26-159,23.

3. Ibn al-Bitriq and Hunayn ibn Ishāq Ibn al-Bitriq's account of rivers seems at first, unlike that of Aristotle, to stress their origin from rainwater rather than from subterranean condensation. He says that some people think that rivers (nahr) flow from a reservoir (vessel) until it is empty. His own theory is ("I say") that vapour rises from the earth, condenses into clouds and then changes into water that falls as rain. When this water gathers under the earth, it becomes the substance from which rivers are formed. Therefore rivers contain more water in winter than in summer, and some rivers run dry in summer because their matter has run out due to lack of rain. Then Ibn al-Bitriq says that water runs deep in the earth and that air changes into water, which adds to the water already there. He follows Aristotle's account that the subterranean water gathers until it gushes out and forms a river or spring (yanbū') and that this especially occurs in mountains and high places; he presents the examples of mountains and rivers from Aristotle. He concludes with Aristotle that the sources ('ayn) of rivers are in mountains and high places, not (only) in hollow places beneath the earth, and that the water is supplied by condensation of air into water adding to the already existing subterranean water.27 Probably Ibn al-Bitriq—or the author of the Greek or Syriac treatise he translated—had no intention deviating from Aristotle, but we get the impression of a different doctrine in the first part of the account because Aristotle's exact meaning was lost by an inaccurate rendering or translation of the Greek text. In Hunayn ibn Ishäq's version of the Meteorology the idea of condensation of subterranean vapour is completely lost. His account differs considerably from that of Aristotle. He says that rivers (nahr), sources ('ayn) and wadis (wādin) are caused by rain. If rain falls on soft earth, it penetrates into it, until it comes to solid earth or to mountains into which it cannot penetrate further; there it remains. If much water has gathered this way, it removes the soft earth around it so that it can flow out; this is how a source is formed. If much water is gathered, the source supplies water the whole year round; if less water is gathered, they run dry in summer. Sometimes a source supplies water during many years and then runs dry, either because of lack of rain, or because the rain flows into another direction. If it flows into that direction in a wide bed, the lack of water continues; if it hits on a mountain or high place, the water is checked and returns, and the 27

Ibn al-Bitriq, Meteor. 42,11-46,9.

source goes back to its former condition. Rivers and wadis may also be formed from melting snow on the mountains. Rivers flow higher when much rain has fallen; then more water flows than is supplied by the source only. Sometimes rivers flow higher when other rivers flow into it, which in turn dry up. The water is fresh in accordance with the place where it gathers and over which it flows.28 Ibn al-Bitriq's account of the variation of moistness of certain places follows Aristotle. He gives the example of Egypt that used to be wet, and then became dry. He adds that trees and crops were planted when the earth became dry.29 There is no corresponding chapter in Hunayn ibn Ishāq. In his account of the sea (bahr) and its saltness (mulūha) Ibn al-Bitrlq mostly follows Aristotle. The opinions of others are mentioned: the opinion of the 'theologians' who say that the sea has sources (yanbū' or 'ayn), the opinion of the natural philosophers, who say that the sea is what is left of the water that originally surrounded the earth after evaporation, the opinion that the sea is the sweat (!araq) of the earth and the opinion that salty earth is mixed into the sea, as water that is filtered (saffā) through ashes becomes salty. The opinion of the theologians is refuted by giving a classification of the waters on earth: water is either flowing or stagnant. Flowing water has sources; stagnant water is either the kind of water to which pools (ajama) belong, or the kind of water that flows from a source and collects in places that are artificially constructed. The sea does not have the characteristics of water coming from a source.30 Ibn al-Bitrlq argues against the doctrine that the sun is fed by the vapour that rises from the earth (above p. 124n3), saying that, according to this view, the sun would increase and decrease in accordance with the vapour that is dissolved to a greater or smaller extent.31 Ibn al-Bitriq's description of the cause of the sea's saltness is not very clear, mainly because Aristotle compares the process of evaporation from the sea (chapter 11,2) as well as that of dissolution of dry exhalation from land (11,3) to processes that leave an undigested, salty residue. However, in 11,3—the chapter under discussion now—he says that the former process does not explain the saltness, but the latter does (see above p. 124n4). Olympiodorus says that both processes cooperate (see above pp. 131-132). Ibn al-Bitrlq does not explicitly state that the sea's saltness is due to admixture of dry exhalation. First, Ibn al-Bitrlq says 28 29 30 31

Hunayn, Jawämi' 94-119. Íbn al-Bitriq, Meteor. 47,8-9. ibid. 513-53,10. ibid. 553-10.

that what we find in the gall-bladder and in the urinary bladder is salty and bitter because it is an undigested residue (fadla) of food that was sweet and fresh. This is similar to water that becomes salty when it is filtered through ashes. The cause of the sea's saltness and bitterness is that there are two exhalations, a moist and a dry one. The light moistness is dissolved from the sea by the sun, whereas the thick, warm part is left as an undigested residue. If heat is mixed with it, it becomes salty water.32 If the heat further increases, the water becomes bitter, like sweat and urine that are salty and become bitter if the heat in them increases. For the same reason, what is left in a process of combustion is salty. If someone says that the sea becomes salty in a similar way as earth becomes salty because it is burnt, this is nonsense, because the earth is not burnt. However, what happens is something like it. For we see that rain may be salty because the dry exhalation is mixed with the moist exhalation that forms the clouds.33 The confusion whether the sea's saltness is caused by evaporation of light, sweet parts of the water or by admixture of dry exhalation or by both, a confusion that originates from Aristotle's text and is apparent in Olympiodorus, is increased in Ibn al-Bitriq's text by a possible confusion of words. The resulting view is that moist, light exhalation is evaporated from the sea; what is left is a residue in which dry exhalation is mixed; it is not specified how and from where this mixing occurs. When this dry exhalation in the water is heated, it becomes salty. We shall find this view again in Ibn Rusd's Middle Commentary, (below p. 154). In Hunayn ibn Ishâq's Compendium the dry exhalation as a cause of the saltness of the sea is not mentioned at all. The sea is salty because the light, fresh parts are evaporated, whereas the salty and bitter parts are left behind. The moistness becomes salty when heat is mixed with it. If the heat increases it becomes bitter, like sweat and urine become salty when they are affected by heat.34

32

This statement might be due to a wrong version of 358a13: θ ΰ γ ά ρ 0ÍV μ η the verbs κρατέω and κεράννυμι (aor. pass, έκράθην) could have been confused. Hunayn has adopted this phrase, see below. If one adopts the hypothesis about the relation between the works of Ibn al-Bitriq and Hunayn such as expounded in the introduction (above p. 9), then this confusion already existed in the Greek treatise on which these works are based. 33 Ibn al-Bitriq, Meteor. 593-60,10. In contradiction to what Petraitis says at 60nl, the account of the sea continues at 61,3-63,2. 34 Hunayn, Jawämi' 122-138. The lines 136-137 are similar in content to Ibn al-Bitriq 59,9-603

κράτηση τό θερμόν;

4.

Pseudo-Olympiodorus

About rivers Pseudo-Olympiodorus says that the supply of water that feeds them is contained in the depths of the earth. Part of it actually exists,35 another part comes to be. Rivers not only rise from the earth, they also penetrate into it. If the flow of a river to the sea is prevented by an obstacle, it forcibly digs a route under the earth. If much water has gathered under the earth it forces a way out and appears as a river. This also explains why there is fresh water in places in the middle of the sea.36 That not all the water is actually present under the earth, but also comes to be there, is shown by the following: (1) A hollow space containing all the water that flows in all rivers to the sea in one year would have to be equal in size as or larger than the earth itself. (2) Just as vapour condenses and becomes water above the earth when the heat flees from the cold, the same must occur under the earth: the heat flees from the cold towards the centre of the earth, dissolving vapour from there that rises and condenses when it reaches the cold surface of the earth. (3) Those who dig the earth looking for water do not find water that has already gathered, but they find moist places where water is formed and slowly gathers drop by drop. (4) Rivers mostly flow from high mountains; these are not the hollow places (tajwīfāt) suitable to receive rainwater and because of the denseness of their material they are not suitable to retain the water. They are suitable for the generation of water because of their cold.37 The earth, like the other elements, does not perish as a whole, but parts may change into other elements. Thus, places that used to be moist because of the presence of a lake or river may be dry now because the water has run out, and vice versa. Indications for this are (1) rivers appearing from the earth and disappearing into it. The place where they disappear becomes dry after having been wet and the place where they appear becomes wet after having been dry. (2) Places that used to be sea become dry land and land is flooded by sea. If a river flows into the sea, it carries heavy particles that become earth where it enters the sea, so that land is formed there by silting (intamara). This 35 That there is water 'actually' existing under the earth is said by Olympiodorus, e.g. in Meteor. 99,12.

«

The previous four phrases follow Olympiodorus, in Meteor. 99,35-100,5. Pseudo-Olympiodorus, Tafsir 102,11-103,8. The last argument follows Olympiodorus, in Meteor. 103,24-30. Note that Olympiodorus says, with Aristotle, that the mountains are suitable to receive water—because of their cavities and clefts (σηραγγώδης is rendered into Arabic as having tajwifāt)—and to retain it—because of their stony material. In the Arabic version the word 'not' is inserted twice, so that just the opposite is stated. Maybe the author of the Arabic survey thought that this would improve the logic of the account which intends to show that rivers do not arise from rain water gathered in a reservoir. 37

means that the water of the sea has to move to another place and that place is flooded. If the river stops to flow and to deposit land, the place is flooded again by the sea and the other place that was flooded becomes dry. An example is Egypt that used to be sea, for shells are found in its deeper parts. (3) Rivers come to be in times of a flood (tūfān), and perish again in times of great drought (harīq). These effects occur in a cycle with a long period, therefore we are seldom able to observe them ourselves.38 The account of Pseudo-Olympiodorus follows Aristotle rather closely. We have seen that several passages follow Olympiodorus. The argument that shells are found in Egypt is also found in Olympiodorus. PseudoOlympiodorus does not mention the terms 'great winter' and 'great summer', but he says that a flood (túfān) is followed by a great drought (conflagration - harīq). Pseudo-Olympiodorus' account of the sea contains 18 paragraphs. It does not follow the order of Aristotle's account. (1) The sea is one of the elements of the world, so either the world and the sea are both generated or they are both eternal. Democritus says that the world is eternal, but that the sea is generated and will perish, because he sees that parts of the sea become dry. His view is not correct, because he concludes something about the whole on the basis of a part. Aristotle's view is that the sea does not perish as a whole, (2) for the moistness that the sun dissolves from it as vapour again returns to it as rain.39 (3) Refutation of the theory that all water on earth has its source in the depths of the earth at its centre (Plato's theory). (4) The sea is the main body of water. Four arguments are given; they correspond to those given by Olympiodorus (see above p. 130). One of them is that the place of the sea contains salty water as well as fresh water. This is shown by several facts: (5) (a) When sailors need drinking water they heat sea water and have the vapour that rises from it absorbed in a sponge. When they squeeze the sponge fresh water is obtained, (b) When one digs a well on the shore it contains fresh water, (c) The fresh water of rivers flows into it. (d) A waxen jar thrown into the sea attracts fresh water to its inside. These arguments are all given by Olympiodorus in various places in his commentary (see above pp. 131, 132 and 133); they are not all in Aristotle. (6) Water may be divided into different classes as follows: It is either enclosed within the earth or emerges at its surface. The former kind is the water of wells (bi'r). Water that emerges is either flowing or stagnant. If there is much 38

Pseudo-Olympiodorus, Tafsir 103,10-104,21. This is the main argument against Democritus. Pseudo-Olympiodorus gives some other arguments; this paragraph follows Olympiodorus 143,26-144,7. 39

flowing water, it is called a river (nahr), if there is little, a rivulet (sāqiya). Stagnant water exists either by itself or get its supply from sources. The former kind is called a lake (buhayra) if it is large, a pool (.ajama) if it is small. Water of the latter kind either comes down from a source, such as water in a dug well, or emerges by itself, as occurs with an earthquake. This division is more extensive than those given by Aristotle and Olympiodorus (see above pp. 123 and 129), but resembles that of Olympiodorus rather than that of Aristotle. This division is given in order to show that (7) the sea has no sources Cayn or yanbü'}, the argumentation follows Aristotle. (8) The sea does not flow like water that flows from sources. Still, we observe that some seas flow because of three causes, (a) The sea will become less deep where rivers flow into it because of the clay carried by these rivers, and the sea will flow from shallow places to deeper ones, (b) In straits the sea is narrowed by the land, so that its flows faster, (c) The wind moves the water in straits, causing waves; they collide with the water in the sea and are pushed back because they cannot set that huge amount of water in motion. Other seas do not flow because of the absence of one of these causes, or because they are in lower places of the earth, and therefore are shallow because other seas and rivers flow into them carrying their silt. Moreover, there is less wind in such low places than in high mountainous areas.40 (9) Due to a river's small surface there is not much evaporation of water. Therefore rivers decrease in summer, but do not run dry. In winter they increase again because their water returns as rain. Due to a sea's large surface, there is much evaporation. Therefore the sea does not increase, although many rivers flow into it. The sea does not decrease either, because water that evaporates in summer returns to it as rain in winter. (10) Vapour that is dissolved from the moisture on earth is not used as nourishment for the sun. The eight arguments that were brought forward by Olympiodorus (see above p. 124n3) are mentioned here. (11) Saltness arises in water when something is mixed into it. If food that is not ripe or not well boiled (undigested) is mixed into fluid, it becomes salty. Therefore urine and sweat are salty. Sweat is less salty than urine, because it remains in the body for a longer time and is longer subject to the process of digestion. Saltness in the atmosphere comes about when smoky exhalation is mixed with water. There are two kinds of smoky exhalation: a light one that rises and a dense one that descends. The former kind is mixed with rain that descends from above. Therefore

40

This survey of why the sea does or does not flow is more extensive than in Aristotle and follows Olympiodorus, in Meteor. 128,14-129,5.

we find that rain is salty at times and in places in which much smoky exhalation is dissolved, such as in hot countries and in autumn. The latter kind is mixed with water inside the earth—therefore we find wells that contain salty water—and with water at the surface of the earth—therefore we find that the sea is salty. Rivers are not salty, because exhalation is not mixed with flowing water.41 (12) Salt water has three characteristic properties: it is dense, heavy and corroding. An indication that it is denser and heavier than fresh water is that a needle floats in salty water and sinks in fresh water. Ships can be loaded more heavily on sea than on rivers. An indication of its corroding influence is that it may cause a wound or take the scab off a wound. (13) In this paragraph Pseudo-Olympiodorus enumerates and refutes four opinions on the saltness of the sea; he combines the arguments given by Olympiodorus in his commentary on 11,1 and on 11,3 (see above pp. 128-129 and 131). (14) Hot springs (hamma) arise when a strong heat burns the earth and water emerges from it. The earth is turned into something like ashes. Therefore a hot spring is salty. Different tastes of hot springs, such as the taste of sulphur, borax and alum,42 arise in accordance with different parts of earth that are boiled and mixed in them and with different amounts of heat that boil the earth. (15) The sea must be salty because it ensures the permanence of its existence, for salty things keep longer. Therefore Egyptians salt the things that they want to keep from putrefaction. The saltness of the sea is due to two causes, (a) The most important cause is the smoky exhalation that is added to it. This does not occur by mixing (mizāj), but by juxtaposition (tajāwur), otherwise the saltness could never be separated. That saltness may be separated appears from the phenomenon that when sea water is heated and the vapour is collected in a sponge, fresh water is gathered when the sponge is squeezed out. Also, fresh water penetrates into a waxen sphere that is thrown into the sea, and in a pit that is dug on the shore.43 (b) The second cause is the admixture of earthy matter, like ashes that are mixed into fresh water make it salty. (16) That the saltness of the sea is caused by admixture of dry exhalation is shown by the following indications, (a) The sea is cold by nature, but potentially hot; a body that approaches the sea is warmed; it lights fire rather than extinguishes it. (b) If the weather is hot, one sees that smoke is dissolved from it and the water becomes fresher. Remember also the phenomena mentioned in the previous paragraph, (c) Fresh water is 41

The account differs from Aristotle, who does not mention two kinds of smoky exhalation. It follows instead Olympiodorus, see in Meteor. 157,12-26, above pp. 132-133. 42 Only alum is also mentioned by Aristotle, at 359b12. 43 These statements follow Olympiodorus, see above pp. 131, 132 and 133.

always added to the sea by the rivers and the rain, still the sea does not become fresh. This means that something else is always mixed in it, viz. smoky exhalation, (d) The heaviness of sea water, which appears from the fact that ships may be loaded more heavily on sea than on rivers, can only be explained by admixture of some earthy substance, (e) The same holds for the density of sea water.44 (17) Indications that the saltness of the sea is caused by admixture of earthy matter are the Dead Sea, hot springs, and the different tastes of sources.45 (18) More indications that the saltness of the sea is caused by admixture of earthy matter are the spring in Chaonia and the burning of reeds by people in Umbria. Smoky exhalation is dense by nature; yet it does not make water turbid. If it were the earthy bodies themselves that were mixed with water, the water would become turbid, as we see in rivers that carry much clay and mud. However, when smoky exhalation is mixed in the sea, the earthy bodies sink to the bottom, whereas it is their taste that is mixed with the water. Therefore it remains clear 46 The account of Pseudo-Olympiodorus is a systematization of the commentary of Olympiodorus, with reordering and additions. PseudoOlympiodorus adduces as cause of the sea's saltness smoky exhalation carrying earthy matter, like Aristotle; also earthy matter by itself contributes to the saltness. The view that it is only the taste of the earthy particles that is mixed with the water, whereas the particles themselves sink, is neither in Aristotle, nor in the Greek commentators. Apart from that, it contradicts the idea that the saltness can be separated, which played an important part in the explanation of the saltness.

5. Ibn Sīnā Ibn Sīnā deals with rivers in the first treatise of the fifth section of the TabViyyāt from his Kitāb aš-Šifā'·, this treatise deals with 'geological' matters. Ibn Sīnā agrees with Aristotle on the formation of rivers, but his approach to the problem is quite different. In the first chapter of this treatise the formation of mountains is discussed. The Latin translation of this chapter forms the first two chapters of the three that were often added to Latin versions of Aristotle's Meteorology under the title De Mineralibus. These chapters were entitled De congelatione et conglutinatione lapidum and De causa montium. The

44 45 46

This paragraph more or less follows Olympiodorus, see above p. 133. These examples are from Aristotle, see above p. 126-127. Pseudo-Olympiodorus, Tafsir 105,4-116,12.

Arabic text has been translated into English by Holmyard.47 The first part deals with the formation of stone, the material of mountains; the second part treats the formation of mountains. We refer to the above-mentioned translation for details on this subject. Here we only remark that one way in which stones are formed is that sticky clay is exposed to heat; then it dries and becomes hard (tafakkur - conglutinatio). The other way is that they are formed from water by solidification (jumūd - congelatio). Heights are either formed (essentially) because wind that arises during earthquakes lifts up pieces of earth or (accidentally) because some parts of the earth are eroded by wind or rivers, while adjacent parts are not. Areas that are now land used to be sea. When the sea receded, the land (clay - tin) was uncovered, exposed to the sun's heat and the clay turned into stone. Rivers and winds eroded the earth over which they moved, creating valleys; the parts that were not eroded became mountains. The second chapter of Ibn Sînâ's geological treatise deals with the advantages of mountains. They play an important part in the formation of clouds, rivers that come from sources, and metals. All these phenomena arise from vapour. Clouds arise from vapour that is carried upwards by heat and then reaches a cold layer of air. Sources arise because a similar process occurs under the earth: vapour within the earth is dissolved by the heat that is confined in the earth by the sun and the stars. The same heat carries it upward to the surface of the earth. Metals also arise from vapour that is enclosed in the earth. If vapour rises from soft soil or from water or the sea, it is dispersed and not kept together. If it rises from solid soil it is confined. Mountains are the most suitable to enclose the heat and to confine the vapour. Vapour gathers in the mountain, until it becomes so much that it forces a way out. It subsequently condenses, turns into water and becomes a source. The mountain acts as a solid alembic, made from iron or glass, that is suitable for distillation. If it were made of thin wood or thin pottery, it would not be able to confine much vapour, and no distillation would occur. If it is made of a solid material, the vapour is not dispersed, but kept inside, so that distillation can occur. Therefore most sources originate in mountains; if they arise in flat land, then it occurs where the earth is solid. The well-known rivers arise from sources in the mountains.48 Ibn Sīnā further shows that clouds and metals are also formed in mountains rather than in other areas. Then, in the third chapter, he 47 48

Holmyard, Mandeville. Ibn Sīnā, as-Šifa, Tab. 5 10,4-11,12.

turns to the different sources of water. He divides water into flowing water from sources ('ayn), stagnant water from sources, wells (bi'r), canals (qanāh) and leakage water (nazz). Flowing water from a source comes about if there is much vapour that is strongly forced to gush forth from the earth and is always followed by more, so that it keeps flowing. Stagnant water from sources comes about when the vapour is driven to the surface of the earth, but is not so strong and abundant that what follows is able to push away what precedes, so that it does not become a flow. Wells and canals are made artificially. If the vapour is not able to break through to the surface of the earth, one removes the weight of earth above it, until the digging has reached the level of the vapour. It becomes a well if no bed is made in which it can stream and if it is not followed by more, otherwise it becomes a canal. Flowing water is better because the motion makes it thinner. On the other hand, it is spoiled by admixture of earthy particles during its flow. Leakage water arises when there is much vapour that does not break through the earth with force. It is slowly driven to the earth's surface, and on its way earthy particles are mixed with it, so that it is spoiled. If a stagnant spring or well is emptied, its water is replaced, for vapour is driven upwards until a certain amount (of water) has gathered. Then a certain weight has been formed that makes it impossible for the vapour under it to push it away. It has become an obstacle, like the earth before the well was dug. If the weight diminishes, the vapour can rise upwards again and remove what covers it from above to a certain extent.49 Ibn Sīnā discusses the sea and its saltness in the fourth section of the TabViyyāt of the Šifā'·, this section is entitled "On action and passion" (or: "On affecting and being affected" - Fī l-afāl wa-l-infi'ālāt). See below p. 311 for the way in which this section fits in with other sections of the TabViyyāt. The chapter on the sea follows upon a chapter in which it was shown that air consists of various layers, due to various admixtures to the element air (see above pp. 55-56). Ibn Sīnā says that water does not consist of various layers, because an admixture is spread through the entire water, as it is less deep and more compact than air. Therefore the sea is salty in its entirety. The water cannot have become salty by itself; it must have happened by admixture of something else, viz. burnt earthy parts that are bitter. An indication is that one may obtain salt from any ashes that are left after burning and from any stone that has been made bitter by calcination (taklīs}. if one boils such a substance in water, then filters the water and lets it boil

49

Ibn Sinā, aš-Šifa,

Tab. 5 13,4-14,10.

further or leaves it in the sun, it solidifies into salt. In this way salt is obtained from alkali (qily), lime (nūra) and ashes and in this way people obtain salt from the ashes of reeds and trees. Admixture of burnt particles is also the cause of the saltness of sweat and urine.50 The sea is not salty because, as some people think, dense water is left behind after evaporation of its lighter parts. Density by itself does not cause saltness, otherwise why would clay not be salty? And why does the sea not become sweet again, since rivers and rain add sweet water to it? Water by itself is homogenous and does not contain light or dense parts. It becomes dense when earthy particles are mixed in it. If it is boiled, then it will finally solidify into salt. If it is distilled or filtered, it becomes sweet: when a waxen sphere is put under salty water, sweet water will filter into it.51 In some places there may be sweet water in the sea; this water is lighter and will evaporate first and turn into clouds. The salty water may also evaporate, but as it is denser, it will not rise far and soon descend as salty rain. It is well known that if salt is boiled in water, salt will rise together with the vapour. Sea water is heavier than other water, due to its admixture of earth; eggs will not sink in it; in the Dead Sea nothing sinks and nothing can live in it. In certain places there are sources of sweet water under the sea. Empedocles has said that the sea is salty because it is the sweat of the earth. Although this is only a poetical saying, it may be interpreted in the way that sweat is moistness from the body that is salty due to admixture of burnt particles and the sea is salty due to something similar.52 The saltness of the sea has a purpose: it keeps the water from turning bad. If it were not for its saltness, the sea water would turn bad and bad water would spread all over the earth. The hot nature of the sea water lights fire rather than extinguishes it. It strongly affects what is washed in it and corrodes it.53 The sea does not necessarily always occupy the same place. In fact, the sea changes its place, but this occurs so slowly that people have never recorded it. The effects are reported through the ages only in tiny fingers of land and small islands. The sea continues to exist because of the supply of water from rivers and sources. The water of the sea does not come from sources under the sea; if that were the case, their number would have to be very large and they would have 50

Ibn Sīnā, aš-Šifa, Tab. 4 205,4-206,3. ibid. 206,4-17. 52 ibid. 207,1-208,1. 53 ibid. 208,2-10. These features are derived from Pseudo-Olympiodorus, see above p. 140, nos. 12, 15 and 16a. 51

been discovered by those who sail the sea. The rivers get their supply of water from sources and from precipitation. It is only in a certain season that precipitation contributes to the water supply. These supplies do not always remain in the same condition at a certain spot. A source may dry up and rain may be absent in a period of drought. Then the rivers dry up. On the other hand, rivers may overflow and take parts of the earth with them. If the supply of water for rivers is interrupted, then the sea will dry up where rivers used to run into the sea, whereas in another place land will be overflown by the sea. This means that the sea has changed its place. It is possible that a gulf is formed when the barrier between the sea and lowland is destroyed. It is known that the area around Najaf, near Kūfa, used to be sea, and the same is said about Egypt, for remains of sea animals have been found there.54 Such changes of the sea have not been recorded, no more than similar changes of mountains. Mountains disintegrate, but are also formed, but this takes such a long time that it is has not been recorded. In that time peoples have been wiped out by floods or epidemics or their language and script have changed so that we do not know what they wrote and said, such as is the case with the script found in the pyramids of Egypt.55 The sea is at rest by nature, but it may get into motion because of winds or because the water is pushed from all sides into a strait in which it will move faster; furthermore, when the sea hits upon the shore it will rebounce and move to a deeper place. Motion of the sea may also occur because rivers flow into it and cause a surge of the water that will move to deeper places. The most prominent reason of the sea's motion is that some parts of it are higher than others. It is said that the sea within the Pillars of Hercules flows because it is shallow and contains narrow places and because many rivers flow into it, whereas the opposite is true for the sea outside the Pillars: it is large, deep and not many rivers flow into it.56 Ibn Sînâ's account of the sea contains many features of Aristotle and some of Pseudo-Olympiodorus. The saltness is caused by admixture of burnt earthy particles. Ibn Sīnā does not specify how this occurs and does not mention dry exhalation.

54

Shells in Egypt are mentioned by Pseudo-Olympiodorus and Olympiodorus, see above pp. 138 and 128. 55 Ibn Sīnā, aš-Šifa, Tab. 4 208,11-210,6. 56 ibid. 210,7-18.

6. School of Ibn Sīnā Bahmanyār devotes a few lines to the formation of stones and mountains in which he follows Ibn Sînâ's treatise in the Šifā'.51 His text on flowing and stagnant sources, and sources that are formed in mountains hardly differs from that in the Šifā'.5S Abū 1-Barakāt discusses mountains, seas, rivers and sources in one of the chapters of the third part of his Kitäb al-Mu'tabar, which is the part in which he deals with the subject-matter of De Generatione et Corruptione. We shall see that he rejects the Aristotelian theory of subterranean condensation of moist exhalation. All water of rivers and sources originates from precipitation in the atmosphere. First, it is explained how stones and mountains are formed: water that surrounds earth is moved by wind and the earth at the bottom is moved along with this motion; the result is that earthy particles are mixed with the water. When people want to make stones for building they throw date pits in streaming water. Earthy particles get attached to each of these pits; they gradually increase in size until a large stone is formed. In the same way earthy particles stick together at the bottom of the water due to the motion occurring there; in the course of time they form an ever-increasing mass until it appears above the surface of the water as a large mountain. Celestial influences and the motion of wind cause the water to dry up, so that these mountains are uncovered; they also cause lakes and swamps to dry up, whereas the earth gets covered with water in other places. This is what has occurred in the country around Najaf, for traces have been found indicating that it was bordered by water not long ago. Similarly, mountains are formed in seas and wherever there is plenty of water; on the other hand they are destroyed by rains and streams that take soil and stones from them downward, by the sun that dissolves dust and by the wind that takes away soil and humps of earth, and in this way elevated places again become lowland, lakes and seas. Due to the cyclical motion of the heaven that causes these effects, the mountains appear as if they were built from different things in layers above one another.59 Some rivers flow because of rain that falls in elevated places and on mountains; they stop flowing shortly after the rain has stopped. Others flow because of snow that melts high in the mountains and keep flowing as long as snow is present there. Others flow because of rain or snow that falls in low places between the mountains and is retained 57

Bahmanyār, at-Tahsīl 718,5-8. Bahmanyār, at-Tahsïl 716,4-5, 716,6-11 and 716,12-717,11 are similar in text to, respectively, Ibn Sīnā, aš-Šifā', Tab. 5 13,6-7, 14,5-10 and 10,8-11,8. 59 Abü 1-Barakāt, al-Mu'tabar II 208,18-209,19. 58

there; the water filters through the lowest and least dense places and gradually gathers until it becomes a river that flows during the whole year. Water may gather in hollow places of the earth in such quantities that it bursts forth as a spring. All these effects increase or decrease in accordance with the quantity of precipitation. Most philosophers claim that sources and rivers arise because air that is confined inside mountains cools, changes into water and becomes a stream. They cannot explain why springs dry up and rivers stop flowing when there is little rain or snow and why they increase when rain and snow increase. Abū 1-Barakāt relates that someone showed him that the wells in the meadows of Hamadān had more water when the air was cold, before rain had fallen, whereas they dried up in hot weather. He said that this occurred because much air changes into water when it is cold, whereas this process does not occur when it is hot. What else could cause the occurrence of water in wells if it had not yet rained? Abū 1-Barakät answered that water filters through the mountains to the meadows below, stays there and rises to the surface of the earth for some time. In a hot season the water in the wells decreases because of evaporation due of the heat of the sun and the longer days; what evaporates is more than the supply that arrives by filtering from the mountains. If the air becomes cold, this evaporation becomes less and the water in the wells increases again. An indication that this is what occurs is that rivers do not behave in the same way as wells in the meadows: they do not increase when it becomes cold, but only when there is rain or snow. Water exposed to the sun is gradually evaporated, but this does not affect streams because they have a continuous supply of water. The rate of evaporation increases when the surface of the water increases, but if many rivers flow into the water, the total quantity of water may remain the same, such as is the case for the sea. If the evaporated quantity is not replaced by water flowing in, the water will dry up. Rivers increase during times of rain and of melting snow, whether they directly arise from rain or from water that filters through the earth and bursts forth from it; they do not increase when it is cold while there is no rain or snow.60 The water of wells also originates from rain and snow that filters from higher places to empty hollow places in the earth. Those who dig for wells find them in certain places of the earth, either low-lying or higher up, not in stony places, but rather in sandy and clayey ones. Wells penetrate into deep cavities and this made people believe that the place of water as an element is under the earth. However, this is not 60

Abū 1-Barakāt, al-Mu'tabar

II 209,19-211,13.

the case, for one may find wells if one digs in more elevated places, whereas one does not find water if one descends and digs in a lower place (near the other, higher place). If the water of wells was the element water that is supposed to be under the earth, the level of this water in relation to the earth's surface would be equal everywhere. Sometimes, when one is digging in a mountainous area, one encounters water, without knowing where it goes and where it emerges; then such water flows from one deep cavity to another, and the next cavity may be in a much lower place than the former one. Those who believe in the change of subterranean air into water say that if one digs a well and finds no water, then after some time one will find water in it, because air will condense into water, which will seep through and gather in the well. Their theory cannot be correct because it does not explain why wells dry up in summer and become full in winter when rain is plentiful. The bottom of the well is cooler in summer than in winter, which means that, according to their view, it would be full of water in summer rather than in winter.61 The water of the sea is the element water and the land is, as it were, an island in it. The cause of the formation of islands is the same as that for mountains. The sea is salty because of the heat of the sun and the motion of the wind that stirs the water; this causes an admixture of earthy particles in it. The different tastes of water from wells, such as those of salt, vitriol, alum, iron, copper and sulphur, are due to the mixture of different soils into their water. Pure water has no taste. Flowing water is sweet because its flowing makes it thin and therefore it is not influenced by the sun. Something may be influenced only when it is at rest, because being influenced needs some time; if it is moving there is no time available for it to be influenced. Most rivers flow from north to south because they flow from snowy mountains and cold areas with much rain to lower areas and to the nearest and lowest seas.62 Fakr ad-DIn follows Ibn Sīnā on the formation of stones and mountains and the advantages of mountains. His text on sources resembles the corresponding text in the Šifā'.63 He adds that people disagree on whether the water of sources is formed from watery particles that are gathered in the depths of the earth, or from air that has changed into water. Fakr ad-DIn claims that the former way is what mostly occurs, although the latter is possible too.64 61

Abü 1-Barakāt, al-Mu'tabar ibid. II 212,8-21. 63 Fakr ad-Din, al-Mabähit_ Sīnâ, aš-Sifà', Tab. 5 13,6-16 and 63 Fakr ad-Din, al-Mabāhit_ II

II 21143-212,8.

62

II 204,19-205,8 and 205,9-12 are similar in text to Ibn 14,5-10, respectively. 205,13-15.

Fakr ad-DIn discusses the sea in the chapter that deals with subjects from De Generatione et Corruptione. He deals with the saltness of the sea, its being heavier than fresh water, the fact that the sea is not always in the same place and the motion of the sea. He states that saltness does not belong to the nature of water, otherwise all water would be salty. For the rest his account is a selection from Ibn Sînâ's text in the Š//â'.65

7. Ibn Rušd Ibn Rusd's account of the rivers in his Short Commentary is Aristotelian; the explanation of how rivers are formed in mountains is more detailed than in Aristotle. He says that the water that exists on the earth may be divided into water under the earth's surface and water above it. In both cases the water may be flowing or stagnant. Stagnant water often arises from rain water if the place where it falls is able to retain it because of the solidity of its material, as if it were a cistern (.sihrij). This kind of water may be formed from air under the earth, in the same way as it is formed above the earth. Such water remains stagnant when it is only weakly driven away from its place of formation. Flowing water mostly arises when there is a continuous generation of water. This occurs in the case of big rivers; they keep flowing for a time longer than human history, therefore it is impossible that in the earth there actually exists water that is sufficient for the flowing of all these rivers for such a long time or even for their flowing from one winter to the next. Such an amount of water would take a space larger than the earth, or if the required space were smaller, the earth would sink. However, it is possible that there are places in the earth that due to their large size contribute to the continuous flowing of these rivers. Places suitable for this are the mountains. Therefore big rivers rise from the mountains. Mountains have many special properties that make them suitable for this. They are moister and colder than other places because they are high and close to the place where rain is formed. Furthermore, because of their density the moistness in them is not dispersed; the cold that always surrounds them keeps their interior warm, as occurs in the bodies of animals in cold weather. This heat evaporates the moistness that is inside the mountains and changes it into hot air that rises upwards. There it

65

Fakr ad-Din, al-Mabāhil II 142,1-143,9 is similar in text to Ibn Sīnā, aš-Šifā', Tab. 4 206,15-17, 205,10-206,2, 206,11-13, 208,2-3, 207,11-13, 208,11-14, 209,2-10 and 210,7-10.

changes into water because of the cold, as occurs in baths. This occurs in the hollow spaces of these mountains. It is the same process as the process of distillation in an alembic.66 Once the amount of water has become large, it pours forth from the mountain as a river. Flowing water may also arise from rainwater, viz. the rivers that flow in winter and then dry up. Some rivers flow because of both causes.67 Ibn Rušd discusses the processes of sea becoming land and land becoming sea after his account of the sea. He says that it is clear that such processes must occur because each element partly changes into other elements. Furthermore, it appears from the fact that shells are found in lowlands, for instance in Egypt and also in Andalusia ("our country"). No records of such occurrences exist because they take place slowly over a long period of time. If they were written down, the records have perished, or they were written in a script that nobody can read, such as that in the pyramids of Egypt.68 Also, peoples who have witnessed such events have perished. The proximate cause of such processes is the presence of rivers and sources. If some part of the earth is moist, rivers will arise and flow to the lower places, where they will flood the land so that it becomes sea, and the reverse will occur if the rivers become dry. Also, sea may become land because of silt carried by rivers; then the sea will flood another part of the land. The remote cause of such processes is the sun's motion in the ecliptic that causes generation and corruption, in accordance with its appoaching and receding. Egypt, for instance, used to be sea, then it dried and now it becomes more dry until it will finally be devastated.69 Ibn Rušd begins his account of the sea by saying that the sea is the element water. Each element has its main body to which all parts move; for water this can only be the sea. The water of all rivers originates from it by means of the rain, and returns to it. It remains in the same state and neither increases nor decreases. The sea does not originate from sources, otherwise water either would dominate in all parts of the earth or the rivers would stop flowing, because their sources would be in the same places as the sources of the sea. Also, almost all parts of the earth would be flooded. The sea, like other elements, is eternal as a species, although parts of it may be generated and corrupted.70

66 67 68 69 70

Ibn Sīnā also compares a mountain with an alembic, see above p. 142. Ibn Rušd, Short Commentary 25,16-27,12. Ibn Sinā also gave this example, see above p. 145. Ibn Rušd, Short Commentary 31,15-33,17. ibid. 28,1-17.

The sea's saltness must not be a property that belongs to it as an element, for taste does not belong to elements, but to composed bodies. An indication for this is that vapour that is dissolved from it becomes fresh; therefore rain, that is mostly formed from vapour dissolved from the sea, is fresh. Another indication is that a hollow sphere made of pitch71 that is thrown into the sea gathers fresh water inside itself. All this makes it clear that saltness is something that is mixed into the water of the sea. In general, saltness is caused by admixture of burnt particles into moistness. This may occur in two ways. (1) Those particles are immersed in sweet moistness from the beginning. Then heat affects the mixture and causes evaporation of the moistness. What remains is a salty, burnt residue. It is like what occurs to the food and water that arrives in the belly of an animal. The residue leaves it and goes to the bladder, whereas the sweet part serves as nourishment for the parts of the body. (2) The bitter particles are mixed with the sweet moistness. Those particles may be either earthy, such as occurs in water that flows over burnt earth and in water that is filtered through ashes, or smoky, such occurs in rain that falls in the beginning of the autumn. Such rain is salty, especially in dry years. We must now investigate which of these ways is responsible for the saltness of the sea.72 It is impossible that the sea's saltness is due to the sun dissolving the sweet part until the part in which the earthy particles are mixed is left, for what is dissolved will return again. It is better to say that the sun is responsible for keeping the saltness—no matter whether it is caused by earthy particles or by smoky ones—constant by means of this process. It seems that the sea's saltness is mostly due to the smoky particles, for saltness is a property of all seas and seas exist in almost all parts of the earth; therefore the saltness must be due to something that is common to all parts of the earth; all parts of the earth have in common that smoky exhalation rises from them. Thus, it is this exhalation that is mixed into the water and makes it salty. That the earth is burnt and turned into ashes does not occur in every part of the earth, and if it occurs, it will mostly occur in parts that are not under the water, seldom in parts under the water. Some seas are more salty than others because they are close to burnt earth, or because the earth dissolves more smoky exhalation. Both causes seem to apply to the Dead Sea. No animals live in it because of its extreme saltness and all animals remain floating because many earthy particles are mixed in it. An indication 71

The Latin translation of the Short Commentary and the corresponding place in the Middle Commentary (Arabic) have: wax, like Aristotle and Ibn al-Bitriq; see Ibn Rušd, Short Commentary, Latin translation 426L; Middle Commentary 93,8. 72 Ibn Rušd, Short Commentary 28,18-30,3.

that the particles that are responsible for the sea's saltness are mostly airy, not earthy, is that the sea is clear. Earthy parts would make it turbid.73 The view that the saltness of the sea is mostly due to smoky exhalation, whereas earthy parts may also contribute to it, is also in Pseudo-Olympiodorus, not in Olympiodorus. The argument of turbidity is used in Olympiodorus and Pseudo-Olympiodorus to argue that no earthy parts are mixed into the sea. Shells in Egypt are mentioned by Olympiodorus and Pseudo-Olympiodorus. Furthermore, some features of Ibn Sīnā appear in Ibn Rusd's commentary. The Middle Commentary on rivers follows Aristotle rather than Ibn al-Bitriq; Ibn Rušd adds several remarks of his own, so that the text has become an elaborate paraphrase rather than just a rendering of Aristotle's or Ibn al-Bitriq's text. For instance, in contrast to what Ibn al-Bitriq says at the beginning of his account of rivers, Ibn Rušd makes it clear that not all rivers come from one or more subterrranean reservoirs that are yearly fed by rain. In refuting this view he gives Aristotle's argument (not in Ibn al-Bitriq) that the size of such a reservoir would have to be larger than the earth itself. He gives as additional arguments that these reservoirs would have to be situated in high places and the size of such reservoirs would surpass the size of the mountains; moreover, the existence of such reservoirs would have become apparent for us because the earth above them would have become darker in the course of time.74 Next, Ibn Rušd gives two causes for the formation of rivers. (1) Rain penetrates into the earth, gathers under the earth in suitable places and flows to other places until it emerges to the surface of the earth. Such rivers flow in winter and dry up in summer. (2) Air turns into water in certain subterranean places and the water emerges from the earth. Such rivers flow in summer and winter, but they increase in winter because of the first cause.75 Rivers have their source especially in mountains, because (1) they are the highest places on earth, so that the water that has gathered in them flows down to the surface of the earth. (2) Their material is rare (porous), acting like a sponge that easily collects water. Additional causes are (3) the cold at the top of the mountains and (4) the density of their tops. Because of the last two causes, air within the mountains turns into water at the top. This water collects because of the porosity in the middle of the mountains and it flows down because of their 73 74 75

Ibn Rušd, Short Commentary 303-31,14. Ibn Rušd, Middle Commentary 73,10-74,4. ibid 74,5-19.

height. A fifth cause is the warmth at the bottom of the mountains (Ibn Rušd does not further explain this fifth reason). These causes are absent in valleys and lowland. If one digs for water in lowland, one will find it depending on the proximity of places where water is formed. Water in low places has its origin in high places.76 The account of the variation of moistness on earth and the account of the sea mostly follow Ibn al-Bitrlq and Aristotle, with additional explanatory remarks. In the discussion of the flowing of the sea Ibn Rušd mentions ebb (jazr) and flood (madd) (neither in Ibn al-Bitrlq nor Aristotle). The text here is not clear, probably corrupt. We get the impression that he means to say that some seas are in a lower place than others and are connected with one another. There is one motion in which the water flows from a lower sea to a higher one—that is flood—and when this motion stops, there is another motion in which the water flows back from the higher to the lower sea—that is ebb. See also the section after the next one.77 Ibn Rušd discusses the view that at first water covered the whole earth and the sea is what is left after evaporation by the sun. This cannot be true, because what is evaporated again falls as rain. Even if in some years more water is evaporated in summer than returns as rain in winter, or vice versa, then, if one considers a large number of years, the evaporated quantity of water will be equal to the quantity that falls as rain. After a couple of rainy years there will always follow a period of drought, because in the rainy period more vapour has returned as rain than has been dissolved.78 This is an extensive version of 355a25-9; it is not in Ibn al-Bitrlq. After having shown that the place of the sea is the proper, natural place of water, Ibn Rušd says that the sea is the origin and source of all water, that is to say, not any sea, but the so-called Ocean (bahr al-muhīt), which is the largest and deepest sea and contains most water. From this sea all rivers and seas originate. The rivers originate from it because the evaporated water from the Ocean returns as rain. The seas originate from it because water flows from it to all seas and all seas flow into it: the seas that are in a higher place flow into it because the Ocean is lower; the Ocean flows into these seas because its water is pushed by the upward motion this water has by itself and which is comparable to the motion of wind; this motion is called flood. The situation is contrary for the seas that are lower than the Ocean. Ibn Rušd remarks that the upward motion of water is due to the wind that 76 77 78

Ibn Rušd, Middle ibid. 84,5-9. ibid. 84,12-85,12.

Commentary

75,8-15 and 76,13-77,6.

arises under the influence of the heat of the moon. When the upward motion has stopped, the contrary motion sets in.79 This is Ibn Rusd's own interpretation of Ibn al- Bitriq's passage on Plato's theory that the sea in the Tartarus is the source of all rivers and seas.80 Confusion already exists in Ibn al-Bitriq, for he says that this theory is impossible and then expounds the theory as if it were Aristotle's doctrine. Ibn Rušd increases the confusion by replacing the Ocean for the Tartarus. The result is this theory of ebb and flood.81 Ibn Rusd's account of the refutation of those who say that the sea is salty because it is the sweat of the earth is more complete than the rather confused account of Ibn al-Bitrlq. It does not completely agree with Aristotle. Ibn Rušd says that these people do not specify how something salty comes out of the earth, whether it is because sweetness has disappeared by heat or drought, or because something burnt has been mixed with it, as when water is filtered through ashes. Both processes occur in animals. Sweat is a residue that has become salty because the sweet part of the food has gone to the parts of the body; what was left is salty. Urine is salty because of admixture of a salty, burnt residue from digestion with the sweet liquids that were drunk.82 Neither process applies to the earth, for the earth is not something that feeds and secretes salty residues. There is some salt in the earth, but this is not sufficient to explain the saltness of all seas, unless most of the earth were burnt or salty.83 After this the saltness of the sea is explained as follows: Two exhalations are dissolved from the earth: a moist one that is subject to concoction or ripening (nadj) and a dry one that is not subject to it. If heat influences the dry exhalation, it turns salty, for dry things that are subject to heat get burnt and become salty; moist bodies are subject to ripening and that is the cause of sweetness. Therefore the residue of food that is not ripened (digested) and is secreted is salty due to the influence of the heat. If the heat exerts an even stronger influence, it becomes bitter. That dry exhalation mixed with water has this effect appears from the fact that rain falling in autumn is salty, as there is much dry exhalation in that season.84 The rest of Ibn Rusd's account of the sea follows Ibn al-Bitrlq. 79 80

81

Ibn Rušd, Middle Commentary 89,5-18. Ibn al-Bitriq, Meteor. 57,9-58,1. *

See Sezgin VII 254-256 for a survey of al-Kindi's explanation of the tides and some8 2 remarks on the ways scholars in antiquity explained them. The same distinction between the formation of sweat and urine is made by Olympiodorus and Pseudo-Olympiodorus, see above pp. 131-132 and 140, nr. 13. 83 Ibn Rušd, Middle Commentary 91,2-16. 84 ibid. 91,17-92,12.

Like the explanation in Ibn al-Bitrlq, this explanation of the sea's saltness is not very clear. One must suppose that moist, sweet exhalation evaporates from the sea and that dry exhalation is mixed into it (from where and how?). When heat acts on this dry exhalation in the water, it becomes salty. This is rather different from the account in the Short Commentary and from Aristotle.85 It is clear that the Middle Commentary is based on Ibn al-Bitriq's version of the Meteorology, but also that Ibn Rušd deviated from it and added to it from other sources, among which Pseudo-Olympiodorus.

85 See Fontaine, "Why is the sea salty?" 1995, 213-7 on how Ibn Falaqera has tried to combine the explanations from the Short and Middle Commentary.

WINDS

1. Aristotle The account of winds (άνεμος or πνεύμα) that was started in 1,13 is taken up again in 11,4. Of the two exhalations the moist one provides the material for rain; the material for wind is the dry exhalation. Thus, the view that rain and wind are the same substance, viz. air—the wind being air in motion and the rain being condensed air—is false. Just as we do not call every amount of moving water a river, but only when it flows from a source, a volume of moving air is not a wind; a wind, like a river, should have a specific source. Air is composed of moist and dry exhalation. The facts confirm that the former gives rise to rain and the latter to wind: if the moist exhalation dominates in a certain season or area, we have a rainy period or region; if the dry exhalation dominates, the season or region is windy. A region may be mainly wet or dry, while locally the contrary is the case, although the whole region has the same position in relation to the sun. This may be due to a local difference in the constitution of the land or to the fact that one of the exhalations has moved to a neighbouring area (359b27-360b26). Furthermore, rain is generally followed by wind and there is no wind during the fall of rain. For when the rain has stopped, the earth becomes dry; then dry exhalation is dissolved that becomes wind. When the dry exhalation is present, the heat is separated and rises to the upper atmosphere; then the wind ceases, while the vapour is cooled and condensed into water. Water is also formed when the clouds are driven together and the cold is concentrated in them by άντιπερίστασις; then the dry exhalation cools and the winds cease (360b26-361a4). The theory also explains that the prevalent winds are north (βορέας) and south (νότος) winds, for the sun in its annual course approaches the regions of the north and south or recedes from them; it does not pass over them. Moisture rises and clouds are formed in these regions at the sun's approaching and rainfall occurs at its receding; therefore most rain falls in the regions north and south of the tropics. Moisture in the earth contributes to the dissolving of dry exhalation; this appears from the fact that, for example, green, moist wood gives off more smoke. Thus, winds will mostly arise in these regions (361a4-22).

The exhalation rises vertically, yet the winds blow horizontally, for the air as a whole is carried along in a circular motion with the celestial motion. Thus, the motion of the wind has its origin above: we see clouds already moving before we feel the wind ourselves. We may define wind as a quantity of dry exhalation that moves round the earth; the origin of its motion is from above; its material comes from below (361a23-361b1). Wind is formed in a way analogous to that of rivers. Small quantities of exhalation gradually collect; they are still weak in their place of origin, but become stronger as they travel further. Thus, in winter the far north is calm, but the north wind gathers strength as it moves away from it (361b1-8). The sun may prevent as well as support the formation of winds. If the exhalation is small and weak, it is destroyed and dispersed by the greater heat of the sun; the sun may even dry the earth so quickly that no exhalation is dissolved at all. Thus, calm weather generally prevails in the hot season. Extreme cold will also destroy the exhalation. Between the periods of hot and cold weather, calm weather may occur too when there is a lack of dry exhalation (361bl4-30). The Etesian winds (έτησίαι - annually occurring north winds) blow from arctic regions, where much snow and water are present. They blow after the sun has reached the summer solstice, not when it is at the solstice, nor when it is far from it. They blow during the daytime only. When the sun is at the summer solstice, the earth is dried too quickly and no exhalation is formed. When the sun has somewhat receded and its heat has become more moderate, snow and ice are melted in the arctic region and then the dry earth gives off smoke.1 The melting stops at night, therefore the wind also stops at night. The question arises why there is no corresponding south wind that blows after the winter solstice. Aristotle interrupts his account of winds and turns to the question which sections of the earth are inhabitable and which ones are not. He says that in the northern hemisphere the region between the tropic of Cancer and the circle that delimits the circumpolar stars (χό δια παντός φανερόν) is inhabitable and that the other inhabitable region is the corresponding section in the southern hemisphere. Outside these regions 1 Aristotle also says (362a20 ff.) that snow is melted after the sun has reached the summer solstice because the greatest heat occurs not when the sun is at its highest point but some time after it, as the heat needs some time to make itself felt. This contradicts the phrases above (362a2 ff.) in two respects: there it is implied that the heat is greater at the summer solstice and that the melting of snow and the formation of winds start when the greatest heat has subsided. See below pp. 164-165 for the commentary of the Greek commentators.

it is too cold (around the poles) "or too hot (between the tropics). Aristotle means to say that the climatic situation in the southern hemisphere must be similar to that in the northern one. He needs this to conclude that there is a south wind corresponding to our north wind. Aristotle's short account of this question, which in itself is not related to the subject of wind, has stirred up an extensive discussion among later authors. We shall give an account of it in a separate chapter, ν/ζ. Chapter 6. Aristotle continues his examination of the winds, after having concluded that the situation in the southern hemisphere must be similar to that of our hemisphere. Thus, corresponding to our north Etesian wind, they will have a south wind blowing from Antarctic regions after the winter solstice. However, this wind will never reach our region, like our north winds will not reach their region. These winds do not travel much farther than where we live: in the sea south of Libya, east and west winds blow alternately. We do have south winds, but they do not blow from the south pole. They do not blow from the winter tropic either, for there is no corresponding north wind blowing from the summer tropic. The south wind with us must be the wind blowing from the hot tropical regions; no moisture is present there and this wind does not arise due to thawing; yet it is stronger than the north wind because the region from where it blows is larger and more open (361b35-363a19). Besides these winds, Aristotle further mentions the 'white south winds' (362a15) that occur after the winter solstice and do not blow as continuously as the north Etesian winds and thus escape notice, and the 'bird winds' (362a23), that start to blow seventy days after the winter solstice. The bird winds are weaker and less continuous than the Etesian winds since less exhalation is dissolved, as less snow is melted due to the greater distance of the sun. Aristotle does not explicitly state the direction of the bird wind, but his account implies that it is a north wind. As will be seen below, the Greek commentators identify the white south wind and the bird wind, and also identify these winds with the south wind that blows from the summer tropic, despite the fact that the latter wind is said to be stronger than the north wind, whereas the other winds are weaker. Aristotle enumerates the different winds in chapter 11,6. The directions on the horizon and the names of the winds that blow from those directions are as follows, ordered in pairs of opposite directions: equinoctial sunset (Zephyrus) and equinoctial sunrise (Apeliotes), north (Boreas or Aparctias) and south (Notus), sunrise at summer solstice (Caecias) and sunset at winter solstice (Lips), sunrise at winter solstice

(Eurus) and sunset at summer solstice (Argestes, Olympias or Sciron). Two more winds are added: Thrascias and Meses, blowing from directions between Argestes and Aparctias and between Caecias and Aparktias, respectively. These winds have no opposites, unless one wants to consider the local wind Phoenicias, the opposite of Thrascias. The directions of Thrascias and Meses are at an angle with the north that is approximately equal to the distance (in degrees) of the circle that delimits the circumpolar stars to the north pole.2 There are more north winds than south winds because (1) we live nearer to the north and (2) there is more rain and snow in the north, so that more exhalation is dissolved there (363a21-364a13). A more general classification divides the winds into northerly and southerly, according to their temperature. North winds are colder than south winds, and west winds are colder than east winds; therefore west winds are counted among the north winds and east winds among the south winds3 (364al3-27). Aristotle concludes his account with the enumeration of various characteristics of winds as follows: Opposite winds cannot blow simultaneously, for one or the other would be overpowered and cease blowing. Winds that are not opposite may blow simultaneously. Opposite winds generally blow in opposite seasons. For instance, Caecias and winds from more northern regions prevail at the spring equinox; Lips prevails at the autumn equinox; Zephyrus prevails at the summer solstice and Eurus at the winter solstice. Aparctias, Thrascias and Argestes have their origin nearest to us; therefore they blow most frequently and most strongly. They can stop other winds and blow them away, and so they bring fair weather, except if they are very cold; then, if they are not too strong, they may 2 Alexander and Olympiodorus mention Libonotus, between Lips and Notus, as the opposite of Meses; similarly, Phoenicias may also be called Euronotus, as its direction is between Eurus and Notus, see Alexander, in Meteor. 110,7 and Olympiodorus, in Meteor. 186,11-187,1. It should be remarked that the wind-directions, as determined by Aristotle, depend on the latitude of the place, because the directions of sunrise and sunset at the solstices are different for different latitudes. We must suppose that Aristotle determined these directions on the horizon of Athens, where they are at an angle of 30° with the east-west direction. It follows that each wind-direction is at an angle of 30° with its neighbouring direction. See Masselink 60-74 for a further discussion of the determination of wind-directions.

According to Aristotle, east winds are warmer than west winds because during the day the sun heats up the east more than the west. His argument is that in the morning the east is heated and that it is kept hot during the rest of the day by the presence of the sun. In the afternoon the west is heated, but immediately after that the sun disappears. Alexander and Olympiodorus follow this 'lamentable' (Webster) argument, see Alexander, in Meteor. 111,2-25 and Olympiodorus, in Meteor. 194,14-25.

freeze the clouds before driving them away. When a wind ceases, it is followed by its neighbour in the direction of the motion of the sun, for the sun is the cause of its formation. Caecias and Lips, though being opposites, are both wet. Argestes and Eurus are both dry; the latter starts dry, but becomes rainy afterwards. Meses and Aparctias are coldest, and bring the most snow. Aparctias, Thrascias and Argestes bring hail. Notus, Zephyrus and Eurus bring heat. Caecias brings thick clouds, for it is cold—as it is partly north—and thus collects and freezes vapour, and it carries much vapour—as it is partly east. Lips brings thinner clouds. Aparctias, Thrascias, Argestes and Meses often bring lightning, for they are cold and lightning is due to cold that is ejected when the clouds contract. Hurricanes (έκνεφίας) 4 most often occur in autumn and also, less frequently, in spring; they are mostly caused by Aparctias, Thrascias and Argestes because these winds often fall on others, as we saw above, and this causes a hurricane. The Etesian winds start from the north as north winds; when they come in more southern regions they get a more westward direction for people living in the west and a more eastward direction for people in the east (364a27-365a11).

2.

Theophrastus

Some commentators refer to Theophrastus' theories of wind; therefore we give a summary account of them here. For further details we refer to Steinmetz and to Daiber's edition and translation of Theophrastus' Meteorology. 5 Theophrastus' opinions on this subject do not form a consistent theory; this is not surprising when one knows his writings on other subjects. According to his treatise De ventis, the sun in its daily motion from east to west pushes the air sideways to the north and south, where it collects and is compressed. Then the air flows back from there to its original place (άνιαπόδοοις, lit.: giving back, restoring) and this is wind. Thus, wind is motion of air (ή toū αέρος κίνησις άνεμος). This is further explained in Theophrastus' Meteorology. There he says that air is collected and compressed on one side and then flows to the other side, since it is forced by the vacuum to move to that side. He compares this to water being sucked up a tube by the mouth: air is extracted from the tube and water fills the tube due to 4

A storm-wind issuing from a cloud, see below Chapter 8, p. 226. Steinmetz 1964 25-33, 41-46 and 55-60; Theophrastus, Meteorology Daiber's commentary 278. 5

ch. 13 and

the force of the vacuum. From De ventis it appears that it is this mechanism of collection of air in a certain place and subsequent άνιαπόδοσις that is responsible for the annual cycle of north and south winds. On the other hand, Theophrastus connects, like Aristotle, the rising of the (north Etesian) wind with the melting of snow in the north, when much exhalation is dissolved. In his Meteorology he says that wind is formed from exhalation which is composed of fine and thick parts and in which the fine parts dominate. Wind arises from water and earth. The wind is strong and continuous when exhalation rises in great quantities and continuously. The wind moves either because it tries to reach an upper place, due to its lightness, or, as was mentioned above, because it is compressed on one side and then moves to the other side because of the force of the vacuum. The wind moves horizontally, because the heavy vapour naturally descends, whereas the light exhalation rises; the resulting motion is horizontal. In De ventis it is said that air is cold and vapourous by nature, and consequently moves downward. The sun's heat that is reflected by the earth moves the air upward. The resulting motion is horizontal. We see that Theophrastus gives two causes for the motion of wind. There is άνχαπόδοσις and the force of the vacuum on the one hand, and there is the natural motion of the exhalation which in some way or another is turned into a horizontal motion. Steinmetz and Sersen claim that if a writer uses the expression 'wind is motion of air', this is an indication of his being influenced by Theophrastus.6 However, the view that wind is moving air is also held by others, e.g. Hippocrates 7 and al-Kindl (see below p. 176). We shall see below (p. 176-177) that the expression 'wind is motion of air' in certain Arabic writers indicates an influence by al-Kindl rather than by Theophrastus.

3. The Greek

commentators

Alexander criticizes Aristotle's statement that the horizontal motion of winds occurs because the air as a whole is carried along in a circular motion with the celestial motion. If this were the cause of the horizontal motion of winds, then all winds would blow in one direction, from east to west, whereas we know that winds also blow in other directions. Then he says that, according to Theophrastus, winds have a horizontal motion because they are not just dry, hot exhalation; if that

6 7

Steinmetz 1964 77-80; Sersen 143-159. See Daiber 1975 77.

were the case, they would rise upwards.8 Alexander does not give more information about Theophrastus' view and the problem of horizontal motion of the winds. Olympiodorus' commentary on Aristotle's account of the winds in 11,4 starts with seven proofs that the wind is not moved air, but dry exhalation; five proofs are Aristotle's arguments, two proofs are added by Olympiodorus himself. (1) Different kinds of things have different proximate matter; rain and wind are different kinds of things, thus, when rain is condensed air, wind cannot also be air. (2) The motion of wind and the motion of air are quite different. Wind is a violent and dominating phenomenon, whereas the motion of air is loose and casual. For example, if air is moved when houses are demolished, the motion does not remain, but immediately subsides. If we move air with a fan, the air moves in accordance with the accidental motion of the fan, whereas a wind has a determined motion of its own, like a river. (3) In rainy years there is not much wind, whereas there is much wind in years of drought. If wind were moved air there would be no explanation for this difference, but it is easily explained when one assumes that wind is dry exhalation. Like Aristotle, Olympiodorus remarks that local conditions may cause deviations from the general order. He says that all towns and regions that lie on the same parallel of latitude may not be subject to the same amount of rain or drought. The reason may be a special location of the town or region, for example on the sea, on a river, or near a lake. For instance, in Alexandria the summer is cool, whereas it is hot in Egypt and Libya. A place enclosed by mountains, where no water flows, will not be affected by rain and drought in the same way as other places on the same parallel. (4) We see that wind and rain generate each another and destroy each another; thus, they are contraries and cannot have the same proximate matter. If a heavy rain falls while wind is blowing, the wind will stop because the rain quenches the smoky exhalation; if rain falls and makes the earth wet, smoky exhalation is dissolved; in these ways rain destroys and generates wind. When there is a strong wind and little vapour, the vapour is dispersed and no clouds can be formed; when the wind is moderate and there is much vapour, the wind makes the vapour thinner and dispels the smoky exhalation that is mixed with it; then the vapour cools and condenses to rain; in these ways wind destroys and generates rain. (5) The prevailing winds are north and south winds. This may be explained as follows: The north and south

8

Alexander, in Meteor. 93,32-94,2. The quotation from Theophrastus is included in Theophrastus 1992, 19932, vol. 1 356 no. 186A.

poles always remain cold. Thus, much moisture is present there. Many showers of rain will occur, so that the earth is soaked. This means that much smoky exhalation is dissolved and strong winds are formed. This does not occur in the east and west, because the sun moves over these regions and thus there is not much cold, not much rain, and not much dry exhalation. (6) On the open sea the wind is less strong than near the coast and in bays. The explanation is that in the latter places the smoky exhalation is near, for it is dissolved from the earth. If the wind were air, it would be equally strong on the open sea. (7) If the wind were moved air, it would blow strongest above the mountain tops, because there the air is moved by the circular celestial motion. However, we know that there is no wind on the mountain tops: lines drawn in ashes are not dispersed.9 Next, Olympiodorus discusses why the wind moves horizontally, whereas dry exhalation rises vertically. He quotes Aristotle's answer that the horizontal motion is caused by the circular motion of the air above the mountain tops which is carried along with the celestial motion. He says that Alexander concluded that if this were true, all winds would blow in one direction, from east to west, and that he therefore preferred Theophrastus' solution. Olympiodorus mentions Theophrastus' view, here and previously in his commentary on the beginning of chapter 1,13. Theophrastus says, according to Olympiodorus, that the smoky exhalation is a mixture of fiery and earthy parts. If these parts were separate, they would move in contrary directions, viz. upward and downward. The combination will move horizontally.10 If the parts were combined to form a real mixture, then it would move with the (vertical) motion of the dominating part. In this case, however, the combination is by juxtaposition of the parts. Each part retains its own power to move, respectively, up and down, and when they are equally strong the result will be a horizontal motion.11 Olympiodorus refutes Theophrastus' view as follows: According to Theophrastus, the origin of the motion of the wind is from below; according to Aristotle, its origin is from above. The wind blows most strongly at its origin. Consequently, the wind will be stronger in lowland according to

9

Olympiodorus, in Meteor. 168,21-171,23 and 172,17-29. Olympiodorus misunderstood Theophrastus. According to Theophrastus, wind is a combination of fine and thick exhalation, in which the fine one is dominating. The horizontal motion occurs because the thick one has a downward motion and the fine one has an upward motion. Their combination results in a horizontal motion. See above p. 161 and Steinmetz 1964 41-43. 11 Olympiodorus remarks that the combination of two motions, one upward and the other downward, resulting in a horizontal motion, is also an explanation of the motion of shooting stars (342a24 ff., see above p. 67). 10

Theophrastus, and stronger in the mountains according to Aristotle. We see that the wind is stronger in mountains, therefore Aristotle's view is better. Next, Olympiodorus quotes Ammonius, who saved Aristotle from Alexander's critique. According to Ammonius, Aristotle did not say that all winds move along with the circular motion of the celestial sphere, but that their horizontal motion is caused by the circular motion. When the smoky exhalation rises to the air above the mountains that moves with this circular motion, it hits upon this air, but it cannot penetrate into it because of the fast motion of that air and the greater density of the smoky exhalation. It is thrust back and gets a horizontal motion.12 The Etesian winds start to blow some time after the sun has reached the summer solstice, when the snow in the arctic region melts. Alexander specifies this, saying that they start blowing not much later than twenty days after the summer solstice. Then he says that one may ask why the snow does not yet melt and cause the formation of the winds at the time of the summer solstice, when the sun is nearest. He gives two solutions: (a) When the sun is at the summer solstice, much snow is directly dissolved into moist exhalation and what is left is melted, but the water that arises is quickly evaporated. Thus, the earth does not become moist by these processes and no dry exhalation is dissolved, (b) When the sun is at the summer solstice, its heat is not directly dominating on the cold earth, so that it cannot melt the snow: the snow is preheated only, not yet melted. The melting starts some time after that. Summer heat is fiercer after the summer solstice for the same reason.13 Olympiodorus also says that the Etesian winds start to blow twenty days after the sun has reached the summer solstice. They start to blow when the snow in the arctic region melts; this occurs some time after the sun has reached its highest point. Although the sun causes the most heat when it is at its highest point, the snow does not start to melt immediately because first the air must be heated and this takes some time. When the air has become hot the snow starts to melt.14 Thus, the first reason given by Alexander is an interpretation of Aristotle's statement in 362a2 ff. that when the sun is at the summer solstice it dries the earth too quickly for dry exhalation to be dissolved. Alexander's second reason and the reason adduced by Olympiodorus are the same as that given by Aristotle in 362a20 ff. The contradiction 12

Olympiodorus, in Meteor. 97,5-34 and 17430-175,30. The quotations Theophrastus are included in Theophrastus 1992, 19932, vol. 1 358 no. 186B. 13 Alexander, in Meteor. 97,30-98,13 and 99,15-16. 14 Olympiodorus, in Meteor. 17630-177,13.

from

between both passages from Aristotle (see above p. 157n1) is not solved by the commentators, although Olympiodorus is aware of it.15 One may ask why there are no annual south winds that blow after the winter solstice, corresponding to the Etesian winds. Alexander says that such winds do exist, namely the 'white south winds' that are also called 'bird winds'. But they escape our notice, because they blow weakly and not continuously; they start to blow seventy days after the winter solstice, whereas the corresponding north wind starts twenty days after the summer solstice. These south winds blow from the tropical region near the summer tropic. This means that the sun, when it is at the winter solstice, is further from their place of origin than in the case of the north wind; therefore exhalation is dissolved more slowly and to a smaller extent. As for the south wind that arises in antarctic regions, Alexander follows Aristotle in saying that this wind exists, but does not reach us.16 Olympiodorus says that south winds corresponding to the north Etesian winds do exist in the southern hemisphere, but they do not reach us, because we live too far away from the place where they arise, just as our north Etesian winds do not reach the southern hemisphere. Furthermore, we do have south Etesian winds; they arise seventy days after the winter solstice. They blow lightly and not continuously because little snow has melted, as the air is still cold, and therefore little exhalation is dissolved. The inhabitants of Alexandria call them rose winds because they blow at the time of the roses; Aristotle calls them white south winds because they blow calmly, or bird winds, because when they blow it is a suitable time for birds to lay their eggs, as if no copulation with a male were required. These south winds have their origin at the tropical region near the summer tropic.17 Aristotle said that the south wind that comes from the tropical region near the summer tropic is stronger than the north wind, because the region from where it blows is larger and more open. On the other hand, there are more north winds than south winds. Olympiodorus, commenting on this, says that in the north more exhalation is dissolved after the earth has become moist by the melting of snow. Thus, many north winds arise, but they are not strong, because they blow from moist regions. In the south less exhalation is available, because there the earth is dry. Thus, there are fewer south winds, but they are strong because they blow from dry earth. Dry matter has well-determined parts; the parts of moisture are fluid. Therefore dryness is related to 15 16 17

See Olympiodorus, in Meteor. 181,11-23; there he repeats his account 176,30-177,13. Alexander, in Meteor. 98,21-101,12 and 105,33-107,12. Olympiodorus, in Meteor. 177,13-27, 182,3-183,3 and 192,28-34.

what is forceful and moistness to what is weak. People compare the north wind to a weak man, the south wind to a strong aggressive man. North winds are able to stop south winds, not because they are stronger, but because they are moister and heavier; their moistness destroys the dryness of the south wind.18 We see that Alexander and Olympiodorus not only identify the white south wind as the bird wind, but also identify these winds as the south wind that blows from the summer tropic. This is the south wind that blows 'with us'; it starts to blow seventy days after the sun has reached the winter solstice. They overlook the fact that, according to Aristotle, the south wind from the summer tropic is stronger than the north wind, whereas the white south wind and the bird wind are weak. Olympiodorus determines the first eight wind-directions as points of intersection of the horizon with the meridian and three parallel circles, sc. the equator and the summer and winter tropics; this results in the same points given by Aristotle. He wants to determine the remaining directions, unlike Aristotle, by the points of intersection of the horizon with two more parallel circles, sc. the circles that delimit the circumpolar stars. However, this is not possible because these circles each have only one point in common with the horizon, sc. the north and the south, and they are not new points, for they are the intersections with the meridian. He gives the following solution, adopted from Ammonius. The five above-mentioned circles intersect the meridian at ten points. Together with the north and south pole we get twelve points. When the meridian is moved in such a way that it coincides with the horizon, one gets the twelve wind-directions. If one follows this procedure, the angle between neighbouring winds is different from what results from Aristotle's procedure.19

4. Ibn al-Bitriq and Hunayn ibn Ishāq Ibn al-Bitriq's account has several passages that differ in content from Aristotle's corresponding passages. These differences are due to a misunderstanding of Aristotle's text rather than a difference of opinion. Some of these misunderstandings may already have existed in the Greek or Syriac version of the Meteorology that was translated by Ibn al-Bitriq;

18 19

Olympiodorus, in Meteor. 1873-14 and 19533-39. ibid. 185,8-189,10. See Masselink 60-74 and 80-81 for a further discussion.

others are due to mistranslations.20 We give some examples. Ibn al-Bitrlq treats the question why winds from the north and south are more frequent than those from the east and west (Aristotle 361a4 ff.). He says that the sun does not pass over the north and the south, but approaches and recedes from these areas, and instead passes over the east and the west. Therefore it dissolves more exhalation from the eastern and western regions than from the northern and southern ones. When in the east exhalation is dissolved, it is dense because of its large quantity. When the sun arrives in the west, this exhalation cools because of the remoteness of the sun and changes into water that falls as rain. The same occurs to exhalation that rises in the west. Rain prevents the blowing of wind, because it quenches the dry exhalation, thus winds do not arise to a large extent in the east and the west. The exhalation that is dissolved in the north and the south is less in quantity and density; therefore it does not often change into rain, but it becomes wind before it can change into rain. This explains why winds from the north and south are more frequent than those from the east and west. This is also confirmed by the fact that rains occur more in eastern and western areas than in northern and southern ones, whereas dry exhalation is dissolved more in the northern and southern regions.21 The differences with the corresponding passage from Aristotle are clear. It seems as if Ibn al-Bitriq's account is about the daily motion of the sun, whereas Aristotle's explanation uses the annual motion of the sun in the ecliptic. The difference was noted and expounded by Ibn Rušd in his Middle Commentary, he remarks that Alexander gave an explanation that was quite the opposite of the explanation he found in Aristotle's text (i.e. Ibn al-Bitrlq's version). This indicates that Ibn Rušd used Ibn al-Bitriq's version of the Meteorology and considered it to be Aristotle's text, whereas he knew the 'true' Aristotle from Alexander's commentary (see below p. 191). Ibn Tibbon also expounds the difference between Ibn al-Bitrlq's version and Alexander's commentary.22 Another example is that Ibn al-Bitrlq says that people have asked why north winds blow after the end of the summer season and after the end of the winter season. This is his version of the question asked 20

An example of mistranslation is 69,6: fī wasat al-jibāl (between the mountains) as translation of 361b28: έν TOiîç à\à μέο0V ώροας (in the intervening periods). Ibn Rušd in his Middle Commentary adopts Ibn al-Bitriq's reading and gives some reasons why no wind arises between mountains: because no suitable exhalation rises there, or because moistness is dominating; see Middle Commentary 107,7-10. Also Ibn Tibbon reads fï wasat al-jibāl and notes that Alexander does not mention this issue; see Otot ha-Shamayim II 312. 21 Ibn al-Bitriq, Meteor. 66,5-67,7. 22 Ibn Tibbon, Otot ha-Shamayim II 273-289.

in 362a11 ff. why north (Etesian) winds blow after the summer solstice, whereas there are no corresponding south winds after the winter solstice. Note that the phrase "after the summer solstice" (μετά τάς θερινάς τροπάς) has become "after the end of the summer season" (ba'da nqidā' fasl al-qayz), and similarly for the winter solstice. Ibn al-Bitriq answers the question by saying that when winter has ended it is followed by spring. Then the sun becomes stronger, its heat melts the snow and dry exhalation rises from the earth, so that the north wind starts to blow. After the end of summer the air becomes cold and confines the exhalation within the earth, where its quantity gradually increases. It increases to such an extent that it finally bursts out and rises from the earth, and the coldness of the air is not able to prevent it from rising because it has increased to such a large amount. These are the reasons that in spring and autumn the north wind blows more than other winds.23 Ibn Rušd in his Middle Commentary again remarks that Alexander gave an account that is different from what he found in Aristotle's text (i.e. Ibn al-Bitriq's version; see below pp. 192-193) and also Ibn Tibbon notes the difference between the accounts of Ibn al-Bitrlq and Alexander.24 Hunayn ibn Ishāq has a similar passage25 in which he uses the same formulations and expressions as Ibn al-Bitriq. Ibn al-Bitriq discusses the different winds and their directions as follows: There are twelve winds. The common people distinguish four winds: the north wind (šimāl), the south wind (janūb), the east wind (saban) and the west wind (dabūr). Between the north and east winds are two other winds, one nearer to the north and the other nearer to the east. Between the north and west winds there are two winds, one nearer to the north, the other nearer to the west. Between the south and west winds there are two winds, one nearer to the west, the other nearer to the south. Between the south and east winds there are two winds, one nearer to the south, the other nearer to the east.26 Hunayn ibn Ishāq gives the same number of winds and the same description. The winds between the four main winds are not given special names; they are all referred to as side winds (nakbāV After the enumeration of winds Aristotle asks why there are more

23

Ibn al-Bitriq, Meteor. 70,4-71,5. Ibn Tibbon, Otot ha-Shamayim II 322-326. 25 Hunayn, Jawāmi' 162-168. Ibn al-Bitrîq, Meteor. 72,2-6. The text given by Petraitis should be corrected in the way indicated in Daiber 1975 80. 27 Hunayn, Jawāmi' 168-178. 24

north winds than south winds (364a6 ff.). Ibn al-Bitriq asks in the corresponding paragraph why there are more winds from the north and the south than from other directions. He answers that the reasons are that (1) these are the inhabited regions and that (2) snow and water are present there in large quantities. When the sun is strong, it melts the snow and then dissolves much exhalation from the earth, so that much wind arises.28 Apparently Ibn al-Bitriq refers to the north winds in the northern hemisphere and to the corresponding south winds in the southern hemisphere, whereas Aristotle refers to the same north winds and to the south winds in the northern hemisphere. Ibn al-Bitriq's answer is the same as that given by Aristotle. Hunayn ibn Ishāq asks the same question as Ibn al-Bitriq; he gives as reasons that (1) the sun remains longer in the northern and southern regions than at the equator and therefore has a stronger influence; thus, more exhalation is dissolved there. (2) Moreover, there is more snow and water present in these regions. Hunayn ibn Ishāq explicitly states that he means the north and south regions of the earth, i.e. north and south of the equator (katt al-istiwā'), not north and south in relation to a person standing in a certain place on the earth.29 Several features of Hunayn's Compendium have been treated above. We mention one more passage, corresponding to Aristotle 360b26 ff., where he states that there is no wind during rain, but that wind starts to blow after rain. Hunayn says that wind seldom occurs after rain. Rain makes the earth moist, so that there will not be any dry exhalation rising from it. Thus, there is a calm during and after rain.30

5.

Pseudo-Olympiodorus

The commentary of Pseudo-Olympiodorus again contains several features that confirm that Olympiodorus' commentary was at least one of its sources. He starts with the definition of wind: wind is a large quantity of dry exhalation that rises from the earth and moves over it. The material of wind is not air, but smoky exhalation. This may be proved in various ways. (1) Different kinds of things must have different proximate matter. Air is matter for rain, therefore air cannot also be matter for wind. (2) Air is moist and hot; the heat comes from the smoky exhalation and the moistness from the moist exhalation. Wind is dry and hot; this appears from the lightness of its motion, its 28 29 30

Ibn al-Bitriq, Meteor. 72,7-11. Hunayn, Jawāmi' 189-198. ibid. Jay/ami' 144-146.

force and its violence. Thus, air cannot be its matter. (3) If air is moved by a fan or a falling wall, it stops moving immediately after this occurrence; wind however, persists for a long time. (4) Much wind arises when the year is dry; much dry exhalation is dissolved in dry years; thus, wind must be dry exhalation. (5) If the sun approaches either the north or the south it dissolves exhalation from these regions. When the sun recedes, rain descends and moistens the earth; then much wind is formed. As for the east and west, the sun passes over these regions; less moistness is present there, and less wind arises from there. Thus, it must be the smoky exhalation that is the matter of wind.31 Not all places are subject to the same amount of rain and wind. For towns that lie in different (climatic) zones, this is explained by the fact that different amounts of exhalation arise in different zones. For towns in the same zone, differences occur due to local circumstances; for instance, if a place is under a mountain, it is protected from the wind and little rain is formed because the clouds rise high. If a town is close to water, the wind it receives is cool because it has passed over the water.32 Rain causes the generation of wind, because much dry exhalation rises from moist earth. Wind causes the generation of rain because (1) clouds are moved by wind from other places and are condensed by it, as occurs in hot places where no moist exhalation is dissolved. (2) The hot wind causes the cold in a cloud to flee inside and to concentrate (άντιπερίστασις), so that rain is formed. (3) The wind separates the smoky exhalation from the moist exhalation; then the moist exhalation cools and changes into rain. Rain destroys wind because it quenches dry exhalation. Wind destroys rain because it makes the moist exhalation thinner due to its heat or because it disperses the exhalation due to its motion.33 The previous three paragraphs correspond to the first five arguments adduced by Olympiodorus in proving that wind is not moved air (see above pp. 162-163). Next follows a section on south winds. One may call a south wind the wind that comes from the southern part of the world; this is a cold wind, because it blows from a cold place, south of the tropics. A south wind may also be called the wind that comes from a place south of the region where we live. That region is dry, because the sun passes over it, and it would follow that no wind blows from there. However, it is possible that the north wind drives moist exhalation to that area, so that the earth becomes moist and wind 31 32 33

Pseudo-Olympiodorus, Tafsir ibid. 117,9-17. ibid. 117,20-118,6.

116,14-117,6.

may be formed. The north wind carries exhalations to that place mostly in summer, therefore the south wind blows especially in summer. Only a little smoky exhalation rises from that area, because of the heat that dominates that place, therefore those winds would be weak. Yet much exhalation gathers because of the large extension of that area.34 In the main, this corresponds to Aristotle and Olympiodorus; the statement that moistness is driven to the tropics by north winds is neither in Aristotle, nor in Olympiodorus. Before the discussion of the question why winds move horizontally, it is explained that when moist and dry exhalation rise together, they are combined by means of juxtaposition ('alā l-mujāwara), not mixture ('alā l-mizāj). As moist and dry exhalation by nature move downwards and upwards, respectively, the motion of a combination of both exhalations may be horizontal. This is similar to what Olympiodorus said in his account of Theophrastus' view on the question why the wind moves horizontally (see above p. 163). Pseudo-Olympiodorus gives three possible explanations for the horizontal motion of the wind. (1) The rising smoky exhalation hits upon the air above the mountains that is moved in a circular motion round the earth. It cannot join this air, but it is thrust back, and this results in a horizontal motion. That it does not penetrate into the moving air appears from the fact that there is no wind on the tops of the mountains. The reason is either the fast motion of the air that prevents other things from entering into it, or the density of the smoky exhalation that makes it unable to rise higher. (2) Part of the smoky exhalation is thin and light, part of it is heavy and earthy. The former part by nature moves upward, the latter part downward; the resulting motion of the combination is a horizontal motion. (3) If the smoky exhalation rises and hits upon air containing much moist exhalation, it is thrust back downward. When it meets other exhalation that is rising, they will start moving horizontally together.35 The first explanation is the one given by Olympiodorus as originating from Ammonius, the second one is Theophrastus' view, as it was (incorrectly) interpreted by Olympiodorus; the third one is not in Aristotle or in the Greek commentaries. As for the question whether the wind originates from below or above (Aristotle 360b25 ff.), Pseudo-Olympiodorus gives an answer that is somewhat different from that given by Aristotle and Olympiodorus. He says that winds start to move either from below or from above.

34 35

Pseudo-Olympiodorus, Tafsīr ibid. 118,23-119,23.

118,9-20.

Those from below start weakly and gradually gain force, while those from above announce their presence by the motion of the clouds.36 Next, it is discussed whether the matter of the winds actually exists within the earth, or whether it is gradually generated. For the first assumption the wind should be strong at its origin and decrease in strength afterwards, like water that has gathered and then gushes forth; for the second assumption the opposite must be the case, and that is what we observe.37 This is a systematization of Aristotle 361bl-5. The explanation of the absence of winds in summer and winter follows Aristotle (361bl4-30) and Olympiodorus.38 Then follows a discussion of the annual (Etesian) winds (jiyāh hawliyya)—the north Etesian wind and its counterpart from the south, that does not reach us—along the lines of Aristotle and Olympiodorus. There is a south wind that does reach us; it starts to blow sixty days after the sun has reached the winter solstice. It is weak because not much exhalation is dissolved in the place from where it originates and because that area has a large extension, for if wind is formed over a large area, it will be a weak wind. This may be compared to a house with many windows. If all windows are open and the wind enters the house through them, a weak wind is felt inside, because the places through which it blows in are scattered. If only one window is open, the wind inside is strong. These south winds are called 'white winds' or 'chicken winds', because when they blow, chickens lay eggs without having copulated with a cock.39 Pseudo-Olympiodorus, like Alexander and Olympiodorus, identifies the white wind, the bird wind and the south wind that blows from the tropics. According to Aristotle, the latter is strong because of the extension of its region of origin, whereas the white wind and the bird wind are weak. Pseudo-Olympiodorus states that the south wind from the tropics is weak, so that he avoids the contradiction that would arise from identifying it with the weak bird wind; this contradiction was overlooked by Alexander and Olympiodorus (see above p. 166). From the extension of the region of origin Pseudo-Olympiodorus concludes that the south wind is weak, contrary to what Aristotle concluded. Olympiodorus agrees with Aristotle that the south wind from the tropics is stronger than the north Etesian wind, but he argues that this is due to its dryness rather than to the extension of its place of origin (see above pp. 165-166).

36 37 38 39

Pseudo-Olympiodorus, Tafsir ibid. 120,4-10. See Olympiodorus, in Meteor. Pseudo-Olympiodorus, Tafsir

119,25-120,2. 175,30-176,5. 120,21-121,18.

The Etesian winds do not yet start to blow when the sun has reached the solstice, but some time later. Pseudo-Olympiodorus' explanation agrees with Aristotle and Olympiodorus. He gives two reasons. (1) When the sun is at the solstice, there is still little exhalation dissolved; it is scattered by the heat and does not rise. (2) The bodies are still frozen and dry and the earth has not yet become moist with melted snow. After some time the bodies have become less dense and more soft, so that they are easier affected by the sun. Then the process of the formation of wind starts.40 The directions of the different winds are determined in the following way. First, a number of circles is defined on the celestial sphere. The horizon ( u f q ) is the circle that divides the celestial sphere into the visible and invisible part. The meridian (katt al-zuhrī) is the circle (in a plane perpendicular to that of the horizon) that intersects the horizon in the north and the south. Furthermore one may distinguish five parallel circles: the circles delimiting the circumpolar stars in the north and south hemispheres (a'zam ad-dawā'ir al-abadiyya az-zuhūr wa-a'zam ad-dawā'ir al-abadiyya al-kafā'\ the summer and winter tropics (inqilāb sayfī, inqilāb šatwī), and the equator {da'ira mu'addil an-nahār). One may construct twelve directions by means of these circles and thus there are twelve winds. The meridian intersects the horizon in the north and the south. From these directions two winds blow, the north wind and the south wind. The five parallel circles intersect the horizon at ten points; this gives ten more winds.41 The east wind (saban) and west wind (dabūr) blow from where the equator intersects the horizon. The nis' blows from where the summer tropic intersects the horizon in the east, the mahwa from their intersection in the west. The azyab blows from where the winter tropic intersects the horizon in the east, the hurjüj from their intersection in the west. The mis' blows from where the circle delimiting the circumpolar stars in the north intersects the horizon in the east, the jirbiyā' from their intersection in the east. The nu'ämä blows from where the circle delimiting the circumpolar stars in the southern hemisphere intersects the horizon in the east, the hayf from their intersection in the east. The Greek names are also mentioned.42

40

Pseudo-Olympiodorus, Tafsir 122,3-16. This way of constructing the wind-directions is in fact impossible, because the circle delimiting the circumpolar stars does not intersect with the horizon, as Olympiodorus already saw (see above p. 166). 42 Pseudo-Olympiodorus, Tafsir 125,11-127,9. The Arabic names of the winds are not all well-established in Badawi's edition. We have adopted the names from Fakr ad-Din, al-Mabāhit_ II 196. See also Sersen 46-48. 41

As there are twelve winds, each of them comes from a section that occupies thirty degrees of the horizon. One may also divide the horizon into four sections only: east, west, north and south. Then three winds may be assigned to each section, in such a way that the east wind and its two neighbouring winds are considered as coming from the east section, and so on. One may also divide the winds according to their temperature and distinguish two classes: cold winds and hot winds. North and west winds are cold; south and east winds are hot.43 East winds are warmer than west winds. The reason is that the sun approaches the east quickly and recedes from it slowly, and the east retains the heat it has acquired during the rest of the day, as long as the sun remains above the earth. The sun approaches the west slowly and recedes from it quickly and the west does not retain the warmth, because the sun disappears under the earth when it has reached the west. The east becomes warmer than the west, although the sun is six hours in the east and six hours in the west as well. This situation is comparable to spring and autumn; spring is warmer than autumn, although they are equally distant from summer. It is also comparable to the initial stage of an illness and its recession: when the illness is setting in, it affects the patient more strongly than when the illness is in its final stage, although the former period is equally distant from the beginning as the latter period from the end of the illness.44 Hot winds are more or less hot, depending on the material from which they originate and on the different circumstances under which they blow. For instance, the temperature of the wind depends on the density of the smoky exhalation; if it is rare the wind is hotter. It also depends on the time of the year: summer winds are hotter. Winds that pass over mountains are colder, due to snow or large areas of water. The situation is analogous for cold winds.45 Winds may be opposite or not in different respects. They may be opposite in respect of the direction from which they are blowing. Winds from opposite directions generally do not blow simultaneously, for it is rare for the same exhalation to arise simultaneously in opposite places. If it occurs and opposite winds start to blow simultaneously, then the stronger wind will overpower the weaker and destroy it. Winds may also be opposite in respect of the times at which they blow. Generally, opposite winds blow at opposite times; this is due to the fact that their blowing depends on the sun and the sun is at opposite tropics at opposite times (sc. summer and winter). Winds may also be opposite 43 44 45

Pseudo-Olympiodorus, Tafsir ibid. 128,3-24. ibid. 1293-12

127,11-128,1.

in respect of effect. Opposite winds may cause opposite effects, or the same effect. For instance, the north wind causes fair weather, the south wind rain. Caecias and Lips both bring moistness in the air. Eurus is dry in the beginning, because the sun in the east dries the small amount of vapour that rises; later, the sun dissolves much vapour and then Eurus brings rain.46 The winds succeed one another: each wind is followed by its neighbour in the sense of the sun's motion. Also, a wind is generally followed by its opposite. They may start to blow simultaneously, but subsequently the weaker is overpowered by the stronger. When the stronger ceases, the weaker starts to blow again. An example is the north wind that always overpowers the south wind because of the large amount of matter that is formed in the north and because its place of origin is nearer to us than that of the south wind.47 Winds may be called by the name of what they carry with them: northeast winds are called snow winds, southwest winds hail winds, northeast winds cloud winds and north winds lightning winds.48 Eastern winds generally blow in spring and winter, western winds in autumn and summer. In autumn and summer the east is very hot; it dries the exhalation before it has become a wind, therefore there is no east wind at that time; the west is moderate, and there much exhalation is dissolved from which wind is formed. In spring and winter the situation is reverse.49 A wind is called a hurricane (έκνεφίας - rīh sahābiyya) in three ways. (1) A hurricane is a wind that blows from a cloud. Its power is such that people are swept away, ships are lifted up and trees are uprooted. (2) One calls a hurricane the wind that arises when two opposite winds blow simultaneously; one wind drives the clouds away, the other, stronger wind stops that wind and drives the clouds back again. (3) One also calls a hurricane a strong wind that drives on the clouds and forces them to strike against the air, so that they leap back, like a ball that hits upon a body and rebounds from it. The first kind is a hurricane in the real sense, the others are winds which people imagine to be hurricanes. Finally, Pseudo-Olympiodorus states that Caecias and Lips bring clouds, and that hurricanes mostly blow in autumn and spring.50

46 47 48 49 50

Pseudo-Olympiodorus 129,14-130,15. ibid. 130,17-131,5. ibid. 131,7-11. ibid. 131,12-20. ibid. 131,22-133,5.

6. Al-Kindī Al-Kindl treats wind together with rain in his treatise "On the reason why in some places it almost never rains" (see above pp. 107-108). He gives two descriptions of the formation of wind. Firstly, he says that there is a horizontal motion of exhalation or air from north to south and vice versa, which is caused by expansion due to the sun's heat; this horizontal flow is wind. This flow may be influenced by local conditions. Secondly, there is a vertical motion of exhalation: it rises from the earth to cold layers of the atmosphere, where it is densified; the moist exhalation becomes water (rain and other precipitation), the dry exhalation becomes earth; these earthy particles push the air by their weight and in this way wind is formed. Neither description of the formation of wind is Aristotelian. That wind is flowing air and that its formation is influenced by local conditions reminds one of Theophrastus' doctrines on wind.51 This made Sersen ascribe a Theophrastian influence on al-Kindl.52 However, in fact there is no similarity between the doctrines of al-Kindï and Theophrastus in this matter. Theophrastus does not explicitly mention expansion as a moving cause of the flow of air; he says that under the influence of the sunrays, air is pushed to the north and the south, where it is compressed; then it flows back to the place from where it was driven. Thus, wind flows from where air is compressed to places where the air is less dense. In al-Kindl's theory just the opposite occurs: it flows from where the air is expanded to places where the air becomes denser.

7. Ikwān as-Safā', al-Qazwīnī and others The Ikwān as-Safā', al-Qazwînî, and Najm ad-DIn maintain with alKindl that wind is air which is set into motion by dry exhalation.53 A1-Qazwīnī and a1-Mas'ūdī quote al-Kindl's concept of air flowing from a place where it has expanded to a place where it contracts.54 Ibn Sīnā also mentions this concept, but he does not consider this to be the way a proper wind is formed (see below pp. 177-178). A1-Mas'ūdī mentions the concept of wind as flowing air55 as well as the Aristotelian 51 52 53 54 55

See above pp. 160-161 and Steinmetz 1964 25-33, 43-47. Sersen 145-147. ibid. 148-149. ibid. 149-150. Daiber 1975 76-77.

theory of winds. We conclude that the above-mentioned authors have mentioned or adopted concepts of al-Kindi rather than of Theophrastus, and that the influence of Theophrastus' meteorological theories was not widespread in the Arabic world, in contrast to the claims of Sersen and Steinmetz. 56 The same conclusion was drawn from the discussion of clouds and precipitation by the Ikwān as-Safā' and al-Qazwini (see above p. 111).

8. Ibn Sīnā Ibn Sīnā says that wind is formed from dry, smoky exhalation, like rain is formed from moist exhalation. Wind may be formed in two ways. The way in which it is mostly formed is that much smoky exhalation rises upward and then descends again, either because it is affected by coldness and becomes heavy, or because the circular upper motion of the air prevents it from penetrating in it and thrusts it back downward. The result is a sideward motion, not necessarily in the direction of the moving air. It could also be in another, or even in the opposite direction, depending on the way the exhalation was rising and on the material that joins it. An arrow that hits a moving body may be deflected in the direction of the motion of that body, or in the opposite direction. The sideward deflection may also be due to what is rising under it; this may prevent the exhalation from descending straightaway, and give it a sideward motion. When such a kind of wind is formed, we generally see the clouds moving before the wind itself. If smoke and vapour are rising from furnaces and we see them subsequently descend from the furthest sky, then it is a sign of an oncoming strong wind." Wind may also be formed—less frequently—before the smoky exhalation has reached a cold region or the moving air. This occurs when the rising exhalation is turned in another direction, either because the way up is very winding, or because it hits upon a cold wind that prevents it from rising further, or because it meets other winds. It may also occur that more exhalation joins with it. If it is continuously joined by more exhalation, a strong wind will be formed, especially if it meets a cold wind that prevents it from rising.58 Sometimes wind blows because air is moving; if a certain part of the air is rarefied by the heat of the sun, it expands, and therefore air 56 57 58

Sersen 143-159; Steinmetz 1964 77-80. Ibn Sīnā, aš-Šifa, Tab. 5 58,4-19. ibid. 59,4-11.

flows. However, wind in the proper sense is formed from smoky exhalation. If air were the matter of wind, then it would not blow for an extended period, but only as long as air is moved by something or is rarefied. Sometimes a wind blows while we know that the sun has rarefied a certain part of the air; sometimes a wind blows from a direction contrary to what we expect from the rarefaction of the air by the sun." The matter of wind is not the same as that of rain. This is clear from the fact that they generally exclude one another: in a year when there is much rain because much moist exhalation is formed, there is little wind, and when there is much wind, the year is dry and there is little rain. However, rain often supports the formation of wind because it moistens the earth, which makes it suitable for smoke to rise. On the other hand, rain may cause wind to drop by preventing smoky exhalation to be dissolved. Rain makes the smoky exhalation moist; it becomes heavy and dense and is prevented from rising or from joining more exhalation. Furthermore, wind may support the formation of rain when it drives together the clouds and densifies them, or when it drives the coldness of the cloud to its inside (άνχιπερίστασις), or when it separates the smoky exhalation so that the cloud becomes colder. When the wind is cold, it also supports the cooling of the cloud. Also, wind may dissolve clouds and make their material rarer because of its heat, or disperse clouds because of its motion.60 In general, winds cause fair weather when they start to blow, because they disperse clouds. There is no general rule that connects the different winds with fair weather or rain that is valid for all countries. One can only give such a connection for each separate region.61 Winds that bring clouds are called 'cloud winds' {rīh sahābiyya). The same name may also be used for winds that are separated from a cloud in the direction of the earth (hurricane - έκνεφίας). They emerge from the cloud under pressure and therefore they are so strong that they overturn everything and cause floods. A whirlwind (zawba'a) is mostly formed from a hurricane when it is driven upward and hits upon a cloud.62 Then it is turned into an eddy and thrust downward. Sometimes 59

Ibn Sīnā, aš-Šifa, Tab. 5 59,12-16. ibid. 59,17-60,9; the account of how rain supports and prevents the formation of wind and how wind does the same for rain follows Pseudo-Olympiodorus, Tafsir 117,20-118,6, see above p. 170. 61 Ibn Sīnā, aš-Šifa, Tab. 5 60,10-12. ft) * It is Alexander who says that a whirlwind is formed when wind from a cloud collides with another cloud. According to Olympiodorus, the whirl is formed within the same cloud that produces the wind, and this agrees with Aristotle's text. See below p. 228. 60

its whirl is increased because it has to follow a winding course, like hair that becomes curly because the pores through which it grows are winding.63 Sometimes a whirlwind is formed from the material of wind that has descended and then hits upon the earth. Then it is turned back, meets other wind, and turns into an eddy. When a whirlwind is formed above, we see its whirls both rise and descend; when it is formed below, we see them rise only.64 When wind gets the form of a whirl, this form remains, due to the thickness and density of the material.65 A whirlwind may also arise when two winds meet. When they are strong, trees are uprooted and ships lifted up from the sea. Sometimes they contain part of a cloud; then it looks like a waterspout (tinnīn) flying in the sky.66 There are twelve winds because one may define twelve points on the horizon. There are four groups (east, west, north and south), each containing three points. The east contains the east of the equator, the east of the summer (the place where Cancer rises) and the east of the winter (the place where Capricorn rises). Similarly there are three points in the west, opposite to those in the east. The points in the north and south are the intersections of the horizon with the meridian and with two circles, parallel to the meridian, that are tangent—not intersecting—with the circles delimiting the circumpolar stars in the north and south hemispheres (ad-dā'iratān ad-dā'imatā z-zuhùr wa-l-kafā').67 Ibn Sīnā says that all these winds have Greek and Arabic names which shall not be enumerated. He only mentions the famous ones, the north, south, east and west winds. The others are called side winds (nakbā'). The north and south winds seem to be the prevalent winds because the sun makes the north and south very suitable for the formation of winds.68 Some people make the west wind fall under the northern winds because of its cold, and the east wind under the south winds because of its heat. According to them there are two essential winds: north winds, i.e. the cold winds, and south winds, i.e. the hot winds. When they say 'north', they mean north in relation to our country. That region is cold

63

This example is in Olympiodorus (below p. 228), not in Pseudo-Olympiodorus. However, Olympiodorus compares the way hair becomes curly to the way a whirl arises in a cloud, not to the generation of a whirl along a winding course. 64 Ibn Sīnā follows Pseudo-Olympiodorus, not Olympiodorus, see below pp. 228 and 232. 65 Ibn Sīnā, as-Šifa, Tab. 5 60,12-61,5. 66 ibid. 61,6-8; c f . Sersen 120. ή7

Ibn Sīnā defines the wind-directions in a way similar to that of Pseudo-Olympiodorus 6 8 and corrects the latter's fault, see above p. 173. Ibn Sīnā, aš-Šifā', Tab. 5 61,12-62,2.

because of the mountains and snow; therefore the winds that pass over it towards us are cold. If they could blow further south, they would become hot because they pass over hot countries. South winds are hot, either because they blow from a hot area, or if we suppose them to originate in a cold area, because they have passed over hot countries. Therefore they are turbid, even if they start in a clear area. They also become turbid because vapour is mixed with them when they pass over the sea that is south of our country. These are the general rules. Of course, occasionally the situation could be different: it is possible that a south wind blows from a cold area near us; then it brings cold. Or a north wind may pass over a sea or a hot desert; then it is hot.69 East and west winds are more moderate, although large differences may occur depending on their country of origin and the sea and mountains over which they pass. East winds reach our country having passed over dry and hot regions. West winds reach us having passed over sea. Thus, the east wind is hotter than the west wind.70 The ancient authors have said that the wind that blows from the summer sunrise (Caecias) brings clouds, and that the one blowing from the winter sunrise (Eurus) is dry in the beginning and then becomes wet. It is dry in the beginning because the sun dissolves exhalation from frozen material; later, when the material has melted much vapour is dissolved and more wind is formed. They have also said that winds from the northeast and from the winter sunset bring snow.71 These rules may be different depending on the country. However, a rule that holds for all winds is that a wind becomes stronger when the sun is in the place from where it blows. Also, the sun is at first not able to dissolve much exhalation in a region that is frozen, but after some time much exhalation will be dissolved. There is less wind in summer due to a lack of material (exhalation) and in winter due to the absence of the efficient cause (the sun). Also, the wind may be weak in spring because the earth is still frozen, and it may be weak in autumn because the earth is too dry.72 Cold winds may be more or less cold and something similar holds for hot winds. This may be due to the difference in temperature of the countries over which they pass, the material from which they are formed, and the season of the year.73

69 70 71 72 73

Ibn Sīnā, aš-Šifā', Tab. 5 62,2-16. ibid. 62,17-633. ibid. 63,4-8. ibid. 63,10-16. ibid. 63,17-64,1; this is similar in content to Pseudo-Olympiodorus, Tafsir

129,3-12.

Opposite winds generally do not blow simultaneously because their efficient cause, i.e. the sun, is not simultaneously present at two opposite places. If opposite winds happen to blow simultaneously, a hurricane (zawba'a) arises, and one of them overpowers the other.74 This mostly occurs in spring and autumn because the sun is not too far from either side where the north and south wind are formed.75 Opposite winds may have the same effect; for instance, the winds from the winter sunset (Lips) and summer sunrise (Caecias) are both wet, the latter because it comes from the north, the former because it comes from the sea in the west. Other opposite winds may have the same effect or not. One wind may have opposite effects in its beginning and its end, like the wind from the winter sunrise (Eurus). This wind is dry in the beginning because the sun in the east dries the moisture that has gathered in the night; later, when the sun has risen, it dissolves much vapour and this is carried by the wind.76 A common feature of all twelve winds is that they start to blow when the sun is close to the area where they originate, but not immediately as soon as the sun has arrived there. This is especially true for the north and south wind. When the sun arrives in the north or the south, it does not immediately dissolve exhalation because the moistness is still frozen. As long as the earth is not moistened, not much smoky exhalation is dissolved, for moistness supports the dissolution of dry exhalation. Therefore these winds start to blow twenty days after the sun has reached the place where they come from. The south wind that does not blow from the pole, but from a dry area at the other side of the sea, does not start to blow until two months after the sun has reached that area, because from dry earth smoky exhalation is dissolved more slowly. This wind is called 'white wind' (rīh baydā'), because it brings clear weather, or 'egg wind' (rih baydiyya), because when it blows, chickens lay eggs without cohabitation with a cock. One would expect that these winds are very rare in summer; they do occur, however, because north winds carry moistness to that area which moistens the earth. Furthermore, these winds would not be strong if the area where they are formed were not extensive. The south wind that is formed in winter, at the south pole, does not reach us because of the distance.77 74 75

7ή

This is similar in content to Pseudo-Olympiodorus, Tafsir Ibn Sīnā, aš-Šifa, Tab. 5 64,1-7.

129,15-130,3.

*

ibid. 64,9-15; this is similar in content to Pseudo-Olympiodorus, Tafsir 130,7-15. Ibn Sīnā, aš-Šifā', Tab. 5 64,17-65,15; this is similar in content to PseudoOlympiodorus, Tafsir 1223-11, 121,3-6, 121,11-18 and 118,9-20; the feature that north winds carry moisture to the tropics to support the formation of south winds is only in Pseudo-Olympiodorus. 77

The winds that blow dependent on the motion of the sun are called annual (Etesian) winds (rīh hawliyya). They mostly blow during the daytime, when the sun is present. The winds are stronger in the countries where they originate; they become weak far from their place of origin. North and south winds are prevalent because the material for the formation of winds is available in the polar regions and there it is made suitable to be dissolved and to rise by the moistness of the earth.78 People might be inclined to think that all the material for winds is present in a reservoir under the earth. If that were true, the wind would blow more strongly in the beginning, when it pours forth from the earth, and would become weaker afterwards, like water that emerges from the earth. In fact, the opposite is the case.79 Furthermore, if the winds were to blow from one reservoir, the occurrence of opposite winds blowing simultaneously cannot be explained. Also, the earth would collapse if there were such a cavity containing the winds, and all winds would get free at once.80 The many similarities with the commentary of Pseudo-Olympiodorus are an indication that Ibn Sīnā used this commentary or its source. At one instance an example from Olympiodorus was used which is not in Pseudo-Olympiodorus. Ibn Sînâ's account of the wind is Aristotelian insofar as he says that the matter of wind is dry, smoky exhalation. However, he does not adopt Aristotle's explanation of the horizontal motion of wind, that it is moved along with the circular motion of the upper air. Ibn Sînâ's view is rather that of Olympiodorus, also mentioned by Pseudo-Olympiodorus, that the rising exhalation collides with this moving air and gets an impulse from it, so that it is deflected into a more horizontal direction. He adds some other mechanisms that may deflect the exhalation, some of them inspired by the account of Pseudo-Olympiodorus, others that are not mentioned elsewhere. He further makes a distinction between winds that start to blow from the upper air and winds that start to blow from a lower layer of the atmosphere. Finally, he briefly mentions the possibility that wind is formed because the sun heats and expands part of the air. This is one of the ways of the formation of wind according to al-Kindl (see above p. 176). Ibn Sīnā says, however, that such wind is not wind in the proper sense. Ibn Sīnā, like Pseudo-Olympiodorus and the Greek commentators, identifies the south wind from the tropics with the white south wind and the bird wind—which he calls egg wind. 78

79 80

Ibn Sīnā, as-Šifa, Tab. 5 65,16-66,1. This is similar in content to Pseudo-Olympiodorus, Tafsir Ibn Sīnā, ai-Šifa, Tab. 5 66,2-9.

120,4-10.

9. School of Ibn Sīnā Bahmanyār says that if the smoky exhalation reaches the cold stratum of the air, it cools off and is squeezed downward and becomes wind. Another cause of wind may be that air is denser in winter; when the sun approaches the north, the volume of the air expands and it becomes less dense; therefore it requires a larger space.81 These ways of formation of wind correspond to ways mentioned by Ibn Sīnā, but the formulation is rather different. Abū 1-Barakāt's doctrine on wind differs from that of Ibn Sīnā, al-Kindi and Aristotle. He starts his account by stating that wind is moving air and gives some causes why air might be moving. First, a living being may move air, like a man may do so with a fan and like a herd of horses, a flock of sheep or a group of birds may move air. The motion of the air lasts as long as the motion of that which causes it lasts; this is not what one calls wind. Secondly, there is the motion of the winds that come from various wind-directions, such as east and west. It is this kind of motion that is called wind. There is no evident cause for their duration. Among them one counts the whirlwind that rises from the earth to the sky in a rotating motion. Older authors said that the matter of wind is smoky exhalation from the earth, just as moist exhalation is the matter of rain. They said that this exhalation rises, then cools and descends and on its way down meets rising exhalation. Then it is deflected sidewards by this rising exhalation.82 Abū 1-Barakāt asks how such a deflection of exhalation by exhalation would be possible; anyway it does not occur with watery drizzle when it descends. How could such a deflection occur with such a force and velocity, stronger than the natural motion of the exhalation? Wind is able to make walls collapse and to uproot trees, whereas descending smoke is not able to destroy roofs. Moreover, if the rising exhalation were able to confer such a force upon the descending one, it would thrust it upwards, not sidewards. Modern authors said that wind is formed when a certain spot of the air is heated and expands and thus moves and pushes the adjacent air away. 83 Such a motion lasts as long as the heating cause exists. If this were the way wind is formed, all winds would move upwards or equally into all directions, since expansion gives rise to such a motion. Also, if cold were to act upon a certain volume of air, it would

81 82 83

Bahmanyār, at-Tahsīl 714,9-10 and 716,1-3. This is the view of Pseudo-Olympiodorus and Ibn Sīnā, see above pp. 171 and 177. Al-Kindi mentions expansion as a cause of wind, see above p. 176.

contract and pull the adjacent air towards itself and then the motion of wind would be downwards, or towards the centre of contraction equally from all directions. This is not what we see for, in fact, wind blows between distant places in one direction. If wind were caused by heat that expands air, it would stop blowing when it cools off, for instance when it passes over a snowy region; similarly, if one supposes that wind is caused by cold, it would stop when it gets hot.84 Thus, the cause of winds is neither the upward motion of smoky exhalation when it is hot, nor its downward motion when it is cold. It is neither air that expands when it is heated, nor air that contracts when it is cooled. There are no other natural causes that could be connected with the motion of the wind. Abū 1-Barakāt concludes that the cause of the motion of the wind must be sought in the wind itself and that it must be something that is different from the elemental natural forces (sc. heat and cold). What moves the wind (rīh) is some heavenly force from the stars that comes to the wind and moves it, such as the spirit (rūh) is also moved by a heavenly force that comes to our world. This force carries the moist and dry exhalations, the dust particles, the seeds of plants, fruits and trees, the moisture, rain and clouds in various directions to all different places of the earth in order the process of generation and corruption to take place. If one sees that a whirlwind arises by itself while the air is calm, and moves on like a rider on his horse with its rotating motion, carrying upwards dust, while it drifts along over the earth from place to place, then one becomes convinced that such a motion cannot be caused by rising heat or descending cold. If one sees winds that destroy mountains and uproot trees, one knows that their cause cannot come from the primary forces heat and cold, but that it must be a heavenly force from the stars. Calling the wind (ar-rīh) a spirit (ruh) and the spirit (ar-rūh) a wind (rīh) is fitting for someone who knows them in the meaning that unites both.85 Someone might ask whether this force in the wind is like soul in the spirit, so that one could say that wind has a soul. The answer is no, if one means that a soul is something that causes volitional motion, that is, motion that can have any direction. One should not call the moving cause a soul, but a natural force. One should not maintain that there are only four natural forces (heat, cold, drought and moistness), for what about other forces in composed bodies, such as the magnetic force? The celestial forces from the stars that cause the motion of winds, in

84 85

Abū 1-Barakāt, al-Mu'tabar ibid. 219,12-220,6.

II 217,11-219,4.

accordance with their approaching and receding and their different positions in the sky, are also responsible for the forces of minerals, plants and animals.86 Abū 1-Barakāt concludes his account by mentioning that one may distinguish twelve wind-directions. Each of these winds mostly blows at a specific time of the year; seamen know these times from the anwā'.S1 Winds are influenced by the places over which they blow, such as seas, mountains, vegetation, snow, moistness, flowing and stagnant water.88 The explanation by Fakr ad-DIn of the formation of wind mostly follows Ibn Sînâ's Kitāb as-Šifā\ but shows some differences. He says that wind arises from smoky exhalation in two ways. (1) The way in which it is mostly formed is that much smoky exhalation rises upward to the cold layer of the atmosphere. If its heat is destroyed by the cold air, it becomes heavy and descends. This downward motion causes an agitation (tamawwuj) of the air and that is wind. If the heat of the exhalation is not destroyed, it continues to rise until it reaches the sphere of fire that is moving with the celestial motion. The exhalation cannot rise further, is thrust back and becomes wind. The explanation of why this wind moves sideways and not necessarily in the same direction as the motion of the fiery sphere follows Ibn Sīnā.89 One may object to this view, saying (a) that the smoky exhalation, which contains earthy parts, is heavier than the moist exhalation. The latter, when cooled, descends vertically as rain, so why does the smoky exhalation not also descend vertically, but moves horizontally to the left and right? (b) Furthermore, the descending motion is natural, whereas the horizontal motion is not natural; a natural motion is stronger than a non-natural. How can such a non-natural motion uproot trees and and destroy walls, when we see that descending particles of dust are not able to destroy a roof?90 Fakr ad-Din answers the first objection, saying that if heavy particles are sufficiently small, they are not able to cut their way through the air vertically downward and they will not descend. If they join and form a larger whole, they will descend. The particles of the smoky exhalation will not join because they are dry; moist particles will join and form raindrops that descend. As for the

86

Abū 1-Barakāt, al-Mu'tabar II 220,7-15. I.e. the rising and/or setting of certain stars that were rence of rain and wind. 88 Abû 1-Barakāt, al-Mu'tabar II 220,15-22. 89 Fakr ad-Dīn, al-MabāhiL II 190,19-191,15 mostly follows 58,6-17. The difference with Ibn Sīnā is that the descending motion, which is wind. 90 This objection was raised by Abü 1-Barakāt al-Bagdàdï p. 183. 87

connected with the occur-

Ibn Sinā, aì-Šifà'. Tab. 5 exhalation sets the air into against Ibn Sīnā, see above

second objection, when the exhalation is thrust back, either by the cold or by the motion of the sphere of fire, the front is pushed by what comes after it, due to the large quantity of material that is dissolved. In this way the material is concentrated and it gets a force that is able to uproot trees and destroy walls.91 (2) Wind may also be formed—less frequently—before the smoky exhalation has reached the cold region or the sphere of fire. This occurs when the rising exhalation is turned into another direction, either because the way up is very winding, or because it hits upon a wind above it that prevents it from rising further.92 The next sections, on wind and rain that may prevent and support one another, and on hurricanes and whirlwinds, follow Ibn Sīnā.93 Then follows Ibn Sînâ's determination of the wind-directions.94 and Fakr ad-DIn's criticism on it. He shows that these directions depend on the latitude of the place where we determine them. First, he observes that for people on the equator there is no 'circle that delimits the circumpolar stars'. Then it is shown that the angle between the north wind and the adjacent winds, that are determined by this circle, is equal to the angle between the equator and the zenith. This angle clearly depends on the latitude of the place where we are. The place with a latitude equal to the complement of the inclinaton of the ecliptic has a circle delimiting the circumpolar stars that coincides with the tropic of Cancer. The latter determines an east wind and a west wind, according to Ibn Sīnā, which would coincide with north winds for this place. If the latitude of the place is larger than the complement of the inclinaton of the ecliptic, the north winds would be closer to the east and west of the equator than the east and west winds that blow from the east and west of the summer. Thus, Ibn Sînâ's determination of the winddirections is not good. The correct way is to divide the horizon into twelve equal parts; each point of division determines a direction from where a wind blows. The Arabic names of these winds are given.95 The text of the next sections (on properties of winds, etc.) is from the Š//ā'.96 91

Fajy ad-Dīn, al-MabāhiL II 191,16-192,19. Fakr ad-Din, al-Mabàhií II 192,20-193,2 is similar in text to Ibn Sinā, aš-Šifâ', Tab. 5 59,4-8. 93 Fakr ad-Din, al-Mabāhii II 193,5-194,9 is similar in text to Ibn Sinâ, ai-Šifâ', Tab. 5 59,17-60,9 and 60,12-61,8. 94 Fakr ad-Din, al-MabàhiL II 194,10-17 is similar in text to Ibn Sīnā, aí-Šifà', Tab. 5 61,12-17. 95 Fakr ad-Din, al-Mabāhii II 194,18-196,13. These names were also given by PseudoOlympiodorus, see above p. 173. 96 Fakr ad-Dīn, al-Mabāhii II 196,14-197,19 is similar in text to Ibn Sīnâ, ai-Sifä, Tab. 5 62,6-18 and 64,1-65,14. 92

10. Ibn Rušd Ibn Rusd's account of the wind in his Short Commentary starts with an enumeration of winds. He says that there are four well-known winds: the east, west, north and south winds. Between these winds are other winds, referred to as side winds (nakbā'). He says that he has found in Aristotle's account of the subject that there are eight such side winds, two between each pair of the main winds,97 so that the total number of winds is twelve. However, he has also found that, according to Alexander, there are eleven winds.98 Observation must decide which of these views is correct.99 Next, it is shown that the matter of wind is smoky exhalation, like moist exhalation is the matter of rain, for contrary things must have contrary matters; rain and wind are contrary: when one is present, the other stops. Furthermore, each of them generates the other accidentally: if rain moistens the earth, the sun dissolves much smoky exhalation from it, just as moist wood produces much smoke when it is burnt; wind drives together clouds from various places and then rain is formed, as occurs in Ethiopia. An indication that wind is formed from smoky exhalation is its fast motion, for a fast and violent motion is a property of hot, dry exhalation. It also appears from the effect of wind, for it always has a drying effect, in contrast to rain.100 Ibn Rušd discusses the cause of the horizontal motion of the wind as follows: The smoky exhalation by nature moves upward, but the wind moves around the earth. This is clear from the motion of clouds. The reason is that if the hot exhalation rises and arrives in a cold, moist region of the atmosphere, it will become colder and moister and be given a downward inclination. Now two inclinations are present, a heavy (downward) and a light (upward) one, and this will result in a horizontal motion along the earth. Such a motion is possible for both components of the exhalation, the heavy and the light one, whereas the natural motion of either component is impossible for the other. Such a horizontal (or circular) motion is neither purely natural for either component, nor purely against its nature. The light smoky part is not able to descend with the moist part; therefore it is given a horizontal motion. An indication that winds start to blow above and then descend

97

Ibn Rušd enumerates these eight winds in the same way as Ibn al-Bitriq, see above p. 168. Thus, 'Aristotle's account' means: Ibn al-Bitriq's version. 98 Alexander, in Meteor. 110,6-10 follows Aristotle in counting eleven winds, although he says that some add the Libonotus as a twelfth wind. 99 Ibn Rušd, Short Commentary 34,2-17. 100 ibid. 35,2-36,1.

is that one sees the clouds moving before feeling the wind itself. Seamen, who are knowledgeable about the formation of winds, know this very well.101 Some people think that the cause of the horizontal motion of the exhalation is that it rises until it hits upon the upper air that moves along with the circular motion of the heaven and then is thrust back from it, so that it is given a horizontal motion. This is impossible, says Ibn Rušd, because if the exhalation would meet this moving air, it would adopt its motion and move with the same motion as the daily motion of the universe, i.e. a motion from east to west only and this cannot be wind. Also, this cannot explain why some winds blow so strongly, like the wind that causes thunder and lightning from the cloud. Their strong and violent motion is due to a contrariness in the material; they occur when the descending moist and cold exhalation meets other exhalation that is rising. This excludes the above-mentioned circular motion.102 Ibn Rusd's explanation of the horizontal motion of the wind is similar to the third explanation mentioned by Pseudo-Olympiodorus (see above p. 171); it is also one of the explanations given by Ibn Sīnā. Ibn Rušd rejects the first explanation of Pseudo-Olympiodorus, also adopted by Ibn Sīnā, that the rising exhalation is given a horizontal impulse from the rotating upper air (see above p. 177). Ibn Rušd then discusses which winds arise and which ones drop in a particular season of the year. Winds generally do not occur in times of extreme heat or extreme cold. Extreme cold makes the surface of the earth dense and this prevents the smoky exhalation from rising; moreover, it is not a property of coldness to dissolve exhalation. Extreme heat causes a kind of burning on the surface of the earth and therefore destroys the smoky exhalation. At other times much wind is blowing, the most prevalent winds being the north and south winds. The reason for this is that they arise from the place where the sun reaches the summer and winter tropics (madār sayfī wa-šatwī or munqalab sayfī wa-šatwī or zawāl sayfī wa-šatwī). East and west winds blow less often because they arise from places over which the sun passes between the tropics, and those are hot places.103 South winds blow sixty days after the sun has reached the winter solstice and north winds blow twenty days after the sun has reached the summer solstice. When the sun is nearest to the north, it melts the snow and moistens the earth and then north winds are formed; the sun 101 102 103

Ibn Rušd, Short Commentary ibid. 37,7-19. ibid. 38,1-14.

36,2-37,6.

needs approximately twenty days for this process. We also observe that the air is hottest after the sun has reached its nearest point, for it takes time for the air to become susceptible to the influence of the heat. The effect does not depend on the effective cause only, but also on what is affected. It is not impossible for a weak effective cause to bring about a larger effect in a specific place than a strong effective cause in another place. The difference is due to a different susceptibility of the places to be affected. The question remains why the south wind starts to blow sixty days and the north wind twenty days after the sun has reached its solstice, in spite of the fact that the position of the sun in relation to respectively the south and the north is the same in both cases. A reason could be that the south wind, having been formed in the same way as the north wind, is very weak at first, so that it does not reach us due to the distance it has to travel, which is larger than the distance the north wind has to cover. After some time it becomes stronger because the influence of the heat has become stronger there and then it may reach us. Another reason could be that the place of origin of the south wind is farther away from the winter tropic than the place of origin of the north wind from the summer tropic. Therefore the sun, after having reached the winter solstice, needs more time to heat the place where the south wind is formed. Ibn Rušd says that he has doubts about which of the reasons is the correct one and settles the matter as follows:104 Aristotle justly said that the region near the equator (mu'addal an-nahār) is uninhabitable due to its extreme heat. Thus, it is impossible for south winds to reach us that have been formed in a place corresponding to where the north wind is formed, i.e. between the winter tropic and the south pole. Such wind would be destroyed when it passed over the tropics, before it would reach us.

104 Things are rather confused here. Alexander, Olympiodorus, Pseudo-Olympiodorus and Ibn Sīnā have said that the south wind which originates in the southern h e m i sphere after the sun has reached the winter solstice and which corresponds to our north wind does not reach us, because the distance is too far (Ibn Rušd's first argument). On the other hand, there is the south wind that does reach us; it originates at the summer tropic, sixty or seventy days after the sun has reached the winter solstice. The long delay is caused by the fact that the sun, when it is in the winter solstice, is farther from the place of origin than in the corresponding case when the north wind is formed (Ibn Rusd's second argument). Ibn Rušd does not distinguish between these two south winds and is not convinced of the above-mentioned arguments. Indeed, if the south wind is formed in the southern hemisphere in the same way as the north wind is formed in the northern hemisphere, the second argument cannot be valid. It seems as if in the next section Ibn Rušd does make the distinction between the two south winds, which solves the problems. Thus, what follows must be a correction Ibn Rušd added in a revision of his text, like the correction added to the discussion of the next paragraph. See also below his Middle Commentary on this question.

Thus, the south wind that blows with us must have its origin at the summer tropic. That region is cold and moist when the sun is at the winter solstice. When the sun approaches that region, returning from its winter solstice, it melts the moistness and wind arises. When the sun has arrived at the summer solstice, the wind stops, because of the fierce heat. This is the account given by Alexander and it is the correct one.105 The specific properties of the four main winds are the following: The south wind is hot and moist; the north wind is cold and dry; the east and west winds are moderate, standing midway between the north and south winds, but the west wind is more moist. The south wind is moist—although it is formed in a dry area—because it comes from a higher region of the earth, from where moist vapour descends that is carried with the wind. The coldness and dryness of the north wind are due to its blowing from cold land and low places. East winds are thought to be hotter than west winds, because the east is warmer than the west. People have found this difficult to explain, considering that the sun has the same position in relation to the earth in the east and in the west. When the sun is in the east, its rays reach the earth in that region approximately at a right angle, whereas they reach the west at an obtuse angle. At midday the sun has the same position in relation to both regions. Then, during the afternoon, the sun has the same position in relation to the west as its position in relation to the east during the morning. This made people say that they could not give a reason for a difference between eastern and western regions and they denied the effect. Those who confirm that the eastern region is hotter give as the reason that the sun, when it appears all at once in the east, has a strong heating power in that region, because of the reason mentioned above and because the area becomes very susceptible to heat. It gradually approaches the west and that region is less susceptible to heat compared to the eastern region and therefore it is less influenced. If the effect really exists, this is a possible reason. Another reason could be that the part of the heaven where the sun is, has a greater effective power than other parts, and thus the regions on earth situated underneath it are more heated by the motion of the heaven and this heating affects the east first. Both ways of heating, i.e. by reflection of the sunrays and by celestial motion, cooperate. It is said that countries with a greater longitude are hotter than those with a smaller one. If this is true, the reason could be that the region with greater longitude is opposite the right part of the heaven and, just as we say that what is right has a

105

Meteor.

Ibn Rušd, Short 100,28-101,9.

Commentary

38,15-41,6.

The

reference

to

Alexander

is

in

stronger effect, we can say that the part of the earth opposite the right part of the heaven is more strongly affected. Ibn Rušd says that he thinks that this is the most important reason why eastern winds are hotter. Then he adds a further reason, namely the following:106 The sun remains six hours above the eastern half and six hours above the western half, but before it rises in the east, it has already heated that area for one or two hours and thus it heats the east seven or eight hours while it is above the earth, and one or two hours while it is underneath it. When the sun goes down under the earth in the west, that area does not profit from the one or two hours' heating while it is below the horizon, because it has already cooled down after the sunset.107 Ibn Rusd's Middle Commentary follows Ibn al-Bitriq's version of the Meteorology more or less phrase by phrase, with some remarks of his own added to it. When he arrives at the statement that the winds from the north and south are more frequent than those from the east and west, he gives the explanation such as given in Ibn al-Bitriq's version; it differs from Aristotle's account (see above p. 167). Ibn Rušd remarks that he has found this explanation in the Aristotelian text which was available to him, but that Alexander gave quite a contrary explanation: the former explanation says that much exhalation rises in the east and west; this exhalation turns into rain, which prevents the formation of wind; in the north and south less exhalation rises, which moreover turns into wind rather than into rain. The latter explanation says that much rain falls in the north and south and this stimulates the dissolving of dry exhalation, whereas in the east and west the heat of the sun disperses the exhalation before it can form wind. Both explanations agree insofar as that the sun heats the east and west more than the north and south. A priori both explanations may be true, because one may claim that the sun dissolves much exhalation from a place when it is near that place and less from a place when it is far from it; one may also claim, however, that a place is dried by the sun when it is near, so that the dissolving of exhalation is stopped, whereas exhalation is dissolved when the sun is far from that place. Observation has to decide which explanation is valid: if there is more rain in the east and west than in the north and south, then the former explanation holds; if the opposite is the case, the latter explanation holds.108 Ibn Rušd does not decide between the two explanations. In the Short Commentary he

106 According to al-'Alawi, this is a correction added by Ibn Rušd in a revision of his book; see Ibn Rušd, Middle Commentary 119n1. 107 Ibn Rušd, Short Commentary 41,7-44,8. 108 Ibn Rušd, Middle Commentary 100,2-102,19.

gives the second explanation (see above p. 188). Ibn Tibbon also discusses the differences between both explanations.109 The explanation of the horizontal motion of the wind is that if exhalation rises into a cold layer of the atmosphere, the moist part will turn into water and the dry part into wind. Each of these exhalations will move to its natural place, viz. upwards and downwards; the result of a combination of these motions is the horizontal motion of the wind.110 Ibn Rušd continues to follow Ibn al-Bitriq's text until he arrives at the the question why north winds blow after the end of the summer season and after the end of the winter season. He gives Ibn al-Bitriq's answer and then remarks that he has found this account in the Aristotelian text available to him, but that Alexander instead asks why north winds blow after the summer solstice—they start to blow twenty days after it—whereas south winds start to blow seventy days after the winter solstice (see above pp. 165 and 168). Two answers have been given. Firstly, the southern area where the south wind is formed is further from our inhabited part of the world than the place where the north wind is formed. Thus, this south wind reaches us only when the heat in the south has increased after the winter solstice to such an extent that the wind is powerful enough to blow as far as our region. This occurs after seventy days. The place where the north wind is formed is nearer to us, so that this wind already reaches us twenty days after the summer solstice. The second answer is that the place where the south wind arises is further from the winter tropic than the place where the north wind arises from the summer tropic. There is no indication that this answer is correct, because it is most probable that the place of origin of the south wind has the same position in relation to the winter tropic as the place of origin of the north wind in relation to the summer tropic.111 If one supposes that the south wind is able to 109

Ibn Tibbon, Otot ha-Shamayim II 273-289. Ibn Rušd, Middle Commentary 104,13-105,10. 111 From here the Latin version differs from the Arabic text: the next section is found not in the Latin version. Instead, the previous discussion is continued. The first answer is dismissed as well, because wind behaves like a river: it is weak at its origin, but the further it moves from its origin, the more it increases in strength. Then follows a long account in which Ibn Rušd tries to find a system in which all winds discussed in this section have a place: the north wind that blows in spring, the north wind that blows in autumn (from Ibn al-Bitriq's account), the north wind that blows twenty days after the sun has reached the summer tropics, and the south wind that blows seventy days after the sun has reached the winter tropics (from Alexander's account). An important part is played by the claim that wind may arise by heat (like the north vernal wind and the north wind that arises after the sun has reached the summer tropics) or by cold (like the north autumnal wind). Furthermore, the principle is applied that if opposite winds blow at the same time, they are formed by opposite 110

pass over the equator, despite the extreme heat that reigns there, the first reason is sufficient. Alexander says that the south wind originates from the summer tropic, while the sun is approaching the vernal equinox. Ibn Rušd concludes his commentary on this subject by saying that both accounts, that of Aristotle (i.e. of Ibn al-Bitriq) and that of Alexander, are not contradictory and may both be valid, as it is not impossible for two contrary winds to blow successively in the same season.112 We have seen how Ibn Rušd is confronted several times with discrepancies between Ibn al-Bitriq's version of the Meteorology and Alexander's commentary, which mostly gives a more truthful rendering of Aristotle's text. He does not choose between both accounts, but leaves the matter undecided or tries to give an account into which both fit. Furthermore, we have seen influences from Pseudo-Olympiodorus and agreements and disagreements with Ibn Sīnā.

causes. Also, opposite winds that blow at different times, before and after the vernal equinox, such that these times are equally removed from the vernal equinox, are formed by the same causes. Using these principles it is shown that the south wind that blows seventy days after the sun has reached the winter tropics, when the sun is approaching the vernal equinox, is caused by cold. See Ibn Rušd, Middle Commentary, Latin version 436I-437K. 112 Ibn Rušd, Middle Commentary 108,6-109,11. The problems here are the same as in the section of the Short Commentary on this subject, see above p. 189n104.

THE INHABITABLE REGIONS OF THE EARTH

1. Aristotle Aristotle discusses the question which sections of the earth are inhabitable and which are not in 11,5, as a digression in his account of wind. He divides the earth into five sections by means of four parallels of latitude: the tropic of Cancer, the circle that delimits the circumpolar stars (to διά παντός φανερόν) 1 and the corresponding parallels in the southern hemisphere. The region between the tropic of Cancer and the circle delimiting the circumpolar stars and the corresponding region in the southern hemisphere are the two inhabitable sections. Outside these regions it is too cold (around the poles) or too hot (between the tropics, where there is (almost) no shade) for habitation. It is absurd to represent the inhabitable world as circular, as maps of the world do, for the inhabitable world would, in fact, form a closed band around the earth if this were not prevented by the ocean beyond the Pillars of Hercules and India. The Greek commentators, Ibn al-Bitriq and Hunayn do not present special features.

2.

Pseudo-Olympiodorus

The commentary of Pseudo-Olympiodorus gives the five sections into which the earth is divided, in accordance with Aristotle, two of them being inhabitable because of their moderate temperature. He adds that when the sun is in the winter solstice, it becomes summer in the southern hemisphere and winter in the northern. The reverse occurs when the sun is in the summer solstice. We can know the situation in 1

More precisely, the projection of this circle on the earth globe; this circle varies with the latitude of the place of observation; we must suppose that Aristotle takes Athens (latitude 38°) as place of observation; then this circle is the parallel with a latitude of 52° degrees. The circle is identical to the polar circle only for places on the tropic of Cancer. Alexander calls this circle ό άεί φανερός κύκλος or Ο αρκτικός κύκλος and its southern counterpart ό άεί αφανής κύκλος (in Meteor. 102,31-33 109,11); Olympiodorus calls them 01 άκροπόλοί παράλληλοι, Ο μέν αρκτικός, ό δέ ανταρκτικός (in Meteor. 183,29-31 187,17).

the southern hemisphere only by analogy, for nobody can go there and nobody from there can come here because the tropics prevent it.2 The inhabitable zone is essentially determined by its latitude only, since all places with different longitude but the same latitude have the same temperature. Therefore one could get to the opposite side of the earth on the same latitude if one were not prevented by the sea in both directions. According to Aristotle, the inhabitable regions are in the middle of each hemisphere; the regions to the south and north of it are not inhabitable due to their heat and cold, respectively. According to Ptolemaeus, the inhabitable region in the north is larger, because part of the tropics is also inhabitable.3 The contents of this paragraph is also found in Olympiodorus, except for the comparison of the views of Aristotle and Ptolemaeus on the inhabitable world.4 The different sections of the earth are related to the direction into which shadows fall. The shadow of those living in the middle of the northern or southern hemisphere falls towards the north or south, respectively. The shadow of those living on the equator falls towards both directions, therefore they are called 'people with two shadows'. If the sun is in the summer solstice, their shadow falls towards the south; it falls towards the north if the sun is in the winter solstice. If the sun is at the equator they have no shadow, because the sun is vertically above their heads.5

2. Ibn Sīnā The division of the earth into inhabitable and uninhabitable sections is discussed by Ibn Sīnā in a special chapter of the first ('geological') treatise of the fifth section of the TabViyyāt. His account has no parallel in previous works on meteorology. He first discusses how the division of the earth into land and sea has come about. In principle the earth should be completely surrounded by water. In fact, this does not occur, because parts of the earth change into other elements and vice versa. Earth that has changed into something else leaves a gap or depth, and when earth is added by a transformation from another element, an elevation or hill is formed. These changes occur under the influence of 2

This phrase is similar in content to Olympiodorus, in Meteor. 184,16-18. According to Ptolemaeus, the inhabitable region extends from 63° north latitude (the island of Thüle, somewhere in Scandinavia) to 16° south latitude (the latitude at which the longest daylight is 13 hours), see Ptolemaeus, Geographia 1,7-10. 4 Pseudo-Olympiodorus, Tafsir 122,18-124,23. 5 ibid. 125,2-8. This section is similar in content to Olympiodorus, in Meteor. 190,24-191,10. 3

the stars, when they are vertically above the matter that changes. Moistness evaporates and rises from the earth and descends again at another place. In this way water changes places: it disappears in one place and arises in another place, until finally the depths of the earth are filled with water and the elevated places are left as dry land. Something else contributes to this effect, viz. that clay is formed between earth and water; when this is dried by the sun it becomes stone and in this way mountains are formed, as we have seen before (see above p. 142). Divine wisdom can be seen in the whole process, for it gives the land animals a place to live and breathe.6 People who have investigated the earth have found that one quarter of it is land. They took as its length one half of the circumference of the earth and as its breadth one fourth of the circumference (90°) in the northern hemisphere, so that the land takes up about one half of the northern hemisphere. There is no clear proof that the other quarters of the earth are water, except the prevalent idea that there must be several times more water than earth in the universe; if an element were to change completely into another, this must result in a quantity equal to the actual quantity of that other element; when water changes into earth its volume decreases, therefore there must be less earth than water.7 One does not need to take into account the excentricity of the sun, in other words, the fact that the southern hemisphere is heated more because the sun is closer to the earth when it is there. This excentricity is so small that one does not perceive its effect, although it is not impossible that conditions for inhabitation near the south pole are better than those near the north pole.8 It seems that the southern border of the northern hemisphere, viz. the equator, is mostly bordered by sea and that one does not reckon with people who live south of it, although people may live there on islands, not connected with the main land. Furthermore, it seems that the northern border is the polar region. It is not yet clear to us whether that area is suitable for the generation and permanent living of men or whether one may travel to it and stay a short time in summer. Maybe it is not suitable for the generation of men, but for that of special animals only. All this is speculation, nothing is established.9 Ibn Sīnā states that the suitability for habitation is determined by the sun that causes heat and cold and then gives Aristotle's division of the 6 7 8 9

Ibn Sīnā, aì-Šifff, ibid. 25,13-20. ibid. 25,20-26,6. ibid. 26,6-14.

Tab. 5 24,7-25,12.

earth into five sections, two of them being inhabitable. He disagrees with it, saying that there are inhabited countries that lie some degrees south of the tropic of Cancer, where the sun reaches the zenith several times, that there are also countries near the equator and that trustworthy people have even reported from countries on the equator, like Ceylon. He will argue that the equatorial region is in fact the most moderate region and the most suitable to inhabitate. First, he discusses some preliminaries concerning the way in which the atmosphere is heated.10 Regarding this question, Ibn Sīnā says that the primary cause is the sun. It does not occur because the sun is hot, or forces some fire to descend to the earth, or because its rays are fiery. The rays are not corporeal nor are they some power that travels through a medium to the earth. Heating is an effect which occurs instantaneously in the body which receives the rays, if between the light source and the receiving object there is a substance that does not prevent this effect, sc. a transparent medium. The heat does not increase in summer because the sun's distance to us is smaller—in fact, its distance is larger, as it is in its apogee (aw/)—but because it is more vertical above us, i.e. nearer to the zenith (aqrab musāmata). If we consider the sunrays as a cone or cylinder, then the axis of this figure heats most and what is struck by it becomes hotter than what is struck by the rays that are removed from the axis.11 What people say about rays that fall in and are reflected sometimes at sharp angles and sometimes at obtuse angles, does not seem to be true, for rays have no existence in the atmosphere; what emits rays of light is visible and the atmosphere is not visible, but transparent.12 However, the heating of the atmosphere cannot be dependent only on the position of the sun in relation to the zenith (musāmatà), otherwise the heat would be greater when the sun is in Cancer than when it is in Leo and it would be the same when the sun is in Gemini and in Leo and also when it is in Taurus and in Virgo. Furthermore, countries near the place of the crossing (majāz) of the sun (equator) would not sometimes be colder than those far from it. All this does not happen. If the sun were suddenly placed in Cancer, then the countries under it would not immediately become extremely hot, but at first they would 10

Ibn Sīnā, as-Šifa, Tab. 5 26,14-27,15. An explanation of this is given by Abū 1-Barakāt, see below p. 200. Note the analogy with the view that the ray along the axis of the visual cone, i.e. the cone of visual rays emitted by the eye, is 'stronger' than the other rays. This is a well-known theme of optics. It occurs in Ptolemaeus, (Opt. II, 20) and al-Kindi; the latter gives an explanation within the framework of the theory of visual rays, see Lindberg 1976 26-30. 12 Ibn Sînā, aš-Šifa, Tab. 5 27,16-28,13. 11

heat up only to a certain extent, just as when we bring a fire in a house, its effect is not immediately very large. The effect increases when the cause remains for some time. Therefore the air is hotter after the summer solstice than before it, although the position of the sun is the same. The sun gradually approaches the countries at the tropic of Cancer and adds heat step by step. After that the sun is in the zenith for some time, as it moves slowly in the solstices; and when it recedes from the zenith it returns to it again after a short time.13 Also, the days are long and the nights are short. This means that the air becomes extremely hot. At the equator the sun reaches the zenith suddenly and recedes from it quickly, and the lengths of day and night are equal. Thus, neither the heat nor the cold becomes very extreme. In our countries, when the sun is far from the zenith, it becomes extremely cold and so there is an alternation between extreme heat and extreme cold, which afflicts and wears out the bodies of living beings. In those places at the equator however, there is no such transition between contraries, but only from a moderate intermediate state to another state not far from it. The bodies of the inhabitants are adapted to extreme contrary conditions in their countries, so that they are not too afflicted by them. For instance, the Turks do not suffer too much from the cold in their country, nor do the Ethiopians suffer too much from the heat. Sometimes the Bedouins in Khurasan suffer from cold, whereas at the same time the inhabitants complain about heat. Ibn Sīnā relates that he once saw in Bukhara, at the time of a heat wave, a Bedouin shivering, wrapped up in clothes and crying for help because of the cold, while the inhabitants of the town suffered from the heat. The conditions at the equator do not change much; it is as if a permanent spring reigns there, unless other factors than the position of the sun also have an effect on the heat.14 Indeed, there are other causes besides the sun's position that influence the temperature of the air in a certain country. High countries are colder than low-lying countries. The northern regions that are free from mountains are colder than those with mountains, because the reflection of the sunrays is different and because of cooling by the north wind. If the above-mentioned conditions are the same, then northern countries are colder than southern. If these conditions differ, a northern country may be warmer than a southern one. Eastern and western countries do not differ if they have the same latitude. People say that eastern countries are warmer than western, because the motion

13 14

This holds for countries that lie a short distance south of the tropic of Cancer. Ibn Sīnā, aš-Šifa, Tab. 5 28,13-30,15.

of the sun starts in the east and ends in the west.15 These people have no understanding at all. For the sun starts its motion in any place on earth in the same way; east and west are relative terms. If the east is indeed warmer than the west, then the reason must be that the sea extends south of it. The sun reaches the zenith above that sea before it reaches the zenith above those eastern countries, and it dissolves much hot vapour from it. The sunrays are reflected in this vapour and they heat it; therefore the atmosphere of a region near the sea is heated more strongly than if there were no sea near it. Those regions are warmer because of this effect, otherwise they would be colder because of the cold of the water.16

4. School of Ibn Sīnā Abū 1-Barakāt discusses cold and heat in the different seasons and climatic zones of the earth in one of the chapters of the third part of his Kitāb al-Mu'tabar, which is the part in which he deals with the subject-matter of Aristotle's De Generatione et Corruptione. He agrees with Ibn Sīnā about the way the sun heats the atmosphere and about the region at the equator being a moderate region, but, actually, his account is quite different from that of Ibn Sīnā in phrasing and formulation. He states that the heat on earth is caused by the sunrays that affect the dense bodies (earth and water). Light, airy, transparent bodies are not affected. The sun itself is not hot, otherwise the upper part of the atmosphere would be hotter than the lower part, as it is closer to the sun; in fact, the higher layers of the atmosphere are colder. The sunrays and the heat that accompanies them are reflected by polished surfaces. If such a surface is concave, the reflected rays are concentrated towards one point and the heat may become so intense that bodies are burned. Surfaces that are not smooth and polished do not reflect the sunrays in this way, but disperse them in all directions, because of their roughness. The heat that is conferred by the sun to the earth differs in intensity in different seasons and at different climatic zones. The causes of this will now be investigated.17 One of the causes of a certain place being hot is that the day is long; similarly, short days are a cause of coldness. This is due to the fact that the longer the sun shines, the intenser the heat becomes. Therefore the 15

This is what Aristotle, the Greek commentators and Pseudo-Olympiodorus say, see above pp. 159n3 and 174. 16 Ibn Sīnā, aš-Šifà. Tab. 5 30,16-31,14. 17 Abū 1-Barakāt, al-Mu'tabar II 202,7-21.

hottest season is the time when the days are longest, viz. summer, and the coldest season is the time when the nights are longest, viz. winter. In the place where day and night are always equal—at the equator—the climate is moderate and neither cold nor heat are extreme. The further a place is from the equator, the larger the difference between the heat in summer and the cold in winter. One might expect that in places where the longest day is longer, the summer is hotter than in places where the longest day is shorter. This not the case; in fact, the contrary occurs: the summer is colder where the longest day is longer and the winter is warmer where the longest day is shorter. Therefore we should adduce other causes besides the length of the longest day.18 In places where the days in summer are longer, the nights in winter are longer too. Therefore the cold in winter is more intense and remains longer in the earth because of the presence of snow. Heating occurs during the short day and the heat does not remain in the earth like the cold does, because cold is a natural property of earth and is retained because of the snow. Thus, the cold is retained after the cause of the cold has disappeared, but the equivalent does not hold for the heat. Therefore, in countries in which there are long periods of snow, the summer is less hot, in spite of the longer days in summer.19 Another cause that determines the heat in a certain place is the height to which the sun rises. People sometimes think that if the sun is close to the zenith, its distance is shorter and when it is far from it, its distance is farther. In fact, the sun moves in its sphere and its distance to the earth is always the same, as the earth is in the centre of that sphere. When the sun is in the zenith, it causes heat in two ways. (1) (A special cause) There are no shadows of mountains or walls. If the sun is not in the zenith, the land in the shadow of a mountain is protected from the sunrays and remains cold. (2) (A general cause) If the sun is in the zenith, then one receives the middle of the beam of sunrays that reaches the earth; if not, then the rays at the sides of the beam strike us. It is known that the rays in the middle are hotter because they are surrounded by other rays that are hot too, whereas those at the sides are less hot, because they are surrounded by cold. Thus, heat is caused by days being long and the sun being in the zenith. These causes oppose one another: in places where the days are long, the sun is far from the zenith, and where the days are short, the sun is close to the zenith.20

18 19 20

Abū 1-Barakāt, al-Mu'tabar ibid. 203,14-204,1. ibid. 204,1-205,10.

II 202,22-203,14.

Another factor that influences heat and cold is the fact that when the sun is at the horizon, in the east or the west, its rays cover a longer distance through the exhalations that are dissolved from the earth than when it is high in the sky. This means that the sunlight is weaker in the morning and the evening than at midday and that the heat is correspondingly less intense at those times. People have explained the more intense heat at midday by saying that at that time the incident rays and the rays that are reflected against the earth's surface are almost along the same line, so that the heat of the incident and reflected rays enhance one another. This is not true, because the incident rays are not hot, for the sun does not heat the transparent air, but it heats the earth. Nor are the reflected rays hot, but what happens is that the incident rays heat the earth and this heat heats the adjacent layer of air and so on. This heat becomes weaker as we rise, up to he cold stratum of the atmosphere (falak az-zamharīr).21 Some people adduce the distance of the sun to the zenith as the only cause for heat and cold and pay no attention to the length of the days. They think that the countries at the equator are intensely hot due to the sun being in the zenith. They say that the heat there is not as harmful and corrupting as the heat in other climatic zones, because at the equator the variations in heat and cold are small, whereas in other climates heat comes after cold and is followed by it. However, in fact, continuous heat is more harmful for plants and animals than alternating periods of heat and cold. Bodies that in winter do not get their share of cold which opposes their internal heat and which is a condition for proper internal moistness, will not be healthy in summer. Any harmful effects of coldness in winter will be compensated by heat in summer and vice versa and in this way a balanced condition is retained, if one considers the matter over an entire year. Due to the equality of day and night, the climate at the equator is always moderate and people are in a balanced condition at any time, which is even better. The former situation is like the situation of someone who is hungry and then eats his fill, or is ill and then recovers, whereas the latter is comparable to the situation of someone who is never hungry or ill. The climate at the equator is like a continuous spring, and the trees bear fruit monthly, not yearly. Thus, the main cause of heat in summer and cold in winter is the length of the day and the moderate climate at the equator is due to the fact that the lengths of day and night are always equal. The main cause of additional heat in summer and cold in winter is the distance of

21

Abū 1-Barakàt, al-Mu'tabar II 205,10-21. The expression kurat az-zamharlr by e.g. the Ifcwān as-Safā', see above p. 53.

is used

the sun to the zenith. That the duration of the shining of the sun determines heat and cold appears from the fact that it is warmer in the afternoon than at midday. That the distance to the zenith is another determining factor appears from the fact that at sunset it is cooler than in the afternoon. If only the duration played a part, then it would always become hotter during the day and it would only start to become cooler after sunset. If only the distance to the zenith played a part, then the heat would be more intense when the sun is in Cancer than when it is in Leo and would be more intense at midday than in the afternoon. In fact, the contrary is the case.22 Other factors that influence the heat are for instance: It is colder on mountains and hotter in depths. In depths the sunrays are reflected to the middle, like in a concave mirror; also, no winds blow there that bring cold air from snowy areas. The mountains are cold, for there the exhalations have thrown off their heat and clear winds blow there. Furthermore, winds influence the heat of the place over which they blow: land winds that carry little rain or snow are hot and dry; winds from cold and snowy mountains and from areas of sweet water are cold and moist. Sea winds are hot and putrefying. Mountains may protect people from the hot, dry land winds and the hot, putrefying sea winds and turn away their harmful effects. If they turn away cold, moist winds, then this makes the area warmer and drier; if they turn away hot winds, then this makes the area cooler.23 Heat is caused by the sunrays, but one cannot say that cold is caused by the absence of sunrays, because cold is something that exists and cannot be caused by something not-existing. Cold is something that exists, for it is an efficient cause: it cools and causes cold, like heat causes heat. Cold by nature exists in earth and water and no special cause is needed for the existence of cold in these materials. If the cause that brings about its contrary and prevents its existence disappears, then it exists in its proper place and influences the adjacent places. Therefore, the cause of the cold in winter is that earth and water return to their natural condition after the heat of the summer has disappeared. Then water freezes, which occurs either because water is frozen by nature and is liquid only accidentally when it is heated, or it is liquid by nature and is accidentally frozen by the cold of the earth. Earth is the coldest element, for it is the most dense element and cold densifies and solidifies.24

22 23 24

Abū 1-Barakāt, al-Mu'tabar ibid. 207,6-24. ibid. 208,1-13.

II 206,1-207,6.

Fakr ad-Dīn gives Ibn Sînâ's account of the inhabitated places of the earth. 25 Ibn Sînâ's view is that, contrary to what the Peripatetics thought, the area at the equator is the most moderate place on earth. He proves this, says Fakr ad-DIn, by using the principle that the degree to which a thing is heated depends not only on the intensity of the source of the heat, but also on the duration of the process of heating. This principle is applied to explain the following facts: (1) Our region is less hot when the sun is in Cancer than when it is in Leo, although in Cancer it is nearer to the zenith. (2) Our region is hotter when the sun is in Leo than when it is in Gemini, although its distance to the zenith is equal in both cases. The same holds for Virgo and Taurus. (3) Iron that is heated a long time in a feeble fire becomes hotter than when it is heated for a short time in an intense fire. (4) Heat is more intense after the sun has reached the solstice than before, although the position of the sun in relation to the zenith is the same. (5) Cold is more intense at dawn, just before sunrise, than in the middle of the night. These facts may be explained as follows: the moment a cause starts to work, it has an effect; the effect of the next instant is added to that of the first one. Consequently the longer a cause works, the stronger the effect will be.26 Then it is explained why the area at the equator is the most moderate place on earth, more moderate than the areas near the solstices. The account is an excerpt of Ibn Sînâ's text.27 Fakr ad-Dīn opposes this opinion of Ibn Sīnā as follows: Let us consider a place Ρ on earth with a latitude equal to twice the inclination of the ecliptic. When the sun is at its closest to P, then its distance to the zenith of Ρ is equal to its distance to the zenith of the equator and it heats these areas with a certain intensity of heat H. Before that the sun was closer to the equator and heating that area, whereas it was further from P, which means it was cold there. Thus, the sun has been heating the area of the equator the whole preceding year with a heat of intensity Η or with a more intense heat, whereas during that time Ρ was heated with a heat less intense than H. If we assume (with Ibn Sīnā) that the actual heat is the result of heat accumulated during the preceding time, it is clear that when the sun is at its solstice close to P, the area of the equator must be hotter than P. When the sun recedes from that solstice, the equatorial region will become even hotter. When Ibn Sīnā says that at the equator the sun is only a short time in the zenith, this is true,

25

Fakr ad-Dīn, al-Mabāhil II 198,4-199,14 is similar in text to Ibn Sînâ, Tab. 5 24,7-25,20 and 26,17-27,9. 26 Fakr ad-Din, al-MabāhiL II 199,14-200,19. 27 Fakr ad-Dīn, al-Mabàhií II 200,20-201,16 is similar to text from Ibn Sīnā, Tab. 5 29,4-30,15.

aš-Šifà',

aš-Šifâ',

but the sun never recedes very far from the zenith, so how could the heat here not be very strong? When he says that day and night are equally long at the equator, whereas the days are longer in summer in other places, then we answer that this has little influence on the heating. For, at the poles the day lasts six months and still it is very cold. Also, in winter nights are longer in places other than the equator and then cold settles in the atmosphere; this prevents it becoming very hot in summer. At the equator cold never becomes strong at night, since the nights are as long as the days.28 Fakr ad-DIn further remarks that someone might argue that if the sun is in the perigee, it is closer to the earth and thus heats it more. Therefore the area of the circle of the perigee must be warmer than that of the equator. The answer is that the excentricity of the sun is not much and will not have much influence on the heating. Ibn Sinä has acknowledged this,29 but it should be remarked that the apogee of the sun moves.30 Now it is at the end of Gemini; suppose it arrives in Libra, then the perigee will be in Aries and the circle of the perigee will coincide with the equator and then this will be the hottest place on earth.31 Finally Fakr ad-Din elaborates on Ibn Sînâ's observation that for people at the equator there is not much difference between the seasons. He says that one might say that they have eight seasons: two summers, viz. when the sun is in the zenith, which occurs twice a year, and two winters, viz. when the sun is in the north and south solstices. In between there are two springs and two autumns. It is discussed when these seasons exactly start, in terms of the position of the sun in the zodiac.32

5. Ibn Ru'sd Ibn Rušd says in his Short Commentary that the inhabited places take up less than one-sixth of the earth's surface, namely about a seventh part. The length of this area has been deduced from observation of lunar eclipses in countries in the most extreme east and west. It has been found that they occur in eastern and western countries with a difference of not more than twelve hours, i.e. 180° in length. As for its

28 29

30

Fakr ad-Dīn, al-Mabāhit_ II 201,18-203,8. Ibn Sīnā, as-Sifa, Tab. 5 25,20-26,3. *

The motion of the apogee along the ecliptic has been observed by l i b i t ibn Qurra, al-Birüni and az-Zarqâli; see Sezgin, GAS VI 26-27. 31 Fakr ad-Din, al-Mabähil II 203,9-17. 32 ibid. II 203,18-204,14. '

breadth, one has found that the furthest country that can be reached in the south is a little more than thirteen degrees south of the equator and the furthest country in the north is sixty degrees from the equator. We shall discuss whether this result is possible from observation or not.33 Aristotle and the Peripatetics have claimed that the inhabitable part of the earth, insofar as it depends on the sun, extends from the northern and southern tropical circles; the area near the equator is not inhabitable because of its heat, whereas the areas in the north and south far from the tropical circles are not inhabitable because of their cold. Ptolemaeus and his followers thought that habitation was possible until below the equator. Ibn Sīnā followed him and thought that the area under the equator was the most moderate climatic zone. He found the opinion of the Peripatetics in contradiction to what people had found by observation and argumentation.34 We say that it is clear that heat is caused by the sun when it is close to the zenith and its rays reach the earth at an angle that is right or approximately right, for then reflection is most strong. The climatic zones differ in being more or less hot, due to the different angles at which the sunrays reach the earth.35 We know there are many inhabitable countries where the sun passes the zenith, for this occurs in southern countries when the sun is in the summer solstice. Thus, we would think that habitation is possible too in the region of the equator. However, this is not a conclusive argument. We know by observation that the most moderate climatic zones for man and most animals and plants are the fourth and fifth zones.36 The other zones are extremely hot or extremely cold. If the degree of heat were determined only by the angle at which the sunrays reach the earth, then would be possible that people lived on the equator, although not in moderate conditions, as Ibn Sīnā says, but in the way people live in countries where the sun reaches the zenith. If there is another cause that increases the heat of the area near the equator, then habitation will not be possible there. This is what we shall investigate now.37 We find that in countries of different climatic zones most heat m

Ibn Rušd, Short Commentary 44,9-45,2. The extension of the inhabitable section of the earth is that given by Ptolemaeus, Geographia 1,7-10. 34 Ibn Rušd, Short Commentary 453-14. See above p. 198 for Ibn Sînâ's view. 35 A climatic zone (iqlīm ~ ΚλίμΟί) is an area north and south of a certain parallel of latitude, where the sunrays fall on the earth at a certain angle. Each climatic zone is determined by the period of the longest daylight M. Traditionally, seven of such zones are distinguished, in which M varies from 13 to 16 hours, with intervals of one half hour. See Neugebauer 725. 36 These are the zones around the parallels of Rhodes and the Hellespont. 37 Ibn Rušd, Short Commentary 45,15-47,2.

occurs in summer, after the sun has reached the summer solstice, as was discussed above (see above p. 189). This heat remains about three months in the moderate countries, such as in our country al-Andalus. In other countries, more to the south, the period of heat is longer than these three months, because they are heated by the sun more strongly. The countries with a latitude that is about half the latitude of the moderate countries, i.e. the countries where the sun reaches the zenith when it is in the summer solstice, will be about twice as hot in summer as the moderate countries, and the heat will remain about twice as long: five or six months. Observation by people who have been there confirms this. Ibn Rušd mentions an observation by himself: he has visited a country at about 30° latitude and experienced that the heat remained four months after the summer solstice. He says that this may also be shown by argument: the shadows of the people there fall to the south during four months and that must be the period of heat with them. It follows that the area near the equator must have one season only, with the most extreme heat. For the sun recedes from the zenith during three months; after that the air might start to become less hot, but then the sun approaches the zenith again and the heat remains. Plants and animals cannot live there, because they are dependent on four seasons for their existence. Thus Aristotle's account is correct, that just as there is an uninhabitable area in the north because of the cold, there is an area in the south that is uninhabitable because of the heat. This may be deduced by a universal argument: if one of the extremes of a contrariness exists—that is, an area that is uninhabitable due to cold—the other extreme must exist too; therefore there must be an area that is uninhabitable due to heat. If there are countries of frost and ice, there must be countries of ebullition and flame.38 Aristotle has explained that in the southern hemisphere there must be an inhabitable area corresponding to our inhabitable area. If the excentricity of the sun has a perceptible effect, the southern inhabitable area will be closer to the south pole and further from the equator in comparison with the northern inhabitable area.39 However, if there were an inhabitable area on the southern hemisphere too, then the largest part of the earth would be land, since the area between the tropics is also dry because of the heat; not much water is present there. The elements in the world must exist in such quantities that they balance one another, otherwise they could not continue existing. A rare, 38

Ibn Rušd, Short Commentary 473-49,4. When the sun is near the winter solstice, it is closer to the earth and heats the southern hemisphere more strongly than the northern hemisphere is heated when the sun is in the summer solstice. 39

easily affected element will exist in balance with a dense element which is affected with difficulty, if its quantity and volume are larger. Therefore the area of water in the world must be larger than the area of land. Consequently, the only inhabitable part of the earth is our part; if there were another inhabitable part in the southern hemisphere, then there would be more earth than water.40 In the Middle Commentary Ibn Rušd quotes Ibn al-Bitriq's text, which states that the earth is divided into two parts, one being inhabitable, the other uninhabitable. The uninhabitable part consists of two parts: one of them is very hot due to the proximity of the sun—this is the southern part—the other part is very cold due to the remoteness of the sun—this is the northern part.41 From this text Ibn Rušd got the impression that it refers to the northern hemisphere only, and that nothing is said about the southern hemisphere. He says that the commentators usually divide the earth into five sections, two inhabitable ones in the northern and southern hemispheres, two uninhabitable ones near the poles and an uninhabitable one between the tropics. He comments that Aristotle did not mention the southern hemisphere, because, according to him, water must be the prevailing element on the surface of the earth, i.e. there must be more water than earth, otherwise the elements are not well-balanced.42 If there were an inhabitable area in the southern hemisphere, then there would not be more water than earth.43 Then Ibn Rušd considers the arguments why some sections of the earth are inhabitable and others are uninhabitable. The region near the north pole is uninhabitable, because snow and ice prevail. Also, the proportion of the lengths of day and night, which is such that at the pole itself it is day during one half of the year and night during the other half, is an unnatural condition for beings that are subject to generation and corruption. Coldness prevails because the sun is far from that region. Also, the parts of the celestial sphere which are opposite that region move slower and therefore less heat is produced. It may be shown that the area near the equator is uninhabitable too. People who live near the summer tropic, such as in Ethiopia, have the sun vertically above them at the time of the summer solstice. They live in unnatural circumstances and can only survive in caves; animals take their refuge under stones and in water. If this is the smallest latitude 40

Ibn Ibn 42 The 43 Ibn results of 41

Rušd, Short Commentary 50,2-51,8. al-Bitriq, Meteor. 71,6-12. same argument is also in the Short Commentary, see above pp. 206-207. Rušd, Middle Commentary 111,1-112,4. This paragraph again indicates Ibn al-Bitriq's distorted version of Aristotle's text.

the

where man can survive, then it is clear that at a still smaller latitude, where the heat is stronger, no habitation is possible. That region is hotter, because there the sun reaches the zenith twice a year. It is nonsense to say that the region near the equator is moderate.44 Furthermore, Ibn Rušd proves that if there is an uninhabitable region due to cold, there must also be such a region due to heat, because if one of the extremes of a contrariness exists, the other extreme must exist too.45 Another argument is that if there is a region that is extremely cold due to the remoteness of the sun and the slowness of the motion of the celestial sphere, there must also be a region that is extremely hot due to the proximity of the sun and the celerity of the celestial motion. There is also an intermediate region where the sun is neither remote nor close and that is the moderate, inhabitable section, where, for instance, Cordoba lies at a latitude of 38.5°. If there is one extreme and an intermediate, the other extreme must exist too. Ibn Rušd concludes by saying that other proofs may be adduced, such as those in the Short Commentary46

44 45 46

Ibn Sīnā said this, as Ibn Rušd remarks in his Short Commentary, see above p. 198. The same argument was given in the Short Commentary, see above p. 206. Ibn Rušd, Middle Commentary 112,9-116,17.

CHAPTER SEVEN

EARTHQUAKES

1. Aristotle Aristotle's discussion of earthquakes (σεισμός) starts with an account of the view of others. Anaxagoras said that ether, which by nature moves up, enters into hollow parts of the earth from below.1 When it does not find a way out, because the upper part of the earth is clogged by rain, it shakes the earth. This view is rather silly, because the earth is a sphere; this is indicated by the fact that we see the horizon change as we move. Also, it cannot be shaken all through by a shock from below. Furthermore, this theory does not explain why earthquakes occur at specific times and in specific places. Democritus says that the earth is full of water; when much rain falls, the hollow places cannot contain the water anymore and it forces a way further inwards. Also, when the earth is drying, water is drawn from the fuller to the emptier places, and the shock of its passage causes an earthquake. Anaximenes says that when the earth becomes wet or dry, it breaks and large masses breaking and falling from elevated places shake the earth. Thus, earthquakes occur during droughts and when there is heavy rain. If this theory were right, however, we would find the earth sinking in many places. Also, earthquakes do occur in places that are not excessively dry or wet. Furthermore, this theory implies that the earth is gradually filled up, so that earthquakes become less frequent and will finally cease (365a18-365b20). Aristotle's own theory starts from the theory of exhalations. The earth is dry by itself, but contains much moisture because of the rain. If it is heated by the sun and its own internal heat, the moist earth dissolves much dry exhalation that becomes wind. If this wind flows inward it becomes the cause of an earthquake. Wind has the strongest motive force—stronger than earth and water, for it can get the greatest velocity, and thus the most forceful impulse. This explains why most of the earthquakes, and the strongest, occur in calm weather: then all exhalation flows inwards. Earthquakes also occur when a wind is 1

According to Anaxagoras, the earth is a flat disc.

blowing, but then they are not so strong, because the exhalation divides itself into an inward and an outward flow. Most earthquakes occur at night, for then the weather is calm because of the absence of the sun and the flow of exhalation retreats inside the earth, especially at dawn, when winds usually start to blow; if they occur during the day, then it will be at midday, for at that time it is calm because the heat of the sun confines the exhalation inside the earth. Thus, at these times the outward flow of exhalation turns inwards again. Furthermore, earthquakes are strongest in places where the sea has many currents, because a violent wind, which would naturally flow outwards, is driven back into the earth by the sea flowing into the hollow spaces under the coast, or where the earth is porous and hollow, because these places can absorb much exhalation. Most exhalation is produced in spring and autumn, not in summer—then it is too hot and dry—nor in w i n t e r then it is too cold; therefore earthquakes mostly occur in spring and autumn. They occur during rain, for much evaporation is produced at such an occasion, and during drought, for drought means that much dry exhalation has been produced (365b21-366b14). Aristotle compares earthquakes with processes in our bodies that are also caused by a kind of organic exhalation flowing in, such as tremors, throbbings, tetanus and spasms and the tremor that we experience after having urinated. Further evidence that confirms his theory: sometimes the wind that caused an earthquake breaks out into the atmosphere and becomes a hurricane; after that the earthquake subsides. In one of the Liparian islands part of the earth swelled up, rose and finally burst. Then wind came out of it, with cinders and ashes. Fire in the earth is generated in this way: exhalation is beaten about in narrow subterranean cavities and thereby catches fire. An indication that winds are moving below the surface of the earth is that south winds are announced by noises from the places where the eruptions occur in the Liparian islands. This occurs because the wind that comes out of the earth is driven back into it by the oncoming sea. This causes a noise, not an earthquake, because the subterranean space is large compared to the quantity of wind (366b14-367a20). More confirmatory phenomena: before an earthquake occurs the sun becomes hazy and dark, although there are no clouds, because the wind, which makes the air thinner, returns into the earth. This also causes calm weather and coldness, because the nature of dry exhalation is warm, although winds are usually not felt to be warm, because the air also contains much cold vapour. This is exemplified by the air we breathe out: near by it is warm, further away it is cold because of

admixture with cool vapour. Another sign that sometimes precedes an earthquake is a cloud appearing in the form of a long straight line. This means that the wind is subsiding above the earth and returning inside it. The effect may be compared to the breakers of the sea that become small and straight when the wind subsides. Also, a lunar eclipse is a sign of an earthquake, for when the earth is approaching its interposition, the heat coming from the sun via the moon decreases, and then the wind returns inside the earth, because exhalation is not drawn out and driven upward anymore 2 (367a20-367b33). The shocks of an earthquake may continue for a long time. This occurs when a large amount of wind is dissolved and the subterranean spaces are narrow, so that the wind does not easily find a way out. The earthquake will continue until the exhalation is exhausted, the shocks gradually decreasing in force. Subterranean noises are due to wind striking hard masses and cavities within the earth. An earthquake may or may not follow. The sound precedes the shock, because it is thinner and more penetrating than the wind itself. If the force of the wind is too weak, because it can easily flow out of the earth due to its thinness, no earthquake ensues, but it causes the sounds called 'bellowing'. Sometimes water bursts out of the earth during an earthquake. This does not mean that the shock is caused by water; it is caused by wind, which forces water out from beneath, as it also may force out earth. An earthquake may be accompanied by a tidal wave (κΰμα). This occurs when opposite winds are present. One of these winds causes the earthquake, the other moves the sea. The sea is pushed back by the wind which causes the earthquake and is heaped up in one place. When this wind gives way, retreating within the earth and causing an earthquake, the whole mass of the sea will burst out, driven by the other wind, and cause a flood on the land. When the flood in Achaea occurred, a south wind was blowing over the land and north wind came from the sea. One wind retreated within the earth and a tidal wave occurred together with an earthquake. Earthquakes are local and limited to a small area, whereas winds are not. Like local rain or drought, earthquakes are local because exhalation in a certain place is joined by that from adjacent places and this concentration in one place in the earth is not disturbed by the influence

2 Contrary to this phrase, Aristotle next says that a lunar eclipse is often preceded by wind, because the heat of the moon decreases as it approaches its eclipse, and it was this heat that prevented the air from moving. See the commentary in Strohm 1984 198. 3 Cf. above Chapter 5 p. 156.

of the sun. As for the wind, the exhalation in the air is influenced by the sun that gives it an impulse so that it flows in a certain direction. The shocks of an earthquake are horizontal, like a tremor (τρόμος), or vertical from below, like a throb (σφυγμός). The latter kind occurs less frequently, because sufficient exhalation collects on the surface more easily than in the depths of the earth. In a vertical earthquake many stones come to the surface of the earth. Earthquakes more often occur on islands near the coast than on islands in the middle of the sea. The sea cools the exhalations, and its weight pushes them back and prevents their formation. The wind causes currents in the sea, not shocks. Because of the extension of the sea exhalation does not collect in it, but comes out of it; therefore the sea is not shaken. Islands near the coast are affected in the same way as the mainland (367b33-369a7). Alexander's commentary has no special features; Olympiodorus' commentary is lacking in the manuscripts.

2. Ibn al-Bitriq and Hunayn ibn Ishāq Ibn al-Bitriq gives the account of how an earthquake arises as follows: the dry exhalation that is dissolved from the earth is the matter of wind. This exhalation arises in two ways: one way is the exhalation that comes out of the earth as wind that is rising upward, and the other is the exhalation that becomes wind within the earth; this wind is moving about (idtaraba), and thus causes an earthquake. Hunayn ibn Ishāq has a similar formulation. He adds that the exhalation that is generated within the earth moves upward by nature, but the solidity of the earth's surface prevents its emergence.4 An indication that an earthquake is caused by subterranean wind is that among the elements only wind has enough motive power to shake the earth, just as it is also able to inflame fire and to stir up water. Furthermore, earthquakes mostly occur when wind is blowing. They occur at night more than during the day, because then the sun is distant; when they occur during the day, then it is at midday and in the morning, for there is much wind blowing at these times. Furthermore, they occur mostly in spring and autumn and in times of rain and drought, because much wind is blowing at such times. They seldom occur in summer and winter, because extreme heat and cold prevent that wind arises. They occur in times of drought, because much dry

4

Ibn al-Bitriq, Meteor. 75,7-10; Hunayn, Jawāmi' 201-206.

exhalation is dissolved at that time, and in times of moistness, because moistness densifies the earth and clogs the places from which exhalation may emerge, so that the exhalation is confined and moves about, thus causing an earthquake. Passages in Hunayn ibn Ishāq correspond to this account, in a similar formulation.5 Note the contrast with Aristotle, who clearly says that earthquakes mostly occur when the wind (viz. the wind above the earth) is not blowing, at night, at midday and in the morning. One may justify the statements in Ibn al-Bitrlq and Hunayn ibn Ishāq by pointing out that at these times much dry exhalation (wind) is dissolved, but that it is confined below the earth and no wind above the earth arises, in other words, that Ibn al-Bitriq refers to subterranean wind. This is how Ibn Rušd in his Middle Commentary can say that both accounts, those of Ibn al-Bitrlq and Aristotle (which he knew from Alexander), are true (see below p. 223). Ibn Tibbon also notes the difference between Ibn al-Bitriq and Alexander.6 The argument that rain and moisture clog the surface of the earth, so that the exhalation is confined within, is not used by Aristotle. Ibn al-Bitriq further says that on a certain island the earth kept rising during an earthquake until a kind of hill (rābiya) was formed; then the place burst and a strong wind emerged, together with ashes. Apparently the earth was burned inside in that place.7 The reason why the sun becomes dark when an earthquake occurs is given by Ibn al-Bitrlq as follows, unlike Aristotle: when the wind that causes the earthquake comes out of the earth, it turns into a thick exhalation that makes the air turbid. Hunayn ibn Ishāq is more explicit: the sun becomes dim after an earthquake because the wind that comes out of the earth carries with it dust and sand that make the air turbid. 8 Ibn Tibbon remarks that Ibn al-Bitrlq differs from Aristotle here. Ibn Tibbon knows Aristotle's explanation from Alexander.9 Furthermore, Ibn al-Bitriq states that earthquakes may occur at the time of a solar eclipse (Aristotle: a lunar eclipse), because then the heat of the sun no longer arrives in the atmosphere, so that the exhalation remains confined inside the earth. Hunayn ibn Ishāq mentions both eclipses, the lunar and the solar eclipse, as occasions during which earthquakes may occur.10 5

Ibn al-Bitriq, Meteor. 75,10-77,3; Hunayn, Jawāmi' 206-208 213-220. Ibn Tibbon, Otot ha-Shamayim II 436-439. 7 Ibn al-Bitriq, Meteor. 77,8-78,1. 8 ibid. 78,4-7; Hunayn, Jawāmi' 222-224. 9 Ibn Tibbon, Otot ha-Shamayim II 470-481. 10 Ibn al-Bitriq, Meteor. 79,1-4; Hunayn, Jawâmï 224-229. 6

Sometimes the earth turns upside down (qalaba), so that what was below becomes above, and sometimes water gushes out that floods the land. Hunayn ibn Ishāq has a similar phrase. Its origin is Aristotle 368a31.11 It again appears that Ibn al-Bitriq's version gives Aristotle's views in a distorted way, because of misunderstandings that possibly already existed in the Greek or Syriac version he translated. The differences with Aristotle's text are noted and discussed by Ibn Rušd (Middle Commentary) and Ibn Tibbon; they knew Aristotle's text from Alexander's commentary. The clogging up of the earth's surface at times of rain is an additional argument, not used by Aristotle in that context.

3.

Pseudo-Olympiodorus

Pseudo-Olympiodorus follows Aristotle, but his systematizing efforts sometimes lead to additions and deviations from Aristotle. First, the opinions of Anaximenes, Anaxagoras and Democritus are mentioned and refuted. According to Anaximenes, earthquakes occur when the tops of mountains fall down; this may occur due to rain or drought. This may be refuted by three arguments. If this view were right, then (1) the earth would become level because the falling high parts would have filled the depressions. (2) There would be no earthquakes in regions without mountains, such as Egypt (different from Aristotle). (3) Earthquakes would occur less and less, because the mountain tops that cause them by falling become fewer. Anaxagoras says that the earth is flat and is carried by air, just as a leaf or a thin plate of gold floats on water. The inner part has a rare, porous structure and the surface is dense and firm due to rain that has fallen on it. When air penetrates into the inner part, it does not find a way out and shakes the earth. This view may be refuted by four arguments. (1) One cannot distinguish an upper and a lower part of the earth. If that were possible, some parts would move down, others would not; in fact, all parts move down; thus, 'under' means: towards the middle of the world. (2) If the earth were flat, places would not differ in respect of how the light of the sun is received and which area of the sky is visible. In fact, we find that the light of the sun arrives in places with different longitude at different times and that in places with different latitude different parts of the sky are visible. It follows that the earth must be a sphere. (3) If the earth is carried by the air, how

11

Ibn al-Bitriq, Meteor. 79,11-80,2; Hunayn, Jawāmi' 229-231.

could it be shaken by it? (4) The theory does not explain why earthquakes occur in specific places and at specific times. Democritus says that the earth is full of water; if more water is added due to rain, a pressure is exerted that shakes the earth, like a jar full of pressed juice ('aslr) is broken by the wind that develops in it. When it is not raining, an earthquake arises when the dry earth attracts moisture and that moisture enters the pores. This resembles the shiver we feel in our body after having urinated, because wind and hot moisture enter the bladder {cf. Aristotle 336b19 ff.). This view may be refuted by pointing out that the mass of water is not confined within the earth, as we have seen before.12 Aristotle's opinion that earthquakes are caused by subterranean wind is confirmed by eleven arguments. (1) Only wind has the motive power to cause such a vehement motion. (2) Earthquakes mostly occur when there is no wind above the earth, for then the exhalation is confined within the earth. (3) Earthquakes mostly occur at night, or in the morning, or at midday; at night, because then the flow of exhalation retreats within the earth; in the morning, because then the sun starts to dissolve exhalation, but it does not yet spread it into the atmosphere; at midday, because the heat confines the exhalation within the earth. The heat accidentally causes the same effect as the cold of the night causes by itself.13 (4) Earthquakes occur in places that are porous and in places where the sea has a strong current. In porous places much exhalation may be gathered. In places where the sea flows on the surface, the earth becomes dense and the exhalation does not find a way out (different from Aristotle). (5) Earthquakes occur more in spring and autumn than in summer and winter. In winter the exhalation freezes; in summer it is dissolved and spreads into the atmosphere. An earthquake in winter is an indication that the atmosphere is more moist than cold, because moistness makes the earth wet and this generates much exhalation. An earthquake in summer is an indication that the atmosphere is more dry than hot, because then much dry exhalation is gathered. (6) Tremors, throbbings and spasms in the body are also caused by wind, such as the tremor we experience after having urinated. The latter is caused by wind that has entered the bladder that has been emptied. (7) Often after an earthquake the earth is split and a wind comes out, sometimes accompanied by ashes and sparks of fire. This once buried one of the cities in Italy. (8) A sound is often heard together with or after an earthquake; this is caused by the wind that 12

Pseudo-Olympiodorus, Tafsir 133,7-135,21. The formulation is an indication that the author thinks of here, see above p. 107. 13

άντίπερίστοίτσίζ

causes the earthquake and moves about in narrow hollows. If the wind has enough space a sound is heard, but no earthquake occurs. (9) Often before an earthquake the air becomes turbid, as the wind stops blowing and exhalation remains in its place, in front of the sun. The dropping of the wind also causes coldness, except when it occurs at midday, due to the heat of the sun (addition to Aristotle). The fact that the nature of wind is hot, whereas we generally feel it as cold due to admixture of other air containing cool vapour, is illustrated by Pseudo-Olympiodorus with Aristotle's example of the air we breathe out (see above pp. 210211). He extends the example, saying that when we breathe out in a bath, the air further away is even hotter than nearby. Furthermore, when we slowly move a fan it cools a little; when we move it more fiercely the cooling is stronger. (10) Often before an earthquake a thin cloud appears as a long straight line. The cloud arises because when two contrary winds are blowing, and the weaker is stopped by the stronger and retreats within the earth, causing an earthquake, the air and the vapour cool down. The cloud is fine because it is near the smoky exhalation and is heated by it a little. The cloud is straight, because it is moved by one constant wind. It is long, because the wind takes up an extensive area. (11) Earthquakes often occur during a lunar eclipse, because the air cools when the light of the moon is absent. Then the pores on the surface of the earth densify, so that the smoky exhalation is confined within the earth.14 The duration of an earthquake depends on the amount of wind causing it and on the hollow spaces in which the wind moves about. If there is little wind, the earthquake is of short duration; if there is much wind it may last for a longer period. If the hollow spaces are straight, the earthquake lasts a short time; if they are tortuous, it may last a long time.15 The different sounds that may be heard when the wind penetrates into the subterranean spaces, like sounds similar to thunder or to the bellowing of a cow, are caused by the different shapes of these spaces. The sound and the earthquake occur simultaneously, but we hear the sound before feeling the earthquake, because the sense of sound is of finer structure than feeling. If we were to see the earthquake we would perceive it quicker than we would hear the sound. This is comparable to our seeing lightning before hearing thunder, although both occur simultaneously; for the sense of sight is finer than that of hearing.16

14 15 16

Pseudo-Olympiodorus, Tafsir ibid. Tafsir 138,19-139,2. ibid. Tafsir 139,4-17.

135,23-138,17.

Sometimes water is thrown out of the earth during an earthquake by the wind causing that earthquake, just as earth is also sometimes thrown out. If two contrary winds blow simultaneously, and the stronger one stops the weaker one that was heaping up the waves, the stronger one will move the waves and cause a tidal wave (tūfān). The weaker one retreats inside the earth causing an earthquake, because it cannot find a way out since the water has closed up the exits.17 Although earthquakes and winds arise from the same substance, sc. smoky exhalation, earthquakes are restricted to one place, whereas winds blow over an extended area. The reason is that (1) the exhalation causing an earthquake moves in narrow subterranean places, whereas for wind an extended area is available. (2) The sun does not have much influence on the exhalation of an earthquake, as it occurs below the surface of the earth, whereas it has a large influence on the exhalation of wind. (3) Subterranean exhalation gathers in one place, whereas wind spreads into various directions.18 Earthquakes may extend horizontally, like a tremor (iktilājī ri'sî), vertically, like a throb (qar'a, intifāk), or in both directions. In a vertical earthquake stones are thrown out together with the wind. An earthquake that goes vertically as well as horizontally is like a ladder (sullam).19 Finally, Pseudo-Olympiodorus mentions that earthquakes seldom occur on islands in the middle of the sea. The reason for this is that (1) exhalation is cooled by the water from the sea. (2) The weight of the water prevents the exhalation from rising; instead, it moves to neighbouring places. (3) The sea is flowing, not something solid, therefore it cannot be shaken and the islands cannot be shaken either. (4) The exhalation that arises in these islands joins with the large amount of exhalation that is already dissolved from the surrounding sea; together they move to neighbouring places.20 17

Pseudo-Olympiodorus, Tafsïr 139,19-140,6. ibid. 140,8-17. 19 ibid. 140,19-141,2. Authors after Aristotle divided earthquakes into different kinds; this division offers some problems. Theophrastus distinguishes three kinds: some earthquakes are like a tremor ( i k f i l â j ī ) , some are inclining with an inclining movement (mail wa-muntaqil al-mayalàn) and some are just inclining (mà'il faqat) (see Theophrastus, Meteorology ch. 15 and Steinmetz 1964 206). The Pseudo-Aristotelian work De Mundo distinguishes inclining earthquakes (έπίΚλίνίΟίΐ), shaking earthquakes (βράσίοα), collapses (ίζημοαίοα) and ruptures (pfjlCTOa) (see 396b36 ff. and Strohm's commentary). Instead of the word έπίΚλίνίΟίΙ one also finds Κλψοαίοα. We shall not go into the problem of how these different distinctions may be related; see Daiber 1992 282 and 290-292. We only remark that the designation 'like a ladder' will refer to επίκλίντοα or κλίμοαίοα, via corruption or misreading of the latter word, as if it were κλίμακες (ladders). 18

4. Ibn Sīnā Ibn Sīnā states that an earthquake is a motion of a part of the earth, caused by something within the earth that moves about and thereby moves what is above it. Then he discusses which kinds of body qualify to move within the earth and to cause the earth to move. It could be a smoky exhalation, such as wind: wind is forceful, for it may break a jar when it is formed from the pressed juice ('asīr) in it. Or it could be flowing water, or air, or fire, or earth. Fire is not present within the earth in its pure form, although it does occur in the form of exhalation or wind that is inflamed. Earth does not move, except by the moving cause we are looking for. Thus, it must be the windy exhalation, that may be fiery or not fiery and that is dissolved below the earth, which shakes the earth in most cases. If air moves by itself, this only occurs in the form of windy, smoky exhalation. Air may also be moved by something else, as occurs e.g. when water that suddenly flows in a depth moves air, or when a pit or cavern that collapses because its support gives way moves air and adjacent earth, or when a roof falls down. It is mostly windy or fiery exhalation that causes an earthquake, for wind has the strongest motive power to move earth. Other possible causes are water that suddenly gets a vigorous flow—this is Democritus' view—and the collapsing of that which supports the ground.21 Sometimes an earthquake is caused by something above the earth, such as a mountain, when the top or a large part of it falls down. Anaximenes said that this is the only cause of an earthquake, but the moving power of earth is hardly enough to cause such a powerful motion that it should be called an earthquake. He said that earthquakes occur in times of rain and of drought. When a lot of rain falls, the mountain peaks become soaked and fall apart. When there is a drought, they crumble and also fall apart. This theory does not explain why earthquakes often occur in countries where no mountain top falls down. If all earthquakes were to occur in this way, they would decrease in force. Moreover, earthquakes in countries without mountains would not sometimes be stronger than those in countries near mountains.22 According to Anaxagoras, the earth is carried by air that supports it; the lower part is rare of structure, whereas the part where we live is denser because of the rain that falls on its surface. If air penetrates in that lower rare part and does not find a way out, the earth is shaken. This view may be refuted by pointing out that the condition of the 20 21 22

Pseudo-Olympiodorus, Tafsir 141,4-13. Ibn Sīnā, aš-Šifa, Tab. 5 15,5-16,4. ibid. 16,5-15.

earth is not like that and that this does not explain why there are earthquakes in certain seasons and not in others.23 Ibn Sînā enumerates the phenomena that accompany earthquakes as follows: They may be useful or harmful. A useful aspect is that the wind causing an earthquake may contain vapourous matter which is carried to a certain part of the earth and then breaks forth from the earth; in this way a source may be formed. A harmful aspect is that such wind may also contain dry, fiery matter which inflames when it moves vigorously; a vigorous motion changes air, vapourous and smoky exhalation into fire: bellows often catch fire when they are made to blow vigorously. If the cause of the earthquake is very powerful, the earth sinks and sometimes burning fire comes out and tremendous noises are heard. If a vast space is available for the wind after it has been in the places where it caused the noise, no earthquake results.24 An indication that earthquakes are mostly caused by wind that is confined within the earth is that when one digs many wells and canals in regions where earthquakes are frequent, so that wind and exhalations have many exits, the number of earthquakes diminishes. Furthermore, earthquakes mostly occur in calm weather, for then the matter of wind is confined inside the earth. Then one often sees long clouds; they arise when two different winds are blowing; one of them dominates and extends further, while the other retreats inside the earth.25 Wind often arises when an earthquake is finished: the efficient cause comes out of the earth. Often there are stagnant clouds in the sky when an earthquake occurs, or the atmosphere is misty. This is due to the fact that the wind which usually blows them away is absent. Sometimes an earthquake arises after opposite winds prevented one another from blowing and prevented their matter from emerging from the earth, so that it remains confined within the earth. Windy matter remains enclosed within the earth mostly at night or in the morning, when cold densifies the surface of the earth; therefore an earthquake often occurs at that time. It may also occur at midday, when the heat dissolves much exhalation, while the surface of the earth is dry and the cold is driven back inside by recoil (άντιπερίστασις - ta'äqub).26 Earthquakes mostly occur in places where the earth is porous inside, while the surface is dense or covered with water, so that the wind cannot emerge

23

Ibn Sīnā, ai-Šifa, Tab. 5 17,1-7. ibid. 17,7-16. 25 This follows Pseudo-Olympiodorus, see above p. 216 no. 10. 26 Pseudo-Olympiodorus is also thinking of ά ν ΐ ί π ε ρ ί σ τ α σ ί ς above p. 215n13. 24

in this connection, see

through it. Flowing water especially prevents the wind from emerging.27 Earthquakes seldom occur in winter, because the cold freezes the smoky exhalation. If they occur, it is an indication that the moisture during that winter is more intense than its cold, for much exhalation is formed when it is moist and not very cold. Earthquakes also seldom occur in summer, because then the exhalation is dissolved and spread by the heat. If they occur, it is an indication that it is a dry year and the surface of the earth is dense by the drought: the pores are closed up and the exhalation is confined. Thus, earthquakes mostly occur in spring and autumn.28 Sometimes eclipses cause an earthquake, because the heat of the rays of light suddenly disappears; the ensuing cold keeps the exhalation inside the earth because its surface is closed up. The cold has an effect that it would not have when it came on gradually.29 Earthquakes differ depending on the motion of the enclosed wind. Some are like a throb (rajfī), when the earth is shaken upwards from below, others are like a tremor (iktilājī ri'sî) that extends horizontally in one direction and again others extend horizontally in two directions. They are called qitqit. If an earthquake spreads horizontally as well as vertically, it is called 'ladderlike' (sullamī). 30 All earthquakes would be like a throb, because the wind naturally moves upward, if some were not forced to move differently because the hollow spaces happen to extend in one direction only.31 The sounds that accompany an earthquake differ according to the different shapes of the spaces through which the wind moves about. Just as vision precedes hearing—for if someone far away strikes two bodies against each other, we see it before hearing it—because vision does not need time, whereas sound needs some time to reach us via the air waves, we hear the sound of an earthquake before we feel the earthquake itself, because the air waves are faster than the waves in the earth, which is denser.32 The many phrases that are similar to passages in Pseudo-Olympiodorus make clear that Ibn Sīnā has used this commentary. Ibn Sînâ's explanation of how light, sound and the tremors of an earthquake reach our senses is more clear and 'modern' than that of Pseudo-Olympiodorus. Although Ibn Sīnā recognizes the Aristotelian cause of earthquakes, sc. dry exhalation or wind, as the main cause of earthquakes,

27

OQ 29

•3Λ 31 32

Ibn Sīnā, as-Šifa,

Tab. 5 17,17-18,12. *

This follows Pseudo-Olympiodorus, see above p. 215 no. 5. Ibn Sīnā, as-Šifa, Tab. 5 18,13-19,2. This mostly follows Pseudo-Olympiodorus, see above p. 217. Ibn Sīnā, aš-Šifa. Tab. 5 19,2-9. ibid. 19,9-14.

he does not exclude other possible causes, such as water and the collapsing of hollow spaces. This could be Theophrastus' influence, who mentions four causes: collapsing of hollow spaces, water flowing in hollow spaces, wind confined inside the earth, and subterranean fire that rarefies air, so that this air tries to come out from inside the earth.33

5. School of Ibn Sīnā Bahmanyār is very short on earthquakes. He does not mention much more than Ibn Sînâ's definition and explanation of earthquakes and the fact that they sometimes occur together with an eclipse. Virtually his whole text on this subject is identical to that of Ibn Sînâ's Κ. aš-Šifā'.34 Abū 1-Barakāt says that an earthquake is a tremor (iktilāj) of the earth caused by motion of air that is enclosed in a large cavity. This occurs either because this air has become hot, or because it moves violently. If the surface of the earth is dense, as in mountainous areas, the motion of air in cavities increases and becomes more violent; then many earthquakes arise. Then it may also occur that the mountain falls into the cavity. If the earth is split, one hears the sound of the wind that comes out of it. If no cavities occur in a certain area, then there will be no earthquakes. If there are cavities, but no mountains, then earthquakes may occur, but less often. If the earth is shaken, it may sink in and water may appear from the sunken depths.35 Fakr ad-Din divides earthquakes into those caused by something within the earth, something above the earth or by a combination of both. The first kind arises in two ways. (1) They most often arise when there is a lot of hot, smoky exhalation within the earth, while the earth's surface is dense without pores and holes. Then the exhalation cannot find a way out, moves about and causes the earth to move. Sometimes it splits the earth, sometimes it becomes fire, and sometimes tremendous noises arise that indicate the forcefulness of the wind. Sometimes the earth is turned upside down or, if there are cavities in the earth, the earth above sinks into them. Fakr ad-Din gives Ibn Sînâ's text, stating that if many wells are dug in areas where earthquakes are frequent, they will occur less often.36 33

Theophrastus, Meteorology ch. 15 and Steinmetz 1964 206. Bahmanyâr, at-Tahsïl 717,12-718,2 is similar in text to Ibn Sīnā, aš-Šifā', Tab. 5 15,4-6 and 18,19-19,1. 35 Abū 1-Barakāt, al-Mu'tabar II 221,3-12. 36 Fakr ad-Dīn, al-Mabähit II 205,16-206,7; lines 206,5-7 are similar in text to Ibn Sīnā, aŠ-SĪfa, Tab. 5 17,17-19.' " 34

(2) A less frequently occurring way is that such earthquakes arise when a lot of water flows in cavities within the earth, or that such a cavity collapses; then subterranean air (exhalation) is stirred up and moves the earth.37 Earthquakes caused by something above the earth arise when the top of a mountain falls down. The text on this subject is similar to that of Ibn Sīnā.38 Earthquakes caused by a combination of something under and above the earth occur when subterranean smoky exhalation tries to rise, but cannot find a way out, because the earth's surface is cold or hot. Fakr ad-DIn here follows the content of Ibn Sînâ's text.39

6. Ibn Rusd Ibn Rušd states that earthquakes are caused by dry exhalation that turns inside the earth and moves about (idtaraba);40 he mentions the following confirmatory phenomena: Among the elements only wind (air) may develop a motive force that is strong enough to shake the earth, just as it is also able to inflame fire and to cause waves on water.41 Earthquakes mostly occur at times when wind is formed, i.e. in spring and autumn, and they are absent when no winds blow, i.e. in extreme heat and cold. This proves that the efficient cause of earthquakes and winds are the same. Often sounds are heard preceding an earthquake. Aristotle relates that on a certain island a hill (rubwa) kept being lifted up, until it burst and a strong wind emerged from it, together with ashes. Apparently the earth was burned in that place.42 Those who witnessed the earthquake of 566 A.H. in Cordoba and its surroundings may confirm that a lot of noise could be heard. Ibn Rusd reports that after the earthquake had started he went to Cordoba; he heard the sounds coming from the west and saw the earthquake occurring when a strong west wind was formed. He says that this earthquake was violent for one year and only stopped after three years. In the beginning many people were killed by the ravage; they say that

Fakr ad-Dīn, al-Mabāhil II 206,8-10. Fakr ad-Dīn, al-Mabàhii II 206,11-18 is similar in text to Ibn Sīnā, ai-Šifā', Tab. 5 16,5-13. 39 Fakr ad-Dïn, al-Mabāhil II 206,19-207,1 follows the content of Ibn Sinā, aš-Šifa, Tab. 5 18,4-8. 40 This word is also used by Ibn al-Bitriq, see above p. 212. 41 This phrase follows Ibn al-Bitriq 75,10-13; the influence of wind on water is not mentioned by Aristotle in this connection. A? The formulation of these phrases is similar to Ibn al-Bitriq 77,8-78,1. 37

38

the earth was split and that something like ashes or sand was thrown out. Further evidence were the mist and the elongated clouds that appeared in the sky.43 Earthquakes occur either by themselves (bi-dātihā) or accidentally (bil-'arad). They occur by themselves when much matter (exhalation) is formed; they occur accidentally when the surface of the earth is clogged, either by dryness or by moisture after continuous rain. Earthquakes may extend lengthwise only, or lengthwise and breadthwise, depending on how the wind that causes them moves. Sometimes the wind causing the earthquake is so strong that the earth is turned upside down (inqalaba) and that the water of the sea causes a flood, as Aristotle relates.44 Some parts of the earth are more suitable than others for the formation of an earthquake; it depends on whether exhalation is easily dissolved or not and whether pores are closed up or not. If both factors are present in a certain place in a way that is favourable for an earthquake, earthquakes will occur continuously. For instance, on an island that is close to the shore windy exhalation is easily formed, while the sea prevents the exhalation from coming out. An example is the place in Andalusia, called Church of the Raven (Kanīsat al-gurāb).45 There one always hears the kind of sound that precedes an earthquake.46 That Ibn Rušd has used Ibn al-Bitriq appears, apart from the corresponding passages mentioned in the notes above, from the importance attached to the clogging of the earth's surface as an (accidental) cause of an earthquake. It is mentioned by Aristotle only in his account of Anaxagoras' theory. In Ibn Rušd's Middle Commentary Ibn al-Bitriq's text is reproduced almost word by word. In this text it is said that earthquakes arise mostly when wind is blowing, more at night than during the day, and if during the day, then at midday or in the morning (see above p. 212). This seems to be different from what Aristotle says in 366a5-24, viz. that they arise in calm weather and that night, midday and morning are such times of calm weather. Ibn Rušd knew Aristotle's text from Alexander's commentary and notes the difference with Ibn al-Bitriq. He then says that both accounts may be true, because at these times indeed the wind above the earth subsides and turns within the earth. These times are suitable for dry exhalation to be raised, but this exhalation is prevented from rising above the earth by cold (at night and in the

43

Ibn Rusd, Short Commentary 52,1-53,18. This phrase follows Ibn al-Bitriq 79,11-80,2. 45 This refers to a church that used to be situated at Cap St. Vincent in Algarve (South-Portugal). See al-Idrisi 180-181. 46 Ibn Rusd, Short Commentary 53,18-54,14. 44

morning) or heat (at midday). At these times the wind above the earth subsides, so that the subterranean wind is strong.47 Ibn al-Bitriq relates that an earthquake occurred on a certain island; the earth was split and a strong wind emerged with many ashes. Ibn Rusd adds that the ashes smothered that city, and even covered a city in Italy. This phrase is found in Aristotle 367a6-8; it is neither in Ibn al-Bitriq, nor in Alexander. "A city in Italy" is mentioned in this connection by Pseudo-Olympiodorus. It is also read, however, by Ibn Tibbon in the copy the Meteorology which he translated. He will have had another copy of Ibn al-Bitriq's text than the one we have.48 Ibn Rusd follows Ibn al-Bitrlq's explanation of the darkness that covers the sun during an earthquake; this explanation is different from that of Aristotle and Alexander (see above p. 213). Contrary to Ibn Tibbon, Ibn Rusd does not say that he has found a different explanation in Alexander. Ibn Rusd concludes this commentary on earthquakes with some remarks on the earthquake in Cordoba which he witnessed, as he did in his Short Commentary.

47 48

Ibn Rusd, Middle Commentary 124,6-125,4. Pseudo-Olympiodorus, Taf sir 137,10; Ibn Tibbon, Otot ha-Shamayim

II 461.

CHAPTER EIGHT

THUNDER, LIGHTNING, HURRICANES, WHIRLWINDS AND THUNDERBOLTS

1. Aristotle Aristotle explains thunder (βροντή) and lightning (αστραπή) by means of the exhalations as follows (chapter 11,9): When moist exhalation cools down, it forms clouds; their density is greatest at their upper limit, because that is the coldest region. (Therefore hurricanes and thunderbolts move downward, although their matter by nature moves upward. This is analogous to fruit stones that move upwards against their nature, when they are shot away from between our fingers.) The hot, dry exhalation moves to the upper region of the atmosphere, but part of it may be caught in the cooling vapour that becomes a cloud. When the cloud condenses, the hot exhalation is ejected and strikes the surrounding clouds, thereby producing a noise; this noise is thunder. It is comparable to the noise of a flame of burning logs, which is caused when the dry exhalation from the wood strikes against the flame. The clouds are not uniform in density and contain hollow spaces; therefore different kinds of sound are produced. The ejected exhalation is usually inflamed and burns with a thin, faint fire; this is lightning. It occurs after the thunder, but we see it preceding the thunder, because sight is quicker than sound: we see the oars of a ship going back before we hear them strike the water (369a10-369b11). Next, Aristotle discusses others' view. Some say that there is actually fire present in the clouds. Empedocles thinks that this is due to the sunrays being caught in the clouds, Anaxagoras that a part of the ether—which he calls fire—has descended into the clouds. They suppose that lightning is the shining of this fire, thunder the hissing noise that occurs when fire is quenched. Thus, lightning actually precedes thunder. The following objections may be raised against these views: Anaxagoras' theory does not explain how fire, which naturally moves upwards, could move downwards and why this does not occur all the time, but only when there are clouds. Empedocles' theory implies that lightning is always present in the clouds instead of giving an explanation of its generation at each occasion. It is the same as if one thought that rain, snow, etc. were present in the clouds and were not

generated. Furthermore, what is the difference between the sunrays being caught in this case and in other cases? They are also caught when water is heated by the sun, but no fire is ejected. The hissing sound that occurs when a flame is extinguished by moisture is, in fact, due to the flame boiling the moisture. A condition of boiling is not present in moisture beforehand, as these views imply. Others, among whom Cleidemus, say that lightning is a visual appearance. They say that it is similar to the brightness we see when the sea is struck with a stick at night and the water seems to shine. If the moisture in a cloud is struck, then a similar brightness seems to arise, and that is lightning. This opinion displays an ignorance of the theory of reflection. The water appears to shine when struck, because our vision is reflected from it to some bright object (369b11-370a21). After having discussed wind, earthquakes and thunder as phenomena caused by the same material, viz. dry exhalation, Aristotle in chapter 111,1 turns to the remaining effects caused by it: hurricanes, typhoons (whirlwinds), firewinds and thunderbolts. A hurricane (έκνεφίας) is formed in a way analogous to that of thunder. Thunder arises when the dry exhalation that is dissolved from a cloud is produced in small and dispersed quantities of rare constitution, frequently and quickly blown out; if it is produced in a compact and dense body, then a hurricane is formed. Its violence is due to the force it has because of the speed of its dissolution. The formation of hurricanes is analogous to that of rain. The matter of hurricanes and of rain (sc. dry and moist exhalation, respectively) both potentially exist in the cloud. Either rain or a hurricane is actually formed, depending on which exhalation is dominating in the cloud (370b3-17). A whirlwind (τυφών) arises when a wind that is dissolved from a cloud collides with another wind.1 The effect is similar to that which occurs when wind is forced from a wide space into a narrow one: the front part of the stream meets resistance, due to the narrowness or a contrary current; it cannot advance forward and is pushed on from behind, thus it moves sidewards where there is no resistance. The same happens to the next part and so on; the result is that the wind forms an eddy (δίνη). In a cloud exhalation is continuously dissolved; it becomes a hurricane when it leaves the cloud. A whirlwind arises when the exhalation cannot free itself from the cloud because of the cloud's density and then moves in circles because of the above-mentioned

1

Alexander's interpretation cloud, see in Meteor. 13432.

is

that

wind

from

a

cloud

collides

with

another

reason, and subsequently descends because the cloud is densest at its upper limit, dragging the cloud along with it. Hurricanes and whirlwinds do not occur in cold weather, because then the dry hot exhalation is quenched by coldness (370b17-371a15). When the wind coming from a cloud has a rare constitution, it may catch fire while it moves downward. Then a firewind (πρησιήρ) is formed. It inflames and colours the adjacent air (371al5-17). A thunderbolt (κεραυνός) arises when a large quantity of wind of rare constitution is squeezed out from a cloud. If its constitution is very rare, it does not scorch and the thunderbolt is called bright. If it is less rare, it scorches and is called smoky. The former moves fast because of its rareness and therefore it passes through an object before it can burn or blacken it. The other kind is slower; it blackens the object, without burning it. Objects that offer resistance are subsequently burnt; objects that do not offer resistance are not burnt. For instance, it has occurred that the bronze head of a spear was melted, whereas the wooden handle was unaffected due to its rareness that allowed the wind to pass through it without affecting it. Also, it has occurred that a garment was not burnt by a thunderbolt, but was made threadbare. Thunderbolts are preceded and followed by wind; this is an indication that the thunderbolt itself is wind too. If a thing is split by a thunderbolt, then this is caused by the blow it gets from the wind. That wind is like smoke and may be inflamed has been shown before and is confirmed by what one saw at the burning of the temple of Ephesus. Smoke and flames issued from the beams of the building, flames were separated from the main fire and moved in various directions (371a17-371b14).

2. The Greek

commentators

Alexander remarks, in connection with the fact that we see lightning before hearing thunder, that sound needs time to reach us, whereas sight is realized without time, because seeing occurs when the object and the medium through which we see it have a certain position in relation to the observer. When this position comes about, vision is realized simultaneously. This is Aristotle's theory from De Sensu et Sensibilibus.2 Olympiodorus' commentary on thunder and lightning is lacking in the extant manuscripts, as well as the first part of the θεωρία on 2 Alexander, in Meteor. 128,35-129,9; see below pp. 244-245 for the theory of vision in De Sensu et Sensibilibus.

chapter 111,1. What is extant starts in the middle of the account of the whirlwinds. A whirlwind may be formed below, on earth, or above, in a cloud. If the wind that is dissolved from the cloud descends to the earth or the sea, and collides with a solid body, the front part cannot advance forward, while it is pushed on from behind by the next part; thus it will move sidewards, where there is no resistance. The same happens to the next part and so on; the result is that the wind forms a whirl that lifts up everything that is in its way: ships, living beings and stones. Something similar may happen in a cloud. The smoky exhalation within a dense cloud cannot free itself from it and remains in the cloud moving around in circles, because it always collides with dense parts of the cloud; finally the cloud bursts open and the exhalation descends to the earth, carrying the cloud with it, and keeping the same form, i.e. a whirl—like curly hair that is curly not because the force that makes it grow is weak, but because the pores through which it grows are twisting.3 How do we decide whether the eddy in a whirlwind moves upward from below, or downward from above? First, we can see it directly, for a whirlwind is visible because of the greater density of the smoky exhalation. If we see the eddy first moving below and then moving above, then its origin is below; if we only see it moving from above to below, then it will have its origin above. We may also observe the bodies that are struck by the whirlwind and are lifted up. If the body is first turned around, subsequently moved sideways and finally lifted up, then the eddy originates from above.4

3. Ibn al-Bitriq and Hunayn ibn 1shāq Ibn al-Bitriq says about the origin of thunder (ra'd) and lightning (barq) that they occur when dry exhalation rises together with moist exhalation. If they arrive in a cold layer of the atmosphere, the moist exhalation forms a cloud and the dry exhalation is enclosed within, and cannot emerge from it because of the cold. The dry exhalation moves about inside the cloud and strikes against its moist parts and finally 3

Olympiodorus, in Meteor. 200,6-201,10, 204,24-205,3, 206,7-10 and 207,16-23. ibid. 201,10-20. The last phrase is not what we expect. We expect that when the body is lifted up after having been moved around, the whirlwind comes from below, and when the body is directly lifted up, the wind comes from above. This is indeed how Pseudo-Olympiodorus describes the matter, see below p. 232. Olympiodorus does not say what we see when the whirlwind comes from below. All this means that some text is lacking in the extant copies of Olympiodorus. 4

breaks outwards through the cloud. This sound of striking is thunder. It is comparable to the sound of moist wood when a fire is lit in it. If the dry exhalation catches fire, this is lightning. The sound of thunder is like the hissing of a hot piece of iron when it is immersed in water.5 The section on hurricanes is corrupted in Ibn al-Bitriq's version and seems not to deal with hurricanes at all. The text runs as follows: if wind is gradually ejected from a cloud and consists of rare parts, no thunder and lightning arise; if this occurs with force, it pushes the moistness gathered in the cloud, and then thunder and lightning arise. The size of raindrops which fall in a thunderstorm depends on how forcefully the moistness in the cloud is struck by the wind.6 The reading that follows Aristotle should be: if wind is gradually ejected from a cloud and consists of rare parts, thunder and lightning arise; if this occurs with force, then a hurricane arises. Aristotle does not mention moistness in the cloud, nor the size of raindrops. Hunayn ibn Ishāq does not discuss hurricanes, whirlwinds, fire winds and thunderbolts at all, but he does discuss the size of raindrops in a thunderstorm; he specifies Ibn al-Bitriq's statement by saying that if the wind pushes against the cloud with force, the moist parts gather and form large drops; if the pushing occurs weakly, then small drops are formed. 7 When Ibn Rušd in his Middle Commentary gives Ibn alBitriq's text, the corruption is partly emended: he says that if wind is gradually ejected from a cloud and consists of rare parts, thunder and lightning arise; for the rest, he follows Ibn al-Bitriq's text, and hurricanes are not mentioned (see below p. 241). Ibn Tibbon follows Ibn al-Bitriq's text, except that instead of the size of raindrops he talks about the size of the thunder.8 Ibn al-Bitriq next discusses whirlwinds (zawba'a), by saying that they arise when a wind blowing from a cloud or another place meets another, opposite wind. Each of them pushes the other aside; then they rise upward as a circular, winding wind, with exhalation and dust.9 In the next paragraph Ibn al-Bitrlq asks why lightning moves downward, although its matter, being fire, by nature moves upward. Aristotle had answered that thunderbolts escape the cloud at its bottom because

5 Ibn al-Bitriq, Meteor. 81,10-82,5. The passage 81,6-82,1 corresponds to Hunayn, Jawämi' 239-246, with many similar phrasings. The example of hot iron immersed in water is also in Theophrastus, Meteorology ch. 1,10-11. 6 Ibn al-Bitriq, Meteor. 85,5-9. 7 Hunayn, Jawämi' 263-266. 8 Ibn Tibbon, Otot ha-Shamayim III, 2-6. 9 Ibn al-Bitriq, Meteor. 86,1-5.

the upper parts are too dense (see above p. 225).10 Ibn al-Bitrlq gives quite a different answer. He says that lightning also contains thick, earthy exhalation. This moves down and carries the fiery part with it, preventing the fire from following its natural course.11 The translation of Ibn Tibbon contains the additional remark that the same cause holds for the downward motion of hurricanes (winds that blow from a cloud); if one inserts this in the text of Ibn al-Bitrlq that we have, it improves the reading of the paragraph as a whole.12 Next follows a paragraph on thunderbolts (sa'iqa). If the lightning has a white colour, because the exhalation from which it arises has a fine structure and is not very fiery, then it does not burn the bodies on which it falls, and no smoke arises. If the wind (exhalation within the cloud) increases and beats the cloud strongly, fire descends from it—that is the thunderbolt—and the smoke is burnt exhalation. An indication is that if there is much exhalation, we see it moving about and pushing away what is around it with an audible blow.13 Ibn Tibbon remarks that the words 'and the smoke is burnt exhalation' do not connect with what precedes, and that a lacuna is present which may be supplied by what is found in the commentators.14 Indeed, Aristotle extensively discusses the effects of a thunderbolt of a denser texture and finally presents the example of the burning of the temple of Ephesus, where he says that smoke is burnt wind (exhalation) (see above p. 227). Then follows a paragraph on the colours of the clouds from which thunder and lightning arise. This is not found in Aristotle. If the heat in a cloud acts strongly, its parts become burnt and its colour is black, because the cloud is compressed and densified, so that it does not accept the sunrays. A cloud becomes white if the fire in it does not act strongly, so that it is not burnt; its parts are not compressed and it accepts the sunrays. Green and red colours occur when the action of the heat is between those that give a white and a black cloud.15 We have again seen that Ibn al-Bitrlq's text differs considerably from that of Aristotle: the formulation and sometimes the contents in corresponding passages are different. Moreover, there are phrases and pas10

Ibn al-'Amid gives a similar account. He compares the non-natural downward motion of a thunderbolts with the motion of bodies thrown by catapults (qa^dßfät), of water spurted away by sprayers (zarraqāt) and of fire shot upward by naphtha-guns (naff at at). See Ibn al-'Amid VII 8. 11 Ibn al-Bitriq, Meteor. 86,7-12. 12 Ibn Tibbon, Otot ha-Shamayim III, 18-20. 13 Ibn al-Bitriq, Meteor. 87,2-10. 14 Ibn Tibbon, Otot ha-Shamayim III, 37-62. 15 Ibn al-Bitriq, Meteor. 87,12-88,2. This paragraph corresponds to Hunayn, Jawāmi' 258-263. Hunayn says that the dark colour arises due to the burning of particles, which makes the colours turbid.

sages that do not correspond to any text in Aristotle, e.g. the statement that lightning contains earthy parts and therefore decends. Furthermore, we conclude that Ibn Rusd and Ibn Tibbon had copies of Ibn al-Bitriq's version of the Meteorology that were different from those we have. Sometimes it is possible to improve our text of Ibn al-Bitriq's version by means of Ibn Tibbon's translation, but the copy he translated was also far from perfect.

4.

Pseudo-Olympiodorus

Pseudo-Olympiodorus' discussion of thunder and lightning starts from the smoky exhalation that is enclosed in a cloud. The light part of that exhalation moves to the upper part of the cloud and leaves it. This causes a cooling of the cloud and subsequently its condensation. Then the denser part of the exhalation in the cloud is squeezed out. If it strikes against another cloud, thunder arises. If it is inflamed after that, lightning arises. The sound of the thunder differs according to the density of the cloud and the size of the hollow spaces in it. Thunder precedes lightning, for the striking against the cloud precedes the inflammation, but we perceive lightning before we perceive thunder, for the sense of seeing is finer than the sense of hearing. PseudoOlympiodorus here gives the example of the oars of a ship. He concludes with a phenomenon, not described by Aristotle: sometimes lightning is seen without any cloud in the sky and without thunder. The explanation is that the cloud that squeezes out the exhalation is under the earth; therefore we see such lightning in the lower part of the sky.16 Pseudo-Olympiodorus' discussion of the hurricane and whirlwind starts with their definitions: the wind that descends from above, all at one time, is called a hurricane (rīh sahābiyya); if it is moving in circles, it is called a whirlwind (zawba'a). A whirlwind may arise below (on earth) or above (in a cloud). The whirlwind that arises above occurs when wind is dissolved from the dense upper part of a cloud and cannot find a way out because of the density of the cloud. Then it collides with the dense parts of the cloud and is thrust back. In this way it moves in circles, until it finds a place through which it can pass to the outside. Then it descends as a whirl to the earth, dragging the cloud with it. A whirlwind arises below, if the wind, when it descends, collides with a solid body. Then it is forced to move back, but this is prevented by other parts of the wind that are descending behind it.

16

Pseudo-Olympiodorus, Tafsir

141,15-142,12.

Therefore it will move sideways, and will get a circular motion. The same occurs to other winds when they are forced from a wide space into a narrow one. When we look for the place where a whirlwind starts—above or below—we may deduce this by considering either its effect or its motion. If we consider the effect of a whirlwind on bodies that are struck by it, then we can say that if a body is lifted up, without having been pushed around first, the whirl comes from above. It comes from below when the body is first moved around along the earth and then lifted up. This wind is so harmful that it lifts up whole animals, uproots trees, and carries away ships. If we consider the motion of a whirlwind, we know that its eddy starts from above if we see it rise and descend; we know that it starts from below if we see it rise only, not descend.17 The contents of this paragraph is similar to that of Olympiodorus' commentary. The comparison of a whirlwind with curly hair is not in Pseudo-Olympiodorus. The relation between how eddies move in a whirlwind and the place of origin of the whirlwind (above or below) is established in opposite ways by Olympiodorus and Pseudo-Olympiodorus. The discussion of the firewind and thunderbolt runs as follows: If the smoky exhalation that is enclosed in a cloud is squeezed out in scattered portions, not all at once but one after the other, the result is thunder and lightning. If it is squeezed out in a compact, dense mass and is not inflamed, then a hurricane or whirlwind results; it is a hurricane if it does not contain rain and a whirlwind if it contains moisture, for the former is more rare, the latter is more dense. If the exhalation that is squeezed out is inflamed and contains moisture, we get the kind of thunderbolt (sä'iqa) that is called πρησχήρ in Greek; if it does not contain moisture, we get the kind of thunderbolt that is called κεραυνός in Greek. If a κεραυνός hits upon something, it passes through it without burning it because of the rare structure of the thunderbolt. There are two kinds of κεραυνός, the one being less dense than the other. The one that is less dense passes through a body without affecting it at all, due to the rapidity of its passing. The denser kind blackens the body that is hit by it, but does not burn it. However, if a body is dense and offers resistance, it is burned. Thus, the bronze head of a spear, the gold and silver coins in a purse and the nails in wood melt, but the wood itself and the material from which the purse is made, is not burned. The πρησχήρ burns and destroys the bodies that are hit by it.18 17 18

Pseudo-Olympiodorus, Tafsīr ibid. 143,16-144,15.

142,17-143,14.

That thunder has its origin in wind is confirmed by the fact that we hear its sound some time after its occurrence. Furthermore, wind blows together with it, for we know that living beings may be damaged by strong thunder, and also that earth may be split; often mushrooms then come out of the earth, and therefore people call them plants of the thunder. That both kinds of thunderbolts originate from wind is confirmed by (1) the rapidity of their motion and action; (2) their inflammation, for flame is nothing but burning dry exhalation, and that is the matter of wind; (3) their being preceded and followed by wind. This may be explained as follows: The wind that causes a thunderbolt, when it comes out of the cloud, is not inflamed in the beginning, because its motion is still weak. After that, in the middle of its being separated, its motion becomes stronger and it becomes inflamed; finally, at the end, it becomes weak again, as in the beginning, and is not inflamed. A process is always strongest in the middle when it reaches its culmination; at the beginning and the end it is weak, as occurs in an illness, for instance. We see that things are moved before they are struck by a thunderbolt; this is an indication that wind precedes a thunderbolt. We see that after the sea has been struck by a thunderbolt, the sea is split into two; this is caused by the wind following the thunderbolt.19 The systematizing of Aristotle's account by Pseudo-Olympiodorus is clear from these paragraphs.

5. Al-Kindi Lightning, thunder and thunderbolts are discussed in al-Kindi's "On the cause of snow, hail, lightning, thunderbolts, thunder and zamharir ",20 These phenomena are discussed following upon his treatment of hail. In our account (above pp. 109-110), we saw that one way in which hail is formed, according to al-Kindi, is that when a cloud cools, wind arises inside a cloud and freezes the watery parts. This wind causes a strong noise. If the cold surrounding the cloud becomes more intense and the cloud is further compressed, the wind shakes the whole cloud. The fast motion of the wind causes parts of the cloud to become rarefied, hot and inflamed. In fact, the earthy part of the cloud, which is closer to the earth, is set on fire when it comes into contact with the wind. This 19 Pseudo-Olympiodorus, Tafsir following a thunderbolt is given Meteor. 202,14-27 208,18-26. 20 al-Kindi, Rasa il II 80-85.

144,17-145,12. The account of wind preceding and by Olympiodorus, with other examples; see in

is lightning. If the impulse of the wind is stronger and there is much matter, it may reach the earth. Then it is called a thunderbolt. If it reaches the earth in its full power, it is able to destroy an object and break it to pieces so fast that it does not burn. If the thunderbolt is less strong and acts less quick, the object that is hit is broken to pieces and then burnt and blackened. If it is even weaker, the object is shattered and catches fire at the same time, and sometimes it is blackened and burnt. Finally, if the thunderbolt is very weak, the object catches fire, but is not destroyed nor burnt. The sound that is heard after lightning and thunderbolt is called thunder. It is the sound of the burning of the cloud and, in fact, occurs simultaneously with the lightning and thunderbolt. But lightning and thunderbolt are observed by vision and vision occurs 'without time' (i.e. vision is simultaneous with the viewed phenomenon), whereas thunder is heard. Sound reaches the senses after some time. If we see someone striking on a piece of wood far away, we hear the sound after some time, depending on the distance. This occurred when we saw fullers beating stones with clothes on the other side of a river. Al-Kindl's account of the discussed phenomena is close to Aristotle, although not completely identical. That wind moves about within the cloud and that the cloud and lightning contain earthy particles is not in Aristotle, but in Ibn al-Bitrlq (see above pp. 228 and 230).

6. Ibn Sīnā Ibn Sînâ's account of thunder, lightning and thunderbolts runs as follows: The exhalation from which clouds are formed is accompanied, especially in hot seasons, by smoky exhalation. Part of this smoky exhalation separates itself from it, while part of it remains together with it and cools when the cloud is cooled. The density of the cloud prevents the smoky exhalation from breaking outward through the cloud, so that it remains enclosed within the cloud. The upper part of the cloud is the most dense, and therefore it is the most difficult part for the smoky exhalation to break out through. The lower part is warmer and less dense. The wind moves about within the cloud and pushes against its sides until it breaks through it and is separated from it. This separation occurs downward, not in the upward direction, which is the natural direction in which smoky exhalation moves, because the places through which the exhalation is able to emerge are in the lower part of the cloud. The cold by which the exhalation is affected contributes to its downward inclination. The sound of the wind moving about

within the cloud is the thunder. We hear the wind when it blows in thin air, so if it blows in a cloud, which is much denser than air, its sound will be even stronger. The thin exhalation is easily inflammable, thus it will inflame when it moves about in the cloud and is subject to friction with and pressure by its dense matter. This is the origin of lightning. If you rub a black body with your hands at night, you will see some weak inflammation and light. Such an effect will arise all the more, when something of fine structure that is a mixture of watery and earthy parts is affected by heat, motion and turbulence. Sometimes inflammation occurs because the heat is enclosed, fleeing from the cold (άντιπερίσιασις). 21 It sometimes occurs that it rains on certains spots where the soil is salt and swampy or sticky and oily. Then greasy, light exhalation rises from that spot that is easily ignited by the sun or by lightning. One sees a luminous blaze on the surface of the earth, like the burning of combed cotton, or rather like the fire that is ignited in the exhalation from a liquid in which salt and ammonia are incorporated. When a bottle of this is put in smoldering fire, exhalation is dissolved; when a lamp is put near the exhalation it is ignited and it keeps burning as long as the exhalation exists.22 There is no lightning without thunder, because lightning only arises when wind moves about in a cloud and is subsequently separated and ignited. Lightning is seen at once, whereas thunder is heard with some delay. Vision occurs when the visible object is present before the observer and the space between them is transparent. This condition is realized at once, without time. Hearing occurs when the air is moved in waves and transports the sound to the ear. This is a kind of motion that needs time. Thus, when we see an ax coming down in a place far away, we hear the sound some time after that. When it occurs close by, the time difference between seeing and hearing is imperceptible.23 Sometimes lightning causes thunder, namely, when inflamed wind is extinguished in the cloud; then the sound of being extinguished is heard. This sound is caused by a violent, rapid motion that occurs by the interaction of fire and moisture. This is similar to what occurs when we extinguish a fire; then a sudden motion of air is brought about which beats against the other air and this causes a sound.24 Lightning is always accompanied by thunder, because the moving 21

Ibn Sīnā, aš-Šifa, Tab. 5 67,4-68,13. ibid. 68,13-18. 23 ibid. 68,19-69,8. This is Aristotle's theory of vision from De Sensu et as ouoted by Alexander, see above p. 227. U ibid. 69,9-14. 22

Sensibilibus,

fire cannot be extinguished without the sound of hissing, and the wind cannot emerge and become ignited without beating the cloud. However, it is possible that thunder occurs without lightning, for wind may violently blow inside a cloud without being ignited. That thunder arises from the beating of clouds against one another, as has been said, is rather unlikely,25 although clouds may be subject to the same motions as winds.26 The sound of thunder differs in accordance with the differences in the winds that break through the cloud, the clouds from which they break out and the positions of the clouds in relation to one another, and it depends on whether the thunder arises from wind which beats and strikes or from inflamed wind which is extinguished. Much thunder and lightning occurs in the north due to its cold and its enclosing of the heat within the cloud.27 Ibn Sīnā mentions other opinions on thunder and lightning, the same as those mentioned by Aristotle. It cannot be a reflection of sunlight, otherwise no lightning could occur at night. It cannot be ether in a cloud, for there is nothing that would drive ether downward to a cloud.28 A thunderbolt is an inflamed hurricane, which descends to the earth—not only its light, but also its inflamed particles, because of its compactness and its containing heavy, earthy parts29 or because it is forced in a downward motion due to the circumstance that the place where it originates is at the lower side of a cloud. Its matter is less rare than that of lightning, which does not last a long time and is quickly extinguished. If the hurricane is not violent, the thunderbolt has a rare structure; sometimes it is just scorching, sometimes it is blackening, sometimes it passes through bodies of rare structure, without burning them and leaving no trace at all. If the thunderbolt is denser and passes through bodies of rare structure, it may leave them blackened; it melts dense bodies. Therefore it melts the pieces of silver or bronze that are put on shields, without burning the shields themselves, sometimes just blackening them. Similarly, it melts the gold in the purse without burning the purse.30 Sometimes an inflamed whirlwind arises; it contains dense matter and becomes a harmful thunderbolt. Sometimes thunderbolts—being inflamed hurricanes—are extinguished and transform into

25 26 27 28 29 30

This is one of the causes mentioned by Theophrastus, see his Meteorology ch. 13. Ibn Sinā, aš-Šifa, Tab. 5 69,9-14. ibid. 69,15-18. ibid. 70,1-6. Earthy parts in lightning are mentioned by Ibn al-Bitriq, see above p. 230. The example of the purse is in Pseudo-Olympiodorus, see above p. 232.

earthy bodies, in accordance with the admixture that they contain.31 Usually a wind precedes the striking of a thunderbolt.32

7. School of Ibn Sīnā Bahmanyār gives a summary of Ibn Sînâ's account: the smoky exhalation that remains in a cloud cools, becomes heavy and tries to get out of the cloud. We hear this as a big noise, which is thunder. Due to the efforts of the exhalation to get out, friction and inflammation occurs, which is lightning. If the matter of the exhalation is dense, a thunderbolt is formed. The lightning is seen before the thunder is heard, because sound is motion that covers a certain distance to the ear, whereas vision occurs directly when the visible object is opposite the observer. For instance, we see fullers moving their hand before we hear the sound of their beating.33 The section on fire that comes about from greasy exhalation follows the text of the Ši/ā'.34 Abū 1-Barakāt says that, according to older authors, lightning is produced when clouds are knocked against one another by wind and fire is struck from them; when this fire is extinguished in the cloud the sound is produced that we hear as thunder. Therefore we see that lightning always precedes thunder. Maybe thunder is the sound of clouds that knock against one another and the sound comes later than the lightning because seeing precedes hearing, for sound travels to the ear by means of a motion that strikes the air and causes a wavelike motion, whereas vision occurs immediately as soon as object and observer are opposite one another.35 Thunderbolts are said to consist of metallic bodies like iron and bronze that are formed in the atmosphere by a mixture of earthy and watery exhalation. Their formation is completed by fire produced from friction. Then they descend as ignited particles; during their descent they join one another like drops of snow or rain, and arrive at the earth as one piece of matter. It burns everything it hits, but is not able 31

In the section where he deals with the formation of stones Ibn Sini relates that bodies of stone, iron and bronze have been found falling to the earth together with thunderbolts, see aš-Šifā', Tab. 5 5,15 ff. 32 Ibn Sīnā, aš-Šifa, Tab. 5 70,6-713M ' Bahmanyār, al-Tahsīl 715,7-14; lines 8 - 9 are similar in text to Ibn Sīnā, aš-Sifā', Tab.34 5 68,3-4. Bahmanyār, at-Tahsīl 713,14-7143 is similar in text to Ibn Sînā, aš-Šifà', Tab. 5 68,13-18. 35 Abū 1-Barakāt, al-Mu'tabar II 221,13-20.

to penetrate into rare things, like clothes; such things are not burnt. For instance, gold and silver coins in a purse may be melted, even when the purse itself is not burnt. It penetrates into the earth due to its weight, for the longer the distance a body falls, the heavier it becomes.36 In his account of thunder and lightning Fakr ad-Din says that smoky exhalation is always contained in clouds, because it rises together with the vapour that condenses into cloud. If the smoky exhalation remains hot, it tries to rise further and tears the cloud apart. The sound of the tearing is thunder. If the exhalation becomes cold, it becomes heavy and tries to descend. Then it also tears the cloud apart and this is heard as thunder. The smoky exhalation is fine and contains earthy and watery parts. The heat and motion make a kind of oily mixture of it that is easily ignited. When this occurs by the friction occurring when the exhalation tears the cloud, then this is lightning. Sometimes lightning causes thunder, namely, when ignited exhalation is quenched in the cloud. The sound of quenching is thunder. The explanation why lightning is seen before thunder follows Ibn Sīnā.37 A thunderbolt is exhalation that descends to the earth, either due to its weight or density, or because something prevents it from rising. Fakr ad-DIn's description of the effect of thunderbolts with a rare and dense structure on different kinds of bodies follows Aristotle. He adds that a thunderbolt falling on a mountain may destroy it completely and when it strikes the sea it plunges into it and burns all animals. If the matter of a thunderbolt is very fine, like wool, it may cut the object hit by it into two, with only a small gap. It is told that a boy sleeping in the desert was struck by a thunderbolt which cut off his legs; they fell from him without blood coming out, because the heat had cauterized the wound.38 The section on the luminous blaze that comes about from greasy exhalation follows the text of the Šifā'.39 In another paragraph Fakr ad-Din describes another effect of the same exhalation: greasy, sticky exhalation may rise until the upper atmosphere, viz. the sphere of fire, forming a continuous flow from the earth until the fire. This exhalation is ignited by the fire and then the ignition descends until the earth. Then we see a blaze descending from the heaven to the earth and when it reaches the earth, it burns everything it meets. The phenomenon is produced in the same way as what occurs when an extin36

Abü Fakr 38 ibid. 39 Fakr 68,13-18. 37

1-Barakāt, al-Mu'tabar II 221,21-222,4. ad-Dīn, al-Mabāhit II 187,5-188,1. 1883-13. ad-Din, al-Mabāhit II 188,15-21 is similar in text to Ibn Sīnā, aš-Šifā',

Tab. 5

guished lamp is placed under a burning one. The smoke of the lamp below rises to the burning lamp and is ignited by it; then the ignition descends and ignites the lower lamp.40 This phenomenon, not mentioned by Aristotle or any commentator, belongs instead to the group that is caused by ignition in the upper atmosphere, like shooting stars and comets. Indeed, the comparison with the lamps is made by Aristotle in connection with shooting stars (see above p. 67). Fakr ad-Din classifies these phenomena together with winds and with thunder and lightning under the same group, viz. the phenomena above the earth caused by dry exhalation.

8. Ibn Rusd Ibn Rusd says that thunder, lightning and thunderbolts belong to the same genus; they are distinguished by their specific differentia. Thunder arises when smoky exhalation in a cloud collects within, when the cloud is condensing, and then is ejected with force. Then we hear a sound like that of moist wood when it is thrown into a fire.41 This exhalation is easily inflammable; when its motion becomes more violent, inflammation will occur, and that is lightning. When this fire descends to the earth, contrary to its nature, then that is a thunderbolt. Thunderbolts differ according to the matter of the wind: if its structure is fine and airy, then bodies of rare structure are not destroyed when they are struck by the thunderbolt. Thus, it may occur that bronze melts, whereas wood that is struck at the same time is not burned and animals are killed without being burned. If the structure of the exhalation is more dense and earthy, it burns the objects that are struck by it. The Peripatetics report that a thunderbolt struck a temple, from which much smoke arose. Ibn Sīnā reports that in places in Khurāsān and Turkey where a thunderbolt had come down, bodies were found that resembled iron and copper and that were formed from the earthy parts of the thunderbolt.42 When he tried to melt them into arrowheads, he did not succeed, but they evaporated, turning into smoke.43 This has not been observed in our country, nor was it mentioned by the Peripatetics. Ibn Hayyān44 relates that during clear weather a large 40

Fakr ad-DIn, al-Mabāhit_ II 190,6-12. The example resembles that given by Ibn al-Bitriq rather than that in Aristotle, see above pp. 225 and 229. 41

42

Earthy parts in lightning are mentioned by Ibn al-Bitriq and Ibn Sīnā, see above pp. 4230 and 236. 3 Ibn Sīnā, aŠ-Šifa, Tab. 5 5,15-20. 44 Ibn Hayyān 988-1076, a historian from Cordoba.

stone, aflame with fire, fell down in a1-Kanbāniya near Cordoba;45 it was like sulphur and was smelling like ammonia.46 We should investigate why the inflamed wind in all the phenomena we have discussed here moves downward with great rapidity along a straight line, whereas it has an upward motion by nature. One might think that this occurs on account of contrariness: one contrary flees from the other via the easiest way, whether it be up, down, right or left, as water and fire flee from one another. However, this is a poetical way of conception. We say that this downward motion is not natural; thus, let us suppose it is a forced motion. Then there must be a body that causes this forced motion and that body must be in motion itself. If we suppose that this latter motion is also forced, the same situation occurs as for the first motion and this continues infinitely. Thus, the motion of the body that causes the forced motion must be natural. The smoky exhalation consists of two parts, a light one and a heavy one. The heavy part moves downward by nature and forces the light part to move down with it. We must suppose that a heavy particle gets a downward motion which is quicker when it is joined with a hot light particle than when it is simple. The cause of this seems to be the effect that occurs when something is adjacent to its contrary. Then these contraries are in danger of being destroyed by one another, but what saves them from one another and maintains their existence is their natural place. Thus, if heavy particles are mixed with the smoky exhalation, they will move down quickly, to their natural place, in order to save their existence. The fiery parts cannot separate themselves in time from the heavy ones and thus by force move down with them.47 It may occur that a wind that is descending does so by moving in circles. This occurs when both motions in the wind (sc. the motion down of the heavy parts and the motion up of the light parts) prevent one another from being executed. It may also happen that a descending wind is opposed by a rising wind. Then the result will be a motion in circles, either upward or downward, depending on which wind, the rising or the descending one, is dominating. Such winding winds are called whirlwinds; they may become so violent that they lift up ships and animals and throw them to another place.48 That wind is the origin of the phenomena under discussion is confirmed by several facts. For instance, winds blow during thunder; these 45

A1-Kanbāniya (Spanish: campana): the Cordoba, see al-Idrïsï 174, 209. 46 Ibn Rušd, Short Commentary 54,16-56,5. 47 ibid. 56,6-57,15. 48 ibid. 57,18-58,7.

plain

in

the south

of

the district

of

winds may cause damage to animals and may split the earth; then plants come out, which are called thunder plants. Also, the rapidity of the motion of a thunderbolt is an indication that it is inflamed wind. Also, the sea is violently moved by wind before and during lightning and thunderbolts.49 We see lightning before thunder. This follows from what is generally the case with seeing and hearing. We see a blow occurring far away before we hear its sound, as when one stands on the side of a river and sees others on the other side striking something.50 The main feature of Ibn Rusd's comment is his explanation of the downward motion of lightning and thunderbolts. That these phenomena are a mixture of light (fiery) parts and heavy (earthy) parts is mentioned by Ibn al-Bitrlq and Ibn Sīnā. Influences by Pseudo-Olympiodorus are also present. In his Middle Commentary Ibn Rusd follows Ibn al-Bitriq. He inserts a passage on Anaxagoras' doctrine on thunder and lightning, which is not dealt with by Ibn al-Bitriq. As for the subject of hurricanes, we already have seen above (p. 229) that Ibn Rušd gives a partly emended version of the corrupted text of Ibn al-Bitrlq. He says that if wind is gradually ejected from a cloud and consists of rare parts, thunder and lightning arise. One expects Ibn Rusd continuing with: if wind is ejected from a cloud with force, then a hurricane arises. Instead of this he follows Ibn al-Bitriq and brings forward the size of raindrops in a cloud that produces thunder. Then follow the paragraphs on whirlwind, and the reason why thunderbolts move downward. We have seen (above p. 230) that Ibn al-Bitriq's paragraph on thunderbolts is corrupt and that Ibn Tibbon has stated that there is a lacuna before the words 'smoke is burnt exhalation'; he indicates how this lacuna may be filled. Ibn Rusd's text runs as follows: Lightning is sometimes seen as white, sometimes as red. The white lightning arises when the exhalation is very rare; when it strikes an object it does not burn it, nor dissolve smoke from it, if smoke is burnt exhalation. This burnt exhalation occurs in red lightning, and therefore this is accompanied by a loud thunder.51 Thus, in Ibn Rusd's text the words 'smoke is burnt exhalation' is connected with what precedes in this way, different from what Ibn Tibbon suggests. We do not know whether Ibn Rusd here reproduces his copy of Ibn al-Bitriq's text, or whether he himself made an emendation of a text he considered corrupt. The commentators never mention red lightning; the denser 49

The examples are also in Pseudo-Olympiodorus, see above p. 233.

kind of thunderbolt is called smoky by Aristotle and the commentators. Next, Ibn Rušd mentions some effects of the white thunderbolt and says that he took them from the commentators. As for the different colours of clouds, Ibn Rusd adds to Ibn al-Bitriq's account the feature that black clouds are earthy, whereas white clouds are not.

CHAPTER NINE

HALOES, RAINBOWS, MOCK SUNS AND RODS

1. Aristotle Aristotle describes these phenomena as follows: The halo (άλως) is a complete circle around the sun, the moon or a bright star; it occurs at night, and during the day at midday and in the afternoon; it seldom occurs at sunrise and sunset (371b23-27). Next to the bright ring of the halo there is a dark ring, which appears still darker because of its contrast with the bright ring (373a26-27). The rainbow (ϊρις) is part of a circle, never being larger than half a circle; it is half a circle when the sun is at the horizon, and the part becomes smaller when the sun rises; when the sun is above a certain height, no rainbow is possible at all. Thus, there is no rainbow at midday in summer. Also, the radius of the circle varies with the height of the sun.1 Sometimes one sees two rainbows at the same time; one never sees more than two rainbows.2 The rainbow has three colours: red (φοινικοΰν), green (πράσινον) and purple (άλουργόν); the outer band is red, the inner purple. Sometimes one sees yellow (ξ,ανθόν) between red and green. When a second rainbow is visible, it appears outside the first one; its colours are weaker and appear in reverse order, i.e. the inner band is red and the outer purple (371b27-372a11). Mock suns (παρήλιοι) and rods (ράβδοι) appear beside the sun, especially at sunset, but also at sunrise; they seldom appear at midday (372al2-17). Before turning to Aristotle's explanation of these phenomena it is useful to review his theory of light and colour. He sets forth this theory in De Anima 11,7, De Sensu et Sensibilibus 3 and Meteorologica 111,2 and 4.3 In De Anima 11,7 he says that vision occurs when two things are present: (a) an object of sight and (b) light. The objects of sight which 1

This is an incorrect observation; in fact, there are variations in the radius of the rainbow, but they do not depend on the height of the sun. 2 In fact, three and more simultaneous rainbows are possible. 3 See Sayili 1939 66-73.

are considered here are the colours of bodies. Other objects of sight are: phosphorescent bodies and fiery bodies; the former are visible in darkness only, the latter in light and darkness. Light is defined as the actuality of the transparent. The transparent inheres in certain things like water, air, glass, etc., as a common property. Transparency exists in them potentially; it is made actual by the presence of fire or some celestial light source. Darkness is absence (privation) of light; then the transparent remains potential. Thus, light is not some substance travelling from the light source to the eye or vice versa; it is not a motion, but a certain condition of the transparent, which comes about and disappears instantaneously. The colour of a body 'moves' the actually transparent, i.e. brings it in a state in which it accidentally obtains this colour (the transparent in itself is colourless); in this way the transparent transmits the colour to our eye. Thus, the actually transparent is necessary for vision; it is the medium which transmits colour between the visible object and the sense organ; no vision is possible without this medium. In the same way as the colour of a body 'moves' the actually transparent, by which it accidentally obtains this colour, fire and other light sources make the transparent actual; therefore one could say that light is the colour of the transparent, in a sense, when it has become actual by fire or some other light source. In De Sensu et Sensibilibus 3 this theory is elaborated somewhat further as follows: Presence of something fiery in a (potentially) transparent medium is light, absence of it is darkness. Transparency exists in substances like air, water, etc., which have no fixed boundary; it also exists to a certain extent in solid bodies, which have a fixed boundary. The transparency in unbounded bodies is light when something fiery is present; the transparency in bounded (solid) bodies is necessarily bounded, and its boundary is colour. Light is, as it were, the colour of the transparent in unbounded bodies. Light in the transparent medium is what colour is in bodies. Light and darkness in the unbounded transparent bodies correspond to the colours white and black in bounded bodies. Other colours arise by mixture of white and black; the substances in which these colours adhere are mixed and form a new substance with a new colour. Besides this, one may get the impression of other colours by juxtaposition or by superposition of white and black, but this does not really generate other colours. In the former case patches of white and black, each being too small to be seen separately, exist next to one another, so that the impression of another colour arises; in the latter case white and black are superimposed, as when one colour is painted

over another or when the sun shining through a haze appears red. Five other colours arise when white and black are mixed in certain proportions; ordered according to a decreasing admixture of white these colours are: yellow (ζανθόν), red (φοινικοΰν), green (πράσινον), blue (κυανοΰν) and purple (άλουργόν); grey (φαιόν) may be classified under black (De Sensu 4, 442a20-25). Note that, as darkness (black) is privation of light (white), mixture of white and black is equivalent to weakening of light; thus, the abovementioned colours are produced successively when light gradually becomes weaker. For the explanation of phenomena such as the halo and the rainbow, Aristotle needs to consider light to follow the rules of geometrical optics, e.g. that light travels along rectilinear paths and is reflected against certain bodies. It is difficult to see how light, when it is considered to be a condition of the transparent, could behave this way. Therefore, in the Meteorology Aristotle adopts a different theory of light and vision, namely that of the visual rays. According to this theory, an object is seen by means of visual rays (sight) which extend (instantaneously) from the eye of the observer to the visible object.4 In terms of this theory, colours arise by a weakening of sight or vision. Weakening of sight occurs when the distance of the object is very large or when the object is viewed by reflection, for reflection also causes a lengthening of the distance sight has to cover. Therefore objects seen at a distance, or seen via reflection, seem to be darker, and white objects may appear coloured if seen via reflection. Weakening of sight to an increasing degree gives rise to the colours red, green and purple successively. For instance, if a white cloud is seen via reflection in water, it appears darker and may get the colours of the rainbow (Meteor. 374b9-35).5 Furthermore, when reflection occurs, the degree of weakening depends on the darkness of the body against which sight is reflected; in other words: the light of the luminous object is mixed with the colour of the mirror, so that the object appears with a different colour (Meteor. 372b6-8). White light reflected from a dark surface becomes red, just as it becomes red when it goes through a dark medium. We see the latter phenomenon occurring when light goes through smoke, e.g. the flame of green wood looks red because it

4

Alexander notes that the theory of visual rays used by Aristotle in the Meteorology is rejected in De Anima, the theory expounded in the latter book is the 'correct' one. However, for the account of the phenomena under discussion it does not make any difference which of the theories is used. See Alexander, in Meteor. 1413-30. 5 In the Meteorology only three colours are mentioned, namely the colours of the rainbow, whereas in De Sensu five colours are mentioned.

produces much smoke, and the sun appears red when it is seen through smoke (Meteor. 374a3-7). Thus, Aristotle gives two causes for the phenomena that light becomes coloured or changes colour when it is reflected: weakening of sight because the visual rays cover a longer distance, and mixing of the colour of the light source with that of the mirror, just as mixing occurs when light passes through a dark medium. Both causes are adduced when he explains why the rainbow is coloured, whereas the halo is white (see below p. 247-248). Finally, weakening of sight may also be due to the weakness or moistness of someone's eyes (Meteor. 373b4-10 and 374a21-22). The laws of reflection are probably known to Aristotle, but he does not explicitly mention them in his works. The way Aristotle describes the reflection which explains the formation of the halo and rainbow even contradicts one of these laws, namely that the incident and reflected rays form equal angles with the reflecting surface. This might be explained by the fact that this law is suggested by a study of the reflected image of an object in a smooth mirror, and Aristotle probably thought that this law indeed holds for such kind of reflection. However, as we shall see below, he thinks that phenomena such as haloes and rainbows are formed by reflection of sunrays against raindrops in a cloud and that these give rise to another type of reflection: the raindrops act as numerous tiny mirrors, which are too small to give a reflected image of the object (sun), only colour is reflected in them.6 For a further discussion of Aristotle's theory of colour and its reception by Arabic authors, especially Ibn Rusd, Ibn Bājja and Ibn Sīnā, we refer to the paper by Gätje. Aristotle's explanation of haloes, rainbows, mock suns and rods starts with the statement that they are all due to reflection (όινάκλασις) of vision against a smooth surface, which acts as a mirror, towards the luminous object (sun, moon) (372al8-21 and 372a29-31). If sight strikes on a smooth surface, for instance a surface of water, it is reflected. Aristotle says in a digression that sight may even be reflected from a layer of air if someone's eyes are very weak. Then this sight cannot penetrate the surrounding air and it is reflected back by that air. Such a person will see his own image before him. That vision of an object may be influenced by air that has a certain condition also appears from such phenomena as promontories in the sea which seem to be raised above it, things which seem larger when a south-eastern wind blows or when they are seen through mist, and the

6

See Boyer 39-41.

sun which seems larger at its rising and setting (373a35-b13). In general, one may distinguish, according to Aristotle, two kinds of reflection: reflection in a mirror which is so small as to be indivisible for our sight—in such a mirror the shape of the object cannot be reflected, only its colour—and reflection in a larger mirror in which colour as well as shape are reflected (372a31-36). As for the phenomena under discussion, it is clearly not the shape of the light source which is reflected; therefore they must be caused by reflection of the first kind, against small mirrors. What takes the function of these mirrors are the small particles into which air and vapour condense in the atmosphere when they form clouds (mist or raindrops). These particles make up one continuous magnitude, so that the phenomena caused by reflection against them appear continuous (373al9-23 and 373b24-28). The halo is caused by reflection of sight towards the light source (moon, sun) against small particles of mist which are formed by condensation of vapour around the light source (373al-3). The halo is a circle around the light source because of the symmetry of the reflection. If A is the eye of the observer, Β the sun and C a point of the cloud where reflection of sight occurs towards the sun, then the relative position of these three points remains the same if one turns the triangle ABC around the axis AB. Then C describes a circle and all points of this circle are also points of reflection of sight towards the sun; that circle is seen as the halo (373a4-18). Of course, this does not explain why there is reflection only in points of the cloud which have a certain distance to AB, i.e. why only a circle in the cloud, not the whole cloud appears bright. When a halo is formed, it means that vapour is condensing into clouds; therefore it is a sign of rain. When the halo is broken up, it is a sign of wind, which breaks up the clouds. When the halo fades away, it is a sign of fine weather because then the dry, hot exhalation dominates the moist vapour and the latter does not condense into mist (372bl5-33). The condensations are more quickly dissolved by the sun than by the moon; therefore haloes around the moon are more frequent (373a27-29). The rainbow is caused by reflection of sight towards the sun against the raindrops which have been formed in a cloud opposite the sun (373b20-34).7 The difference with the halo is that the halo arises from a reflection against air or mist around the sun. When a halo is formed, sight has to cover a shorter distance, and is reflected from air (mist), 7

See also Boyer 41-44 and Sayili 1939 for an account of Aristotle's theory of the rainbow.

which is lighter in colour; when a rainbow is formed, sight covers a longer distance, and is reflected from drops of water, which have a darker colour. Thus, the halo appears as white and the rainbow as coloured, because in the latter case sight is weakened to a greater extent. A condensation of exhalation into raindrops around the sun does not persist, since it is dissolved by the sun or turns into rain; otherwise a halo could appear coloured, just as the rainbow (374a1-18). We can see a full circle with rainbow colours around lamps when the air is moist; it is formed in the same way as the halo, when the moistness in the air and the smoky soot act as a dark mirror. Because of the weakness of sight (i.e. of the light of the lamp) and the darkness of the mirror, we do not see red, but only purple (374al9-30). We also see rainbow colours in the sprinkling of water that occurs when oars are raised out of the water or when we just sprinkle water in a room. The colours appear in the same way as in the rainbow, i.e. in the direction opposite the sun. (374al9-30). A relatively small extent of weakening of sight gives red; more weakening gives green, then follows violet; these are the colours of the rainbow (374b31-34). Thus, according to Aristotle's theory, the outer (red) band of the primary rainbow corresponds to the place where the weakening of sight by reflection against the raindrops in the cloud occurs to a relatively small extent; the inner band corresponds to the place where this weakening is strong. One would expect that the difference in colour between the bands is explained, in accordance with Aristotle's theory of colour, by relating this difference either to a difference in blackness of the raindrops8—however, this would badly agree with the symmetry (circularity) of the band—or to a difference in distance to be covered by sight. However, Aristotle brings forward a completely different argument: he says that the reflection of sight to the sun is strongest in the outer band because it is the largest band (in area); therefore the outer band is red (375al-5). This means that reflection would even be stronger in the bands of the secondary rainbow, in contrast to what is observed: the colours in the secondary rainbow are weaker and in opposite order. Aristotle's explanation of the colours of the secondary bow is in accordance with what we expect from his theory: sight has to cover a greater distance to reach the sun via reflection in the secondary bow than in the primary one; therefore the colours are fainter. Furthermore, this distance also differs when one compares the bands within the secondary bow: sight covers a smaller

8 As for the rods, the existence of different colours is indeed explained by a d i f f e r ence in composition of the cloud, see below pp. 250-251.

distance when it is reflected in the inner band; therefore that band is red. A third rainbow is not formed because the distance sight has to cover would be even larger than for the second one, so that it would appear still fainter 9 (375a30-b15). The yellow colour, which appears between red and green, arises by contrast. If a colour is placed next to another one it may give a different impression. For instance, the moon rainbow looks whiter, since it appears at night and against a dark cloud. Similarly, the part of the red band next to the green band appears whiter than the other parts, and thus gives an impression of yellow (375a7-22). The circular form of the rainbow is explained in the same way as that of the halo. In his explanation (Meteor. 111,5) Aristotle uses a hemisphere on the circle of the horizon, with its center Κ at the observer's eye. The sun as well as the reflecting cloud are on the surface of this hemisphere, each on different sides of K. Consider the vertical plane through the sun and K; its intersection with the hemisphere is a semicircle S. First, Aristotle considers the case that the sun is at the horizon. If Η is the sun and M a point of the cloud on S at which sight from Κ is reflected towards H, then the relative position of these three points remains the same when one turns the triangle HMK around the axis HK. Then M describes a circle in a plane perpendicular to HK (and also perpendicular to the plane of the horizon), and all points of this circle are also points of reflection of sight towards the sun. Exactly one half of this circle is above the horizon; that semicircle is seen as the rainbow when the sun is at the horizon. Next, Aristotle considers the case that the sun is above the horizon. This means that the triangle HMK is turned around Κ in its own plane. When Η rises to a certain distance above the horizon, M descends towards the horizon the same distance. The rainbow is again obtained by turning HMK around HK; then M describes a circle in a plane perpendicular to HK. The part of this circle which is above the horizon is now less than a semicircle. When H rises further above the horizon, M finally arrives at the horizon, and no visible rainbow can be formed. Therefore there is no rainbow in sunmer around midday.

9 Webster suggests that Aristotle means to say that the second rainbow is a reflection of the first one, i.e. that it is formed after a second reflection of the visual rays. This would explain that the colours are fainter and appear in reversed order. However, Aristotle gives no indication against which mirror this second reflection would occur. Alexander mentions that, according to some people, the secondary rainbow is a reflection of the primary one, against a cloud that is further away; however, according to Aristotle—says Alexander—the secondary rainbow is formed in the same way as the primary one by reflection to the sun against a cloud that is further away; see in Meteor. 159,9-21.

Aristotle's explanation of these properties of the rainbow is correct. However, what remains to be explained is, just as in the case of the halo, why reflection only occurs in points of the cloud which form (part of) a circle through a certain point M, i.e. why only (part of) a circle in the cloud, not the whole cloud, appears coloured. The rays KM and MH do not fulfill the law of equal angles for reflection, if the surface of the hemisphere is considered to be the reflecting surface. Thus, point M is not determined by the requirement that these rays must follow this law. Aristotle brings forward another requirement for M: point M is determined by the condition that there must be a certain fixed ratio between the distances MH and KM; thus, reflection occurs only when M is such that MH/KM = c. He gives a geometrical proof that for a given ratio c there is only one point M on the semicircle S, such that the distances MH and KM have this ratio c, and he gives a method to determine M when c is known. M is found as the intersection of the above-mentioned vertical semicircle S, which has Κ as its centre and HK as its radius, with a circle in the same plane, namely the circle that is the locus of the points for which holds that the distances to the fixed points Η and Κ have the ratio c. The centre and radius of this circle are determined by a certain method described by Aristotle. The same method is found in one of the works of Apollonius, as Heath has remarked. We refer to his work for an account and discussion of this method.10 Aristotle does not discuss a way to determine c; his demonstration shows that he had knowledge of certain rather advanced mathematical theorems, which were later to be discussed in the works of Apollonius, but it does not contribute to the explanation of the rainbow. In order to explain also the secondary rainbow Aristotle has to assume—which he does not do explicitly—that reflection occurs for two values of the ratio c; he gives no clue how he thought this would relate to physical reality. The (non-existing) effect that the radius of the rainbow varies with the height of the sun is mentioned (see above p. 243), but not explained by Aristotle. Rods are fragmentary light phenomena near the sun (374al6-17). They are formed by reflection of sight against small particles of water in a cloud near the sun when the cloud's constitution is uneven, either because it does not have the same density everywhere, or because its different parts are more or less watery. Because of the inhomogeneous

10

Heath 181-189.

composition of the cloud, different colours appear in the rods: red, yellow and green (377a30-b14). Mock suns are formed in the same way as rods, but in this case the reflecting cloud is homogeneous, so that the mock suns do not appear with different colours; moreover, the reflection occurs against particles of dense mist, which have not yet reached the stage of water, so that it appears white, like the halo. Therefore mock suns are a sign of rain, especially when they are in the south, since the air in the south is more liable to turn into rain than in the north (377bl5-28). Mock suns and rods appear in a place where on the one hand the cloud will not be dissolved by the sun and where on the other hand sight is not too much weakened. Thus, they do not appear very close to the sun, for the sun will dissolve clouds close to it; this will always occur for clouds which are below the sun, close to the earth. They do not appear very far from the sun either, for if the cloud is too far, sight will be weakened too much; this always occurs when the cloud is above the sun at a distance, when it is below the sun, high in the sky, and when the sun is high in the sky. Only when the sun is low and when the cloud is beside the sun, there is a distance at which neither the cloud is dissolved nor sight is weakened too much; it is there that mock suns and rods appear (377b28-378a11).

2.

Theophrastus

In the Arabic version of Theophrastus' Meteorology from the phenomena considered in this chapter only the halo around the moon is discussed. Theophrastus adduces a cause for the halo which is partly different from that given by Aristotle. Vapour and moonlight play a role, but the halo is not a phenomenon caused by reflection of moonlight only. He says that the halo (dä'ira) arises in air filled with vapour when moonlight causes a wavelike motion in it, in the same way as when we throw a stone into water and circular waves are formed. The rays of the moon rarefy and 'sweep away' the air vertically under the moon, so that there the air becomes thin, whereas the air gathers and becomes thick in a circle around it. Something similar occurs when one blows through a pipe on a surface covered with dust: the dust is swept away and gathers in a ring. Wind disperses the thick air; then the halo disappears. This also occurs when it rains.11 Thus, according to Theophrastus, vapour is densified in a circle 11

Theophrastus, Meteorology

ch. 14,2-13 and Steinmetz 1964 1970-200.

around the moon; it is against this dense vapour that light is reflected, so that it becomes visible as a circle of light. The halo is not only a phenomenon κατ έ'μφασιν, but also καθ' ύτιόστασιν. This theory gives a tentative explanation why only a circle in the cloud, not the whole cloud appears bright, something left unexplained in Aristotle's account. Theophrastus' explanation of the halo is quoted by Ibn Suwār ibn a1-Kammār in his Treatise on Meteorological Phenomena.n

3. The Greek

commentators

When a halo appears, only a circle in the cloud, not the whole cloud is bright. Alexander's explanation for this is similar to that of Theophrastus: The area of the cloud that is vertically under the luminous object (sun or moon) becomes rarefied under the influence of its light. If the cloud is uniform, this occurs equally in all directions up to a certain distance from the axis joining the observer and the luminous object. Beyond this distance the cloud remains as it is and small particles of mist gather there. Thus, a circular area of thin vapour is surrounded by a ring of particles of mist against which visual rays are reflected to the luminous object. This causes the effect of the halo. Alexander says that this is Aristotle's theory of the halo.13 Alexander then states that Posidonius follows Aristotle, but that almost all others say that the halo does not arise by reflection, but by refraction of visual rays, as occurs when we see something through water. They assume that the cloud is concave and spherical and that the star above the cloud is seen as a circle due to dispersion of the rays in the cloud. Sosigenes has sufficiently shown the falsehood of this theory.14 Aristotle mentions the possibility of reflection of visual rays against layers of air and then gives some examples (373b10 ff.) of other effects air may have on vision, such as that promontories in the sea seem to be raised above it and that things seem larger when a south-eastern wind blows and when they are seen through mist. Alexander asks why Aristotle adds these phenomena here, since they are not due to reflection. The answer is that these examples are added in order to show that vision is influenced by thick and moist air in the same way as it is influenced by water. Then it follows that visual rays may be reflected from air, as they are reflected from water. It is well known that things appear larger if they are seen through water. The examples show that 12 13 14

Ibn Suwār, Treatise on Meteorological Alexander, in Meteor. 142,28-1433. ibid. 143,7-14.

Phenomena

see below p. 336,1-11 and p. 276.

the same effect occurs when they are seen through thick and moist air (the south-eastern wind is moist; mist is moist and thick). Thus, moist air and water have the same effect. The reason why things seen through water amd moist air seem larger is that they are seen less clearly. This is further explained as follows: Things that are far away are seen less clearly; therefore we are used to think that things that are seen less clearly are far away. Also, if a thing is farther away, it is seen at a smaller angle and with a smaller size than when it is closer to us. If something is close and consequently it is seen at a larger angle, but at the same time it is seen less clearly because of the medium through which it is seen, so that we think it is farther away, then we see it as larger than usual. For if it really were at the distance we think it is, we would see it at a smaller angle; now it seems to be at that distance, but it is seen under a larger angle; therefore it appears larger to us. Another reason is that if we see something through a moist and thick medium, the impression is spread out and becomes more vague, as lines drawn on moist paper spread out and become vague.15 Also Olympiodurus recognizes that Aristotle's examples of promontories that seem to be raised above the sea in moist weather and of things that seem larger through mist are not examples of reflection against air. Things appearing larger are an indication of refraction, not of reflection. He settles the matter by saying that Aristotle usually does not attach much importance to examples. When a promontory seems to be raised above the sea, this may be explained by the fact that mist gathers at its foot and hides it; one cannot discern this mist from the water of the sea and thus one only sees the top of the promontory rising from the sea.16 Alexander says that Philippus, Plato's associate, showed that the rainbow is an optical phenomenon. He argued that if an observer changes its place and goes to the right or to the left, the rainbow also goes to the right or to the left. This is a characteristic of what exists κατ εμφασιν. What really exists moves in the opposite direction of the motion of the observer. Also, if we approach the rainbow or recede from it, the rainbow should approach us or recede from us, as images in a mirror. Whether this effect is observable for such a large distance remains to be seen.17 Having discussed the primary and secondary rainbow with their different order of colours Alexander asks why, if the outer band of the 15 16 17

Alexander, in Meteor. 149,5-150,12. Olympiodorus, in Meteor. 232,21-233,10. Alexander, in Meteor. 151,32-152,16. See also Boyer 62-65.

primary rainbow and the inner band of the secondary rainbow are both red, the area between both rainbows is not red as well. He does not give a real explanation, but just says that only certain specific parts, not all parts of the cloud, are suitable to reflect visual rays.18 Olympiodorus, in an introduction to his discussion of phenomena such as the halo and the rainbow, shows that these phenomena are κατά εμφασιν, and, more particularly, that they are formed by reflection (κατά ανάκλασιν), not by refraction (κατά διάκλασιν). He says that, according to Alexander, the halo is indeed formed by reflection, but that the rainbow is formed by refraction, and moreover, that the form of the rainbow is κατά εμφασιν, but that its colours are κατά ύπόστασιν. This is not what we find in Alexander's commentary. Alexander explicitly says that the halo and rainbow are both due to reflection,19 and he does not say that the colours of the rainbow are κατά ύπόστασιν. Olympiodorus' mistake could be explained by the fact that he did not know Alexander's commentary directly.20 Olympiodorus shows that the halo and rainbow are phenomena κατά εμφασιν as follows (he takes the halo as an example): (1) The luminous object is always seen in the centre of the halo, whatever the place of observation may be. If it were a phenomenon κατά ύπόστασιν, one would see the sun or the moon not always at its centre, if one views it from different places. (2) If it were a phenomenon κατά ύπόστασιν, it would not disappear when a cloud passes before the luminous object. This does happen, however, and thus the halo needs a luminous object for it to occur. (3) Such an exact mathematical form as the halo (a perfect circle) cannot consist of unstable substances such as air and clouds.21 Next, Olympiodorus discusses reflection and refraction.22 He says that when we see things at which we do not look directly, this is due to reflection and refraction. For instance, in a mirror we see things that we do not look at and also things that are hidden by other things and cannot be seen directly. If we drop a ring in an empty vessel, we cannot see it because the sides of the vessel obstruct the view. When we fill it with water, we can see the ring. This is due to refraction. The visual rays are broken in all these cases. Reflection and refraction differ in the following respects: (1) In the case of reflection object and

18

Alexander, in Meteor. 160,21-33. ibid. 152,18. 20 Steinmetz 1964 42. 21 Olympiodorus, in Meteor. 210,15-38. 22 See also Boyer 67-73 for an account of Olympiodorus' discussion of the rainbow and halo. 19

observer are in one plane and the mirror is opposite that plane; in the case of refraction the surface that breaks the visual ray is between the object and the observer. (2) Reflection occurs at equal angles, refraction at obtuse angles. Olympiodorus elaborates on the matter of reflection, and derives the law of equal angles from the statement that nature never works in vain; in the case of reflection this means that the visual rays travel from the observer via the mirror to the object along the shortest route. He shows that if the reflection does not occur at equal angles, the distance covered by the visual rays is always longer. As for the statement on refraction, he gives no quantitave data, but only says that if a visual ray strikes upon the surface of a different medium, it is bent towards to the normal on that surface, the angle between the broken ray and the surface being obtuse. (3) Things seen by means of reflection seem to be smaller, those seen by refraction seem to be larger. The reason is that the visual rays converge to the object after reflection, but diverge after refraction. We see things under water as larger than they really are, and the rising sun, when it is seen through mist, seems to be larger. It is clear that the halo and rainbow are formed by reflection, not by refraction, for if they were formed by refraction, the sun and the moon would appear larger when the rainbow or halo is present; this is not the case. Also, the halo and rainbow would appear between the sun or moon and the observer, which is not the case either.23 Olympiodorus enumerates the differences between the halo and the rainbow as follows: (1) The halo is mostly a complete circle around the luminous object; sometimes it is smaller; then it may be more or less than a semicircle. The rainbow is always a semicircle or smaller. It is a semicircle when the sun is at the horizon; then its radius is at its smallest. When the sun is above the horizon it is less than a semicircle and its radius is larger. (2) The halo mostly occurs around the moon, rarely around the sun. The reverse is the case for the rainbow. (3) The halo mostly occurs at night, the rainbow mostly during the day. (4) The halo mostly occurs at midday, seldom near the horizon; the rainbow mostly occurs near the horizon, seldom at midday. Only in winter the rainbow may occur at midday. (5) The halo is single, the rainbow is double. (6) The halo has one colour, sc. white; the rainbow has three colours: red, green and purple. (7) The halo is formed by reflection in a smooth and homogeneous cloud, whereas when a rainbow is formed, reflection occurs in an uneven and inhomogeneous cloud, that is, a cloud of which some parts are more near its surface than other parts

23

Olympiodorus, in Meteor. 211,1-214,28.

(i.e. some parts are closer to us and other parts are further away) and of which some parts are denser than other parts.24 About rods and mock suns Olympiodorus remarks (1) that they occur beside the sun. (2) They only occur during the day. (3) They occur near the horizon more often than at midday. They differ in that rods are formed in uneven and inhomogeneous clouds, just as the rainbow, whereas mock suns are formed in even and homogeneous clouds, just as the halo. Therefore the latter are white and the former have three colours.25 In order to explain how phenomena such as the halo and the rainbow are formed, Olympiodorus makes the following assumptions: (1) For reflection to occur one needs a mirror that is dense, smooth and transparent. It must be dense, so that the visual rays do not penetrate into it, but are reflected from it towards the luminous object. It must be smooth, for it it were uneven, the rays would not stay together, but would disperse in all directions. It must be transparent, so that the image of the luminous object may appear in it. (2) In very small mirrors, only the colour, not the form of the object appears, as when water is sprinkled and the sun is reflected in the drops. (3) Sight is weakened by reflection, as also occurs when we see something through mist or with a weak power of sight. In such cases a bright object seems darker or becomes coloured.26 Olympiodorus gives the proof that the halo must be circular in the same way as Aristotle. Then he asks why the halo is a ring and not a whole disc, i.e. why reflection occurs only at a certain specific distance from the axis connecting the luminous object with the observer. He answers, unlike Alexander (see above p. 252), that the visual rays that reach the luminous object along this axis or at a small angle with it, are more powerful than those that are emitted at a larger angle with the axis.27 These rays are not reflected, but continue their path straight through the cloud. Thus, in the middle of the halo one does not see a reflection, but the real object itself. The visual rays that are emitted at a larger angle with the axis are weaker and cannot penetrate into the cloud; they are reflected and give rise to the ring of the halo. Furthermore, one may ask why the halo moves with the daily motion of the moon when it is formed in clouds that do not move with that motion; as one knows, clouds are formed in the lower region of the atmosphere, the part that does not move with the daily motion, 24 25 26 27

Olympiodorus, in Meteor. 217,27-21835. ibid. 218,35-219,7. ibid. 219,8-23. This is a well-known opinion in optics, see above p. 197n11.

unlike the upper atmosphere which moves along with it. The answer is that the halo is a phenomenon κατά εμφασις and does not have an existence of its own. It is formed when we look at the luminous object and the air is in a suitable condition for its formation; when we divert our sight from it, it does not exist anymore. It is always formed as a circle around the moon, whatever the place of the moon. If we look at it again after the moon has changed its place, it is not the same halo as before; that one perished when we stopped looking; when we start looking again a new halo is formed, in another place.28 Further properties of the halo are explained: The halo is formed close to the earth because that place is calmer. It is formed more often around the moon than around the sun, because the sun dissolves exhalation that has gathered, whereas the moon does not. For the same reason the halo more often appears at night than during the day. It more often occurs when the sun is at the meridian than when it is near the horizon, because it needs some space to exist. The diameter of the halo is estimated to be 40 degrees;29 when it is formed at the meridian sufficient space is available.30 Next follows Olympiodorus' account of how the rainbow is formed. (1) Both the halo and the rainbow arise by reflection, but the halo is white and the rainbow is three-coloured. The reason is that (a) the halo arises from reflection from a white, rare cloud that is less moist, whereas the rainbow is formed by reflection from clouds that are dense, moist and dark, (b) When a halo is formed the visual rays cover a smaller distance to the luminous object than when a rainbow is formed; therefore they are less weakened. Olympiodorus remarks in passing that, according to Aristotle, the celestial bodies themselves are uncoloured, for colour only occurs in the world of the four elements. (2) The rainbow is formed opposite the sun, not around it. In order to understand this one should know that the rainbow is formed while the cloud is condensing into small drops, before these drops have coagulated and turned into rain. If such a cloud is near the sun, it is quickly influenced by the sun's heat; then the dry exhalation which is mixed with the cloud is quickly separated and the cloud turns into rain, so that there is no opportunity for the rainbow to be formed. If such a cloud is opposite the sun, the period in which small drops are present is longer and a rainbow may be formed. (3) The ring that occurs around a lamp has one colour, sc. purple, and is a complete circle. It is formed by reflection against the soot. If 28 29 30

Olympiodorus, in Meteor. 222,29-223,17. Modern works usually give 44 degrees. Olympiodorus, in Meteor. 229,30-230,9.

someone sprinkles water in a fine spray opposite the sun, a rainbow is formed due to the reflection against the drops, with a purple colour. Thus, a rainbow is not necessarily situated opposite the light source, nor is it always three-coloured, nor is it always a semicircle or less. (4) Light seen through something slightly dark or via reflection from a slightly dark mirror gives a red colour; therefore the rising and setting sun, seen through the mist of the air close to the earth, appears red. Furthermore, visual rays that travel a long distance become weaker. Therefore distant objects appear darker, jagged objects appear smooth and big things appear small.31 The same occurs for objects that are close by, but are seen via reflection, for in that case, too, the distance covered by the visual rays is long. Therefore we see a white cloud reflected in water as dark. The weakening of sight that occurs when we see a dark colour is a negation of sight to a certain extent; it is not a complete privation, for we still see different colours when sight is weakened. (5) The explanation of the different colours is as follows: The visual rays are reflected in the cloud from different layers of drops. Those that are reflected from the layer that is closest to us are weakened to the least extent and the error (απάτη) occurring to vision is only small. It is as if they were passing through a slightly dark medium. Therefore that part of the cloud is seen as red. The visual rays that fall on a layer of the cloud that is further away are weakened more; therefore that part of the cloud is seen as darker: its colour becomes green. The rays that fall on a still further part of the cloud cause a still darker colour that part becomes purple. (6) Olympiodorus says that Aristotle gives two reasons for the order of the coloured bands in the inner and the outer rainbow: the distance of the bands explains the order in the outer rainbow and the size of their surface explains the order in the inner one. The argumentation for the inner rainbow is that the band with the largest surface, that is, the outer band, receives most visual rays; therefore it is seen most clearly, 31 That is, sight errs in the perception of certain properties of visible objects that are far away. It is clear that visible objects have certain properties that are perceived by sight. Ptolemaeus mentions seven of such properties: corporality, size, colour, shape, position, motion and rest (Ptolemaeus, Opt. 11,2). Ibn al-Haylam extended this list to 22 properties, among which roughness and smoothness. Errors in the perception of visible properties have been a subject of discussion in antique and Arabic works on optics. Ibn al-Haylam devotes a whole Book of his Optics on errors in direct vision (Ibn al-Haylam, K. al-Manāzir III; see also the commentary in Sabra 1989 vol. II 106-111.) However, this discussion does not show much relation with the way the subject is discussed by the authors under consideration here. We shall come across such a discussion in Olympiodorus (see next four paragraphs), Pseudo-Olympiodorus (see below pp. 270-273), Ibn Suwār (see below p. 276) and Ibn Sīnā (see below p. 279).

with only little error. Thus, this band is seen as red. Fewer visual rays fall on the next adjacent band because it is smaller in size. Vision is less clear and more error occurs; this band is seen as green. Similarly, the next band is seen as purple. The order of the colours in the outer rainbow is explained by the distance of the band to the observer, as has been shown above (previous paragraph). Ammonius comments on this explanation, says Olympiodorus, and criticizes Aristotle for giving two explanations (the size of the bands and their distance) for one and the same phenomenon (the order of colours). Instead of Aristotle's explanation, Ammonius gives the following account: The eye emits a cone of visual rays. The rays along and near the axis are the strongest rays, whereas those forming an appreciable angle with it are weaker. Thus, what is hit by rays close to the axis is seen with little or no error; the error is larger in what is seen by rays further from the axis. If we look at the double rainbow, the axis of our vision falls in between both rainbows, between the two red bands. In that region no colour arises because no error occurs. Because of the same reason the halo is a ring, not a whole disc, as was shown above. The adjacent regions of the cloud are struck by rays that are at a small angle with the axis; they are slightly weaker and moreover travel a larger distance; therefore these regions appear as red. The next adjacent regions are struck by rays that are weaker and travel a further distance. These regions are green in each rainbow. Finally, the purple colour belongs to the regions that are struck by rays that are at an appreciable angle with the axis and therefore are still weaker. Olympiodorus claims that this is what Aristotle in fact also says, although he appears to say something else. Aristotle says that (375a2 ff.) "the outer band of the primary rainbow is red because most vision is reflected to the sun from the largest band." We must interpret 'most vision' not as largest number of visual rays', as was done above, but as 'strongest vision'. The rays falling on this band are stronger than those falling on other bands; therefore this band is red and the other bands have a darker colour. Aristotle did not mention the reason why the rays striking this band are stronger (sc. because they are closer to the axis of vision); therefore people thought that the colour was determined by the number of rays that struck that band.32 Olympiodorus' explanation of the yellow colour between red and green follows Aristotle. He adds an argument from Ammonius showing that the yellow cannot arise from reflection such as the other three colours. If yellow were to arise by reflection it would not be between 32

Olympiodorus, in Meteor.

233,22-239,28.

red and green, but before red, since it is lighter than red, closer to white, and sight errs less when it sees yellow than when it sees red.33 Then Olympiodorus considers Aristotle's proof that the rainbow has a circular form (see above p. 249). The sun and the reflecting cloud are on the surface of a hemisphere on the circle of the horizon. The observer Κ is situated in the centre of that hemisphere. Olympiodorus comments on this and says that Κ cannot be in the centre, because then the distances of the sun Η and the cloud M to Κ would be equal, which cannot be true. This is also clear from the fact that if Κ were the centre, then, according to the laws of reflection, the ray KM would be reflected against the cloud back to K, not to H. For if we draw the tangent SMT in M, then KM is perpendicular to SMT. If we take Κ not in the centre, then the ray KM does not strike the cloud under a right angle, but it forms with SMT an acute angle KMS and an obtuse angle KMT. Then the reflection occurs towards the sun H, in accordance with the law of equal angles, along MH, in such a way that the angles HMT and KMS are equal. Olympiodorus claims that Aristotle knew that Κ should not be in the centre of the hemisphere: Aristotle says that the ray KM is reflected from the hemisphere towards Η "over the greater angle" (375b24), that is, the reflected ray MH falls within the greater (= obtuse) angle KMT, in such a way that HMT equals KMS.34 Olympiodorus gives Aristotle's method to determine point M if the condition is given that the distances of M to H and Κ have a fixed ratio c. As we have seen above (p. 250), this method uses the circle that is the locus of the points of which the distances to the fixed points Η and Κ have the ratio c. Aristotle describes how the centre Ρ and the radius of this circle may be determined. Olympiodorus and other commentators interpreted Aristotle's text differently: they thought that the centre Ρ was meant to be a pole of the (semi)circ1e that formed the rainbow, i.e. that if MP is turned around the axis HP, M describes the circle that forms the rainbow. In fact, Ρ may have this function, but it is not its function in Aristotle's description of how M is determined. That Ρ was interpreted as a pole of the rainbow appears from Olympiodorus' account of some commentators' (he does not mention names) proof that Ρ falls within the horizon. He does not approve this proof, because it uses the fact that the distances HK and KM are equal. Olympiodorus already criticized this assumption in another context, see previous paragraph. Then he gives a proof from Ammonius that does not use this assumption.35 33 34

.

35

Olympiodorus, in Meteor. ibid. 248,8-249,14. ibid. 255,19-258,5.

2453-6.

Aristotle mentioned, but did not explain the (non-existing) effect that a rainbow which is a full semicircle has the smallest radius, whereas rainbows which are smaller than a semicircle have a larger radius. Olympiodorus says that this may be explained by considering the fact that rainbows smaller than a semicircle do not rise high above the earth. Therefore they are seen through a vapourous atmosphere and seem to be larger. Another explanation—a better one, says Olympiodorus—is that a rainbow which remains close to the earth is seen by visual rays that travel a shorter distance than a rainbow which rises higher. The latter seems smaller because the distance covered by the visual rays is longer.36 Concerning rods, Olympiodorus quotes Alexander. He says that, according to Alexander's interpretation of Aristotle's text rods are formed by reflection of our vision against clouds towards the sun. Then Olympiodorus quotes Ammonius, who says that if this were the case, the phenomenon would be circular, just as the halo and rainbow. Instead, rods are formed by reflection of our vision against a cloud towards another, white cloud. The truth of this view appears from the following: (1) Aristotle compares the formation of rods with the reflection of white clouds in water. The image may contain coloured rods and they arise without reflection to the sun. Similarly rods arise in the sky without the sun playing a role. (2) Rods are formed in clouds of uneven constitution, says Aristotle. This agrees with their being formed by reflection from a dark cloud to a white cloud. The colours arise because of the unevenness of the reflecting cloud.37 In the main, Olympiodorus does not differ from Aristotle's explanations. He systematizes Aristotle's account and gives a "different interpretation" of certain passages, as he formulates it. In fact, these different interpretations are deviations from Aristotle's theory. Also, Olympiodorus presents some additions to Aristotle's geometrical proofs in relation to the rainbow.

4. Ibn al-Bitriq and Hunayn ibn Ishāq In the Arabic version of Aristotle's Meteorology Ibn al-Bitrlq discusses the halo (hāla\ also referred to as istidāra), the rainbow (qaws quzaha) and rods ('amid, pi. a'mida). Mock suns are not mentioned. These phenomena are explained by reflection of light; however, the explana-

36 37

Olympiodorus, in Meteor. 261,18-28. ibid. 263,20-264,26.

tion does not use the language of the theory of visual rays, such as Aristotle's explanation, but that of rays of light which are emitted by the light source and then reach our eyes. The same holds for Hunayn's Compendium. The description of the way in which the reflection occurs is confusing in the texts of Ibn al-Bitrlq and Hunayn. One gets the impression that the rays of the light source are reflected against air towards moist vapour that rises from the earth and gathers in the atmosphere; the phenomena are seen in that vapour, as when light is reflected against water towards a wall, one sees this reflection on that wall. In another passage however, it seems to be the cloud or a spray of fine particles that is the reflecting surface. Also, the description of the colours is confusing, as will appear from the account below. After a description of the halo and its occurrence, following Aristotle's text (371b23 ff.), Ibn al-Bitrlq gives the cause of the halo as follows: Moist vapour rises from the earth and then condenses38 in the atmosphere. When the sun shines on it, its light shines in the air behind or in that vapour; then the rays are bent off ('utifa), and are reflected towards the rising vapour, so that one sees a circle.39 Ibn Tibbon exactly follows Ibn al-Bitriq's text, but Ibn Rusd's Middle Commentary deviates from it in certain details, resulting in a more comprehensible text, see below p. 292. The halo as a weather-sign is described somewhat differently from what is in Aristotle: it is a sign of moistness and water when it becomes denser; it is a sign of fine weather, dryness of the earth and winds when it becomes weaker and is dissolved, since it is the hot, dry exhalation—which is the matter of winds—that dissolves the moist exhalation, which is the matter of rain and clouds.40 Ibn al-Bitrlq does not give Aristotle's geometrical proof for the circularity of the halo (373a4 ff.). A comparison of Ibn al-Bitrlq's text on the halo with that of Ibn Tibbon's translation shows that in several places Ibn Tibbon's copy of his text was better than the copies extant now. In other places, however, his copy seems as corrupt as our copies. This appears, apart from the already mentioned case of yaktuf and yaktur, (see above 38

We suggest an emendation of the text in Ibn al-Bitriq, Meteor. 88,9: yaktuf (condenses) instead of yaktur (increases), as the Greek text always has 'condensation' in this connection, although Hunayn has yaklur (Jawāmi' 274) and Ibn Tibbon also reads yakfrr (Otot ha-Shamayim III 79). Ibn Rusd writes: yaktur wa-yatakätaf (Middle Commentary 140,18-19). 39 Ibn al-Bitriq, Meteor. 88,8-11. 40 ibid. 89,1-5. The Arabic text is corrupt here; the correct content is inferred from Hunayn (Jawàmi' 277-280>, Ibn Tibbon (Otot ha-Shamayim III 83 ff.) and Ibn Rusd (Middle Commentary 147,2-9) agree with the emended text.

p. 262n38) from one of the last phrases of this section on the halo. The (Arabic and Hebrew) text reads: "The moon is sometimes also surrounded by a halo due to the sun's heat that dissolves the vapour drawn up by it, as a halo that shines from its light."41 Ibn Tibbon adds a commentary, saying that the commentators Alexander, Ibn Sīnā and Ibn Rusd wrote the opposite, namely that the halo mostly appears around the moon, and rarely around the sun. Indeed, Aristotle says that the halo more often occurs around the moon than around the sun because "the sun, being hotter, quicker dissolves the condensations of the air" (373a327 ff.). Thus, Ibn al-Bitriq's above-mentioned phrase, that is meaningless such as it is, should be emended in the sense of Aristotle's text. Ibn Rusd gives a text that agrees with Aristotle.42 Ibn Rusd gives in his Middle Commentary a better reading than Ibn al-Bitrlq in all above-mentioned cases of a corruption. This does not prove that Ibn Rusd's copy of Ibn al-Bitriq's text was better than the ones we have: he may have improved the text on the basis of Alexander. The discussion of the rainbow starts with a description corresponding to Aristotle's text 371b27 ff. Concerning the variation of the radius with the height of the sun Ibn al-Bitriq adds: "It appears with a smaller size at sunrise and sunset because the sunrays which shine on the clouds decrease."43 Thus, there is an effort to explain this effect, although the explanation is not clear. Ibn Rušd adds: "I think he means the rays that participate in the reflection"44 The description of the colours is peculiar: the outer band is wine-red (kamrī) and the inner band, which is the smallest in size, is greenish (yalī al-kudra). Between the wine-red and greenish colours one may see red (humra) and yellow (sufra). In a later passage the colours are said to be wine-red, a mixed colour, green and white i.e. yellow (see below p. 265). 45 Ibn Tibbon also had this text and he complains about its confusedness and incompleteness. He infers a better text from Alexander and Ibn Rušd's Short Commentary46 Ibn Rusd's Middle Commentary (not used by Ibn Tibbon) has a better version: the outer band is wine-red (kamri), that is, light-red (ašqar), the next one is green, and the one adjacent to green (yall al-kudra), which is the smallest in size, is purple (urjuwānī). Between the green and wine-red colour one may

41 42 43 44 45 46

Ibn Ibn Ibn Ibn Ibn Ibn

al-Bitriq, Meteor. 89,7-8; Ibn Tibbon, Otot ha-Shamayim Rusd,' Middle Commentary 147,11-13. al-Bitriq, Meteor. 89,12-14. Rusd, Middle Commentary 147,20. al-Bitriq, Meteor. 90,5-91,4 and 95,3-7. Tibbon, Otot ha-Shamayim III 113-138.

III 88 ff.

see yellow (sufra). 47 We see, with Ibn Rusd, that a comprehensible text arises that agrees with Aristotle, if one interprets the expression y all al-kudra not as 'greenish', but as 'adjacent to green', along with some other emendations. The cause of the rainbow and the rods is the same: they arise because the sunrays are reflected towards the rising vapour, just as rays that shine upon water and are reflected towards a wall. If the air is coloured by the coloured light that shines in it, and that colour is reflected to another body, then that colour shines on that body, similar to what occurs when water reflects rays of light to another body. Reflection may occur from all smooth, gleaming bodies.48 Again, Ibn Rusd gives a clearer version: The cause of the rainbow and the rods is the same: they arise because the sunrays are reflected from the moist vapour to our vision, just as rays that shine upon water and are reflected towards a wall on which the sunrays do not shine directly. If the air has almost the nature of water and is coloured by the coloured light that shines in it and that colour is reflected to another body, i.e. a body opposite that air, then that body is coloured by it, similar to what occurs when water reflects rays of light to another body opposite it.49 After the statement that reflection occurs from all smooth, gleaming bodies, Ibn Rusd and Ibn Tibbon give some more text on this subject, which is not in the extant copies of Ibn al-Bitriq's text. It is stated that small mirrors only reflect colour and larger ones reflect form and colour. It may occur that a colour is seen in a mirror not as it really is, but more turbid. This is due either to the fact that this colour is mixed with the colour of the mirror, if that mirror is not very clear, or due to weakness of sight.50 Ibn al-Bitriq further remarks that the colours of the rainbow arise when the sky produces rain51 and the remnant of vapour that is enclosed in a cloud changes into a spray (rašš) of small particles. This occurs before the cloud produces large raindrops. If the sun shines opposite this spray and cloud, they become something as a smooth mirror and transfer the colours that shine upon them to the air. If the reflecting surface is more watery, the colours become darker; if the air

47

Ibn Rusd, Middle Commentary 148,6-8 and 14. Ibn al-Bitriq, Meteor. 91,4-6 and 12-16. 49 Ibn Rusd, Middle Commentary 149,9-12 and 151,7-11. 50 ibid. 151,16-152,7; Ibn Tibbon, Otot ha-Shamayim III 152-159. 51 These are the words in Ibn Rusd's Middle Commentary (154,5) and in Ibn Tibbon's translation (Otot ha-Shamayim III 174); Petraitis' text has: 'becomes turbulent' 0idtaraba) (92,10), instead of 'produces rain'. 48

is clear and the cloud is nearer to us, the colours are lighter.52 The red colour of the rainbow arises just as the red colour of the sun when it is seen through much vapour. A white, i.e. yellow, colour arises from reflection against the spray that is formed from the cloud. This colour does not remain long.53 Ibn Tibbon's text on what is explained in the last three phrases is more extensive; it is a very corrupted version of 374a4-22. He finds it very confused and therefore he also gives Alexander's commentary on this subject.54 Ibn Rusd also had this more extensive text in the Middle Commentary, and he presents it in a somewhat more comprehensible version, but could not turn it into completely consistent explanation (see below p. 295). The phrase on the white colour that does not remain must be a misinterpretation of 374a12 ff., where it is said that a spray of drops around the sun does not remain, but is soon dispersed or turned into rain, otherwise a halo would be possible with rainbow colours. Ibn al-Bitriq again describes the colours of the rainbow: they are wine-red, green, a mixed colour55 and white, i.e. yellow. The outer band of the rainbow is wine-red because most sunrays fall on that part of the cloud. Then comes a mixed colour, then green. Finally comes white, i.e. yellow, adjacent to the inner side. If the rainbow is seen in a blackish cloud, there is a colour between red and green, that verges on white. If it is seen in a white cloud, then the green verges on the just-mentioned colour.56 Ibn al-Bitriq gives nothing of Aristotle's mathematical proofs that the rainbow is circular and that never more than a semicircle is seen. Ibn al-Bitriq mentions the latter fact, and says that the sunrays that shine on the cloud only form half a circle. He adds (not in Aristotle) that one never sees a rainbow in the south because the sun is never in the north. Rainbows are only seen in the east, west and north and in between these directions. One does not see the rainbow in summer when the sun is in the south, in the middle of the sky. The reason is that at that time the sunrays equally shine on both the east and the west horizons and do

52 This phrase corresponds to Aristotle's explanation of the difference between the light colour of the halo and the darker colours of the rainbow (373b33-374a4). By 'clear air' Aristotle means the reflecting surface (mist) when a halo is formed; this mist is lighter than the cloud in which a rainbow arises, see above pp. 247-248. 53 Ibn al-Bitriq, Meteor. 92,10-93,12. 54 Ibn Tibbon, Otot ha-Shamayim III 189-229. 55 It appears from Ibn Rusd's Middle Commentary that he read: "green, which is a mixed colour", see below pp. 296-297; this would give a more sensible text. 56 Ibn al-Bitriq, Meteor. 95,3-11. Ibn Tibbon has the same text, but Ibn Rusd's Middle Commentary gives quite a different text, see below p. 296.

not shine in a particular direction more than in another. Then there is no direction where a rainbow might arise.57 The account of the rods follows Aristotle, but one peculiar statement occurs: Ibn al-Bitriq says that once near the Bosporus two rods were seen on either side of the sun that rose together with it and remained until sunset. Aristotle has this very same statement concerning mock suns (372al5-16). Mock suns are not mentioned by Ibn al-Bitriq at all.58 We have seen again that Ibn Tibbon had a another copy of Ibn al-Bitriq's text than the copies we have and that the extant text may be improved using Ibn Tibbon's translation. This does not remove the confusion in the description of the colours, something about which also Ibn Tibbon complained. In his Middle Commentary Ibn Rusd often inserts explanatory remarks of his own between the text of Ibn al-Bitriq. This makes it difficult to decide whether Ibn Rusd, whose text is generally better than that of Ibn al-Bitriq, had a better copy or improved the text from his knowledge of Alexander. The discussion of the halo in Hunayn's Compendium is very similar to that in Ibn al-Bitrlq's version. He gives the description of the halo, its cause, and its function as a weather-sign. Hunayn's discussion of the rainbow also contains phrases that correspond to those in Ibn al-Bitriq. See Daiber 1975 9 for a list of corresponding phrases from these paragraphs. Hunayn adds a paragraph (not in Aristotle; it may correspond to Ibn al-Bitriq 93,6-8) about the clarity of rainbows. He says that if moistness is dominating in the reflecting vapour (cloud), the colours become turbid and darkish; if dryness dominates, the colours are clear and whitish. Something similar may be observed in fire: if it arises from moist wood, its color is a turbid red, if it arises from dry wood, it is a clear yellow.59 Aristotle does not discuss the clarity of rainbows in this way, but the thoughts in this paragraph are Aristotelian: see 373b32-374a10 about the differences between the halo and the rainbow.

5.

Pseudo-Olympiodorus

Pseudo-Olympiodorus first gives an introduction about vision. He uses the concept of visual rays emitted by the eye of the observer. He says that if the object is straightly opposite the observer and is seen in the place where it actually is, then vision is real. Vision that is not

57 58 59

Ibn al-Bitriq, Meteor. 96,2-14. ibid. 97,2-10. Hunayn, Jawāmi' 281-293.

real 60 occurs when the visual rays are broken (inkasara). This may occur in two ways. (1) The first way is refraction (inkisār wā-taštīt or inkisār wa-tabaddud, lit.: breaking with dispersion). This occurs when we look at something moist. Then the object seems larger than it is, such as when a piece of wood is under water. If it is two cubits long we imagine that it is, for instance, ten cubits. (2) The other way is reflection (in'ikäs, or inkisār wa-rujÜ, lit.: breaking with reversion). This occurs when we look at something dark, like smooth, polished things. Reflection may occur from air, clouds, or the smoke of a lamp. Reflection from air may occur for someone with a healthy sight if the air is turbid. It may occur for someone with a weak sight even if the air is clear. A powerful vision is able to push aside the thin air close by, not the thick air further away. A weak vision is not even able to push aside the air close by. Therefore we see stars at their rising and setting larger because they are seen through thick, turbid air.61 We see a ring around a lamp in the smoke that rises from it because vision is reflected to the lamp from the turbid air around it. Reflection against clouds occurs in the case of the formation of the halo, the rainbow, mock suns and rods. Reflection from water occurs when we are rowing and water is sprinkled into drops by the oars; then rainbows are formed too.62 Furthermore, one should know that (1) the mirror that produces these effects should be smooth, dense and transparent. (2) Tiny, dispersed mirrors reflect the colour of a luminous body, not its shape. Even very small luminous objects may be viewed by the senses, but their form is not viewed. Colour and form are only seen in large mirrors. (3) The colour of a body seen through a mirror is not always the same as the real colour. This is due either due to a corruption or to a weakening of vision.63 The properties of the halo (da'ira) are described by Pseudo-Olympiodorus as follows: The halo is mostly formed around the moon, more seldom around the sun and very rarely around the stars. Its form is mostly a complete circle; sometimes it is only part of a circle, more or less than a semicircle. It appears more often at night. It is mostly seen at the meridian. The halo is formed rather close to the earth, not at a great height. Around the halo one sees something like a ring of air.

60

He means that what is observed does not correspond with the real object of vision. This does not connect with the previous phrase because it is an effect of refraction, not reflection. 62 Pseudo-Olympiodorus, Tafsir 145,14-146,10. 63 ibid. 146,12-21. Olympiodorus mentions the same three points, see in Meteor. 219,8-23, above p. 256. 61

Sometimes more than one halo is seen; they have different sizes and are in different places. The colour of the halo is always white. The middle of the halo is such that the halo appears as a ring. The appearance of a halo is a sign of rain, fair weather, or wind.64 Then follows the explanation of these properties. The halo is more often formed around the moon because the vapour gathered there is not easily dissolved. The sun dissolves the collected vapour, so that the halo seldom appears around the sun. It is very rarely seen around the stars because they do not collect much vapour; if a halo is formed at all it is very weak and thin; it is weak because the light of the star is weak and thin because the diameter of the star is small.65 The circular form is explained in accordance with Aristotle's and Olympiodorus' account. If it is not a complete circle, but only part of a circle, the reason is that not all parts of the cloud equally reflect vision.66 The halo more often occurs at night because during the day the vapour that gathers is dissolved by the sun's heat. The reason that it appears more often at the meridian than near the horizon is its size. People have estimated its diameter by means of the stars that are closest to its circle and found it to be 45 degrees.67 This means that a complete circle of such a size cannot be formed when the luminous object is near the horizon. If it occurs, part of it is hidden.68 The reason why the halo is formed rather close to the earth, not at a great height, is that winds close to the earth are not so strong, so that they do not disperse the clouds. The colour of the ring that surrounds the halo is seen as black because of the whiteness of the halo; the explanation is that if a thing is close to its contrary, it shows off more clearly, and white is contrary to the colour of the vapour in the surrounding air. The colour of the halo itself is white, no other colours such as the rainbow has. The reason is that (1) when a rainbow is formed, the reflection of vision occurs at a great distance. This causes an error in the observation of the colour of the luminous object. (2) When a halo is formed, the mirror has a more rare constitution, so that it does not change much the colour of the luminous object.69 More haloes may be formed if there is more than one mirror (cloud) in different places and at different depths. Then vision is reflected from 64

Pseudo-Olympiodorus, Tafsir 147,1-15. ibid. 147,17-148,2. 66 ibid. 148,4-149,11. 67 The text has: 45 galwa\ a galwa, lit.: a bowshot, is a measure of length. There is a corresponding passage in Olympiodorus (see above p. 257) where the diameter of the halo is said to be 40 degrees ( μ ο ί ρ α ) . 'Degree' must be the correct expression here. 68 Pseudo-Olympiodorus, Tafsir 149,13-18. 69 ibid. 149,20-150,7. 65

each of them to the light source. This may be compared with someone who stands before several mirrors or with someone standing before one mirror opposite which several other mirrors are placed. This effect has been used by people to make an instrument that shows the number of hours of the day that has passed. They arranged the mirrors in such a way that in the first hour one image was visible, in the second hour two images, etc. When there are more haloes, the largest is seen closest to the earth, the smaller ones are seen higher above the earth. The reason is that if the mirror is close, the object is seen as larger because it is seen at a larger angle.70 The reason for the fact that the middle of the halo is seen in such a way that the halo appears as a ring, is, according to some people, that the part of the cloud which is at the centre of the halo is directly opposite the observer and is consequently seen by 'real' vision. This opinion implies that the halo is not something imagined, but really exists. Others say that the rays along and near the axis of the cone of visual rays cover a smaller distance and therefore penetrate into the cloud without being reflected, so that the luminous object is observed directly. The other visual rays cover a longer distance; they are weakened and then reflected to the light source; in this way an image of the light source is formed.71 The account of the halo as a weather-sign follows Aristotle. Then follows Pseudo-Olympiodorus' description of the rainbow. He says that the rainbow always occurs opposite, not around the luminous object. It is never a complete circle, but always a semicircle or smaller. It mostly occurs during the day, seldom at night, and mostly in the east. It generally has three colours: date-colour (busrī)—this colour resembles that of red dates and the comb of a rooster—leek-green (karrātj), and purple (urjuwānī)—this resembles the colour of the blood of the marine animal from which purple dye is made.72 Sometimes also yellow is visible. Green is always in the middle, red and purple are on the sides. At most two rainbows may occur simultaneously; three is not possible. If there are two rainbows, the order of colours in each of them is opposite. When the days are long, no rainbow occurs at midday; when they are short, rainbows may occur at all times of the day.73 70

Pseudo-Olympiodorus, Tafsir 150,9-24. The discussion of multiple halos is not in Aristotle, nor in Alexander, nor in Olympiodorus. 71 ibid. 151,2-10. The latter theory is the one propounded by Olympiodorus; see in Meteor. 222,29-2233 and above p. 256. The former theory must be the one by Alexander or Theophrastus (see above pp. 251-252), since these theories imply that the halo is not (only) an image, but has a substantial existence. 72 Alexander mentions that purple is the colour of the blood of the sea murex (πορφυροί ΐής θοίλοίοοίας), see in Meteor. 161,8.

This description is followed by the explanation of the mentioned properties. The reason why the rainbow always occurs opposite, not around the sun, is that it is formed by reflection from dense, thick clouds. If the sun is right above such a cloud, the dry exhalation is separated, and the cloud turns into water and becomes rain; then no rainbow is formed. If the cloud is opposite the sun, then the sun's influence is less strong and the process of the cloud changing into rain is slower; in the meantime a rainbow may be formed. The reason why no rainbow is formed opposite a star is that a cloud which is suitable to produce a rainbow is not suitable to reflect the light of a star, due to its density. Moreover, the colours of a rainbow would not be visible because of the darkness of the night. Finally, Pseudo-Olympiodorus mentions the reasons why a rainbow seldom occurs opposite the moon, following Aristotle.74 The reason why a rainbow is never a complete circle, but always a semicircle or smaller, is that the sun is not vertically above the reflecting cloud, as in the case of the halo, but opposite it. It is a complete semicircle when the sun is at the horizon; then the radius of the rainbow is small; the sun and the centre of the cloud are on opposite ends of a diameter, whereas the observer is on that diameter in the middle. When the sun is above the horizon, the rainbow is less than a semicircle and its radius is larger; when the sun rises, the above-mentioned diameter turns in a vertical plane around the observer, so that the centre of the cloud descends below the horizon. Thus, the centre of the rainbow is hidden, and what is visible of it above the horizon is less than a semicircle. It inclines to the horizon more than a rainbow which is a complete semicircle; therefore it is seen through much vapour and appears larger; this is the reason why its radius is larger when it is less than a semicircle.75 The reason why the rainbow is coloured, not white as the halo, is that the reflecting cloud is thicker, denser and darker; this causes the (reflected) light of the sun to become darker. Moreover, the visual rays have to cover a longer distance because the cloud is opposite the sun; therefore they are weakened, so that an error (galat) will occur to our sight. Thus, one does not see the white colour of the light source such as it is, but one sees a colour that is a mixture of white with black. If white is dominating over black, one sees red. If black is dominating, one sees a green colour; if white is almost not present, one sees purple 73

Pseudo-Olympiodorus, Tafsir 152,6-21. ibid. 152,23-153,20. 75 ibid. 153,22-154,26. The explanation of the latter question follows Olympiodorus (not in Aristotle), see in Meteor. 261,18-25 and above p. 261. 74

(banafsajī).16 Sometimes one sees a light red colour {ahmar nasi'). That colour is not due to reflection, for if that were the case, it would appear before red and green. The colour arises because vision errs when two colours are adjacent. Like in Olympiodorus' account, the same examples are brought forward as those in Aristotle to show that colours make a different impression, depending on the surrounding colours. Pseudo-Olympiodorus adds that sellers of clothes and jewels perform their activities not during all hours of the day, but only at suitable hours, namely when there is sufficient daylight. Thus, the three usual colours are seen because vision errs, as the visual rays are weakened by the reflection. The fourth colour is seen because of the error occurring when red and green are adjacent. Other colours are not seen, for if a colour were formed that is closer to white than red, we would not distinguish it from white. For a colour to be formed that is closer to black than purple, the cloud would have to be denser and darker; then it will turn into rain and the rainbow will be dissolved.77 When there is one rainbow, its outer, largest band is red because from that large ring most visual rays are reflected to the luminous object. Then vision is powerful and only a small error will occur to it. White, which is the colour of the luminous object, dominates over black and the result is red. The middle band is green because less visual rays are reflected from that area, so that the power of vision is less and vision errs to a larger extent. The smallest band is purple because vision will be weak and a large error will occur to it.78 If there are two rainbows, the colours of the inner one are clearer. The reason is that (1) vision is reflected one time only and is therefore less weakened. (2) Vision covers a smaller distance when it travels to the sun via the inner rainbow and is therefore less weakened. As for the outer rainbow, vision is reflected twice: one time in the first rainbow and another time in the second one. Therefore vision is weaker. Also, the distance covered by the visual rays is longer, which also causes more weakening. It is clear that a third rainbow cannot be formed: vision would become too weak to observe it.79 The order of colours in the outer rainbow is opposite to that of the inner one: the smallest band is red, the largest one is purple. The red The phrase on the red colour is not in the text here, but taken from a later passage, p. 156,5; probably the text is corrupt here. Ibn Suwār has a clearer text: "We see red when white is dominating over black; we see green when neither black nor white are dominating and we see purple when black is dominating." See his Treatise on Meteorological Phenomena, below p. 360,10-13 and below p. 276. 77 Pseudo-Olympiodorus, Tafsir 155,2-25. 78 ibid. 156,2-13. 79 ibid. 156,15-157,2.

bands of each rainbow are near one another. This order may be explained as follows: If vision strikes upon the area between the rainbows, no error will occur to it: here the 'real' colour is observed. The reason is either that the sun shines vertically on this area of the cloud and dissolves it, so that no mirror is present against which reflection may occur or that the axis of the cone of visual rays falls on that area and rays along or near that axis are so strong that vision does not err. The outer band of the inner bow and the inner band of the outer bow are both not far from that area; therefore only a small error will occur to vision of these bands and we see them as red. The next bands are further from that area between the rainbows; more error will occur to vision of those bands; they are seen as green. This holds for the next bands to a greater extent; therefore they are seen as purple.80 Besides this explanation of the order of colours that is common for both rainbows, there is also an explanation for their order that is different for each of them. For the inner rainbow the different quantities of visual rays reflected from each band cause different degrees of error and thus different colours, as has been explained above. For the outer rainbow the different distances of the bands cause different degrees of error. Therefore the inner, smallest band, which is closest to our vision, is red; then follow green and purple in an increasing order of distance.81 When the days are short, the rainbow may occur at any time of the day. In the morning and the evening the sun and the (centre of the) reflecting cloud are at opposite ends of a diameter of the horizon. When the sun rises above the horizon to a certain height, the centre of the reflecting cloud descends under it to the same amount. A rainbow may be formed opposite the sun, if the sun does not rise too high, so that the centre of the cloud does not descend too far below the horizon. Thus, a rainbow may be formed at midday when the days are short. However, when the days are long, a rainbow cannot be formed at midday, since the sun rises so high, and thus the centre of the cloud descends so low below the horizon, that a rainbow is not visible above the horizon.82 In the next paragraph, Pseudo-Olympiodorus discusses the ways in which vision may err. Visual rays that travel a long distance are necessarily weakened and therefore far objects are not seen such as they really are, but we err in their observation. If the object is 80 This explanation of the colours is mentioned by Olympiodorus as originating from Ammonius, see in Meteor. 238,20-239,14 and above p. 259. 81 ibid. 157,4-158,14. 82 ibid. 158,16-159,7.

observed directly (via straight, not broken rays), error may occur in the following ways: (1) Large objects seem small, as if they had the size of an ant. For example, in reality the sun is 170 times as large as the earth, but we see it as smaller than a date. (2) Objects that are jagged are seen as not-jagged. Thus, square objects that are far away seem to be round. (3) Objects with a rough surface seem to be smooth. A stony place of the earth seems to be even. (4) Spherical objects seem to be flat. Therefore stars seem to be flat. (5) Objects that are far from one another seem to be near. Therefore we see the planets as if they were not far from the fixed stars.83 Error may overcome vision also when it is reflected (inikās wa-rujū') or refracted (inikās wa-tabaddud). If vision is reflected, white objects seem less white; for instance, white clouds reflected in water seem less white. If vision is refracted, objects seem to be larger than they really are; for instance, a piece of wood of ten cubits that is under water seems to be hundred cubits. Also, their colours seem to be hidden. Vision is more violently moved by an object that is near than by something far. Also, it is more violently moved by white objects. Therefore, being near and being white are related and white is the affirmative of vision: we see the light of the sun as white. If one looks at something that is drawn as white, it seems to protrude from the plane. If painters want to paint protruding parts, like hands or a nose, they make them white. Vision is hardly moved neither by what is far, nor by what is black; thus, black and far are related attributes and black is the negation of vision: if no light falls on an object, it is seen as black. Things that are drawn as black seem to be in the depth of the picture. Therefore painters make parts black that lie deeper in the body, like eyes.84 The halo and rainbow on the one hand and the solar and lunar eclipses on the other hand are similar is certain respects and dissimilar in other respects. They are similar because in each of these phenomena three things are on a straight line: the luminous object, vision and the centre of the phenomenon concerned. They are dissimilar because the halo and the rainbow are imagined phenomena, since they are formed by reflection. The eclipses occur due to the real relative positions of sun, moon and earth.85 The mock suns (šumūs) around the sun are similar to the sun in form and colour. They occur when the sun is near the horizon. At midday, when sun is in the middle of the sky, they do not occur 83

Olympiodorus says that distant objects seem darker, less jagged and smaller; see in Meteor. 236,7-9, above p. 258. 84 Pseudo-Olympiodorus, Tafsir 159,9-160,22. 85 ibid. 161,2-10.

because the sun dissolves the clouds, so that no mirror is present against which vision may reflect. Or they seldom occur at midday, for Aristotle relates that he once saw two mock suns rising with the sun and staying with it the whole day, until sunset (372a15). The reflecting cloud must be besides the sun; it must be white, since that is the colour of the mock suns; it must have a dense constitution, otherwise vision is not reflected. It is a sign of rain because a dense cloud quickly turns into water. More particularly, if mock suns appear in the north, when the sun is at the summer tropic and there is a north wind, they are not so much a sign of rain, because the strong north wind dissolves the clouds. If they appear when the sun is in the south, at the winter tropic, and a south wind is blowing, they rather are a sign of rain, because the south wind is weak and does not push away the cloud.86 Rods Casan, pi. 'isīy) are elongated phenomena that are red-coloured or have various colours. They are seen in clouds and are formed when vision is reflected from a cloud to another cloud that has a different colour and density. The various colours arise because the colour of the visual rays is mixed with the colour of the reflecting cloud. The reflecting cloud must not be dense because vision is not reflected to the sun. Also in water one may see something like rods; this occurs when vision is reflected against water to a cloud.87 Several paragraphs have corresponding sections in Olympiodorus' commentary. Some subjects are not discussed in Olympiodorus, such as the fact that more than one halo is possible, that the halo is not always a complete circle, that purple is the colour of the blood of the murex and that the halo and rainbow are comparable to eclipses in a certain respect. The example of painters who bring depth in their paintings using light and dark colours is not in Olympiodorus here. It occurs in connection with the formation of chasms and trenches and is mentioned by Olympiodorus and Pseudo-Olympiodorus there. The example goes back to Ptomelaeus (see above pp. 72 and 79). The secondary rainbow is explained by Pseudo-Olympiodorus as a reflection of the primary one; Alexander explicitly denied this (see above p. 249n9).

6. Al-Kindī on the blue colour of the sky The subject of one of al-Kindi's treatises is the blue colour of the sky.88 Although the subject is not treated in any of the other works 86 87 88

Pseudo-Olympiodorus, Tafsir 161,12-162,9. ibid. 162,11-21. Al-Kindi, Rasâ'il II 103-108; German translation in Wiedemann 1915.

consideration here, except for a few remarks by Bahmanyār (see below p. 283), we discuss it here, because it is a clear example of how Aristotle's theory of light and colour is applied in the Arab world. Al-Kindl says that colour is only observed when light is reflected from a solid body (in Aristotelian terms: a bounded body, see above p. 244; al-Kindl uses munhasir), that is, an earthy body. Water, air and fire do not accept colours. The sunlight does not directly reach places in the shadow. These places are darker and cooler. This indicates that it is not only the distance of a place to the sun that determines its heating by the sun. That there is some light in places in the shadow is due to reflection of sunlight against particles that arise from earth and water and are mixed with the air. These particles transmit some light and heat to that area. Such particles rise into the atmosphere until a certain height, that is, until they have has lost all heat that made them rise. It is due to these particles that the sky has a colour. This is comparable with fire that has a colour due to the fact that earthy parts rise from the fuel together with the fire. Pure fire has no colour.89 Air adopts a colour insofar as solid bodies are present in it and this colour is between darkness and light, viz. blue. It is comparable to the light of a luminous body seen through a transparent coloured body. What is observed is a mixture of the colour of the luminous body and the transparent body.

7. Ibn Suwār ibn al-Kammār Ibn Suwār ibn al-Kammär in his Treatise on Meteorological Phenomena mostly keeps close to Aristotelian explanations. This is not surprising, as his treatise is the first part of a more extensive work—the other parts not being preserved—the second part of which is his translation of the Aristotelian text from Syriac and the third part a detailed commentary on the text. The first part (the treatise under consideration) is a general survey and commentary on the text, in which he also discusses commentaries of Olympiodorus and Alexander. Also Theophrastus is mentioned. An edition and translation of the treatise in given below (Supplement 1); here we shall only give a brief outline of the contents. Ibn Suwār starts his treatise with an introduction, in which he mentions Aristotle's theory of the two exhalations, the smoky and the watery exhalation, which are the origin of all atmospheric phenomena. 89

This theory of fire is adopted by Ibn Sinā, see above p. 81.

These exhalations are intermediate states of elements changing into one another: when earth changes into fire, it first changes into smoky exhalation; when water changes into air, it first changes into watery exhalation. The watery exhalation gives rise to phenomena such as clouds, rain, etc. on the one hand—these are really existing substances—and to haloes, rainbows, mock suns and rods on the other hand—these are images perceived by our vision, such as images seen in a mirror. That the latter phenomena are images caused by reflection, is shown by arguments that are also in Aristotle, Alexander and Olympiodorus.90 In his account of the halo, Ibn Suwār treats some questions that are not in Aristotle, or, if they do occur there, he treats them more extensively. These questions are: why the halo is sometimes not a complete circle, why it very seldom appears under the stars, why it appears as a ring, not as a disc, why it also appears when the moon is not full, and how more than one halo may be formed. As for the first two questions and the last one, Ibn Suwār more or less follows Pseudo-Olympiodorus; these questions are not discussed by Olympiodorus; for the third question Ibn Suwār quotes Alexander and Olympiodorus, but it is also treated by Pseudo-Olympiodorus; the fourth question is not discussed in any of the commentaries before him. Besides Aristotle's proof that the halo is circular, he gives Theophrastus' proof, as being the 'physical proof'.91 Ibn Suwär's account of the rainbow mostly follows Aristotle. The explanation of the colours, in which it is stressed that they arise due to errors of sight, is clearly inspired by Pseudo-Olympiodorus, although also Olympiodorus mentions the concept of error. The explanation of the order of the colours is basically Aristotelian, but some details seem to be original in Ibn Suwār. He says that the colours arise by a mixture of the light of the sun with the dark colour of the reflecting cloud. The latter colour is observed as varying from rather dark in the outer band to very dark in the inner one, due to the varying size (surface) of the bands. Similarly, the reflected sun light is observed as varying from rather white in the outer band to not very white in the inner band. The mixture of the varying degrees of darkness (of the cloud) and whiteness (of the sunrays) results in the colours red, green and purple. Why the colours in the secondary rainbow have the reverse order compared to the order in the primary rainbow is explained in a way adopted from Olympiodorus; this explanation is also in Pseudo-Olympiodorus. Ibn 90 91

Ibn Suwār, Treatise on Meteorological Phenomena, see below pp. 318-326. ibid., see below pp. 328-346; c f . also above pp. 251-252.

Suwār gives Aristotle's geometrical proof that the rainbow is a semicircle when the sun is at the horizon and less than a semicircle when it is above the horizon. Besides this he gives a 'physical' proof, by comparing the sizes of the rainbow with the lunar phases.92 This last view does not occur in any of the commentaries before him. It reminds one of Pseudo-Olympiodorus' comparison of the halo and rainbow with eclipses (see above p. 273). About rods (qudbān, sg. qadlb) and mock suns (šumus) Ibn Suwār is very brief; he follows Aristotle. He also mentions Olympiodorus' theory of rods93 It is clear that Ibn Suwār knew and used Aristotle and his Greek commentators Alexander and Olympiodorus. He also knew Theophrastus' Meteorology, he translated it.94 Furthermore, it is clear that Ibn Suwār also knew the commentary of Pseudo-Olympiodorus; several passages originate from this commentary, that do not have a correspondence in Olympiodorus. On the other hand, a few passages originate from Olympiodorus that do not have a correspondence in PseudoOlympiodorus. Therefore we cannot conclude that for Ibn Suwār the commentary he knew as Olympiodorus' commentary was in fact the one of Pseudo-Olympiodorus—assuming that the latter text, such as we have it, is complete. We have seen that a few features of Ibn Suwâr's commentary do not occur in earlier commentaries and may be original.

8. Ibn Sīnā Ibn Sînâ's account of the halo, rainbow, mock suns and rods in the Kitāb aš-Sifā' has been translated (with a few passages omitted) into German by Horten and Wiedemann; we refer to their work for more details of the text.95 First, Ibn Sīnā presents an introductory chapter on the theory of vision. He says that the phenomena under discussion are imaginary, that is, they are observed as an image of a body in some other body, that image not being something imprinted in the matter of that other body; these phenomena are comparable to the image of a man in a mirror. There are three theories on how these images are formed: (1) The theory of visual rays. According to this theory, rays are emitted by our vision towards a smooth surface, the mirror. The rays are reflected against this surface, and continue their straight path until 92 93 94 95

Ibn Suwār, Treatise on Meteorological Phenomena, see below pp. 346-376. ibid., see below pp. 376-378 and above p. 261. This work was edited and translated by Daiber in 1992. Horten and Wiedemann. See also Boyer 77-79 for an account of Ibn Sînâ's theory.

they fall on some other body. Then we see that body together with the mirror, so that it appears as if the form of body were in the mirror. In actual fact, this is not the case, since we see the image moving if we move and we see the image at some distance of the mirror. (2) A second theory says that if a body is opposite an observer and a transparent medium is between them, and the body is actually luminous, then an image is formed in the eye. There is nothing that is emitted from the body, traverses the medium and reaches vision. The medium is a transmitter of the image, in the sense that it enables the body to form an image in the eye. The cause of the transmission to the eye is the light that falls on the body, not the medium. If the observed body is smooth, then the image of another body may also be transmitted to the eye. That other body is in relation to the smooth object what the smooth object is in relation to the eye; the smooth body does not receive in itself something (an image of that other body) that is imprinted in it, but it transmits the form. One might think it surprising that a body could influence vision without being in contact with the eye, merely by being opposite it. However, sound also influences the sense of hearing from all places opposite the ear, and nobody finds this surprising. (3) The third theory says that the image of the observed body is seen as if it were imprinted in the mirror. There is no thruth in this theory, for if a form is imprinted in something, then this form does not change its position in relation to that thing if the observer moves to another position in relation to it. If a wall becomes green because light is reflected to it from a green body, this green colour does not move on the wall when we move. But if we see a tree reflected in water, it does move when we move. What really occurs in the case of reflection is that both forms (of the object and of the mirror) come together in the eye, so that it appears as if the image is in the mirror. As for the first two theories, whether something is emitted from vision and is then reflected against a mirror to the object, or whether the object exerts an influence on vision via the mirror, the result is the same, for the figures and lines one has to draw are the same. Aristotle used the theory of visual rays because it was the most well-known theory.96 Some physicists have proposed rather unreasonable ideas concerning these phenomena in the clouds, due to a prejudice against the theory of visual rays, hanging on to the school of the Peripatics, without sufficient insight. They said that the halo is formed by a kind of wave-like motion in the clouds caused by the light of the luminous 96

Ibn Sīnā, aš-Šifa,

Tab. 5 40,5-43,5.

body, or that the light dissolves the middle of the cloud, leaving unaltered the parts on the edge at equal distances from the middle.97 According to such theories, the halo would have a real substantial existence. The difference between real forms that are imprinted in matter and imagined forms that appear in a mirror is that the latter move when we move, whereas the former stay in the place where they are.98 Also, images seem to be behind the mirror, real forms do not. If the smooth body is transparent, the images are not seen; if they are seen, then what is behind the smooth body is not visible. If there is a coloured body behind the transparent body, the image is seen.99 If mirrors are so small that vision cannot divide them, then they do not transfer the form of a body, only its colour. If a body is seen through a medium and there is a second medium behind it that is separated from it by a plane, then we see the body as larger than it really is, especially when the second medium is liquid, for instance if one sees something in water. Then the colours of the body are insufficiently transmitted and become less bright. If the body is above the water and such a plane acts as a mirror, then it seems smaller and darker than it really is. Such errors in vision may occur either in the size of a body, or its form—then something jagged seems to be round or flat when it is far—or the position of its parts—then something rough seems smooth when it is far—or its colour, or its position in relation to something else—then we err in the distance of far away bodies in relation to each other, e.g. between the moon and the fixed stars.100 If the light of a luminous object is reflected in a mirror that is not far from it, we observe the colour of the body as it is; if the mirror is dark and far from the luminous object, then other colours will be formed.101 It is possible that not all parts of a reflecting body transmit the same image to vision; some parts may transmit another image, or no image at all.102 In the latter case those parts are ruled out in their contribution to the reflection. This may occur, either because these parts miss something that enables them to transfer the image, or because another,

97

These are the theories of Theophrastus and Alexander, as we saw above, pp. 251-252. See also Pseudo-Olympiodorus, above p. 269. 98 See Ibn Suwār, Treatise on Meteorological Phenomena, who quotes this from Alexander, see below p. 322,17-22. 99 Ibn Sīnā, aš-Šifa, Tab. 5 43,6-44,2. 100 -phjj parage about the ways in which vision may err is similar in content to Pseudo-Olympiodorus, above pp. 272-273. 101 Ibn Sīnā, aš-ŠifāTab. 5 44,3-45,6. 102 This will serve to explain that the halo and rainbow are like a ring, i.e. that reflection does not occur from all parts of the reflecting surface.

stronger light falls on them, which prevents them from transmitting light that is weaker. It may also occur that the image is not powerful enough to be transmitted; this occurs when the object is very far or has a weak colour. If all reflecting parts of the mirror have a similar position in relation to the observer and the luminous object, then the lines from the observer to the mirror fall on the mirror at the same angle on all sides, and the same holds for the lines from the mirror to the luminous object. Then the form of the image is a circle: if the point where a line from the observer falls on the mirror is rotated around the axis that connects observer and luminous object, a circle is formed, which is the image of the luminous object. Such an image changes its place when the observer changes its place. That is how we know that they are images.103 After this introduction, Ibn Sīnā starts the account of the halo, repeating his rejection of the theories that imply that the halo has a substantial existence. One of these theories says that the cloud is spherical; when the moon's rays fall on it, a cut (qat') is formed that must be circular; another theory says that the light of the moon causes a wave-like motion in the cloud, like a stone thrown into water, and that the middle is dissolved by the light.104 These theories cannot be true, because the halo is seen in different places by observers in different places. Therefore it must be an image. The cloud transmits the light of the luminous object while it is not changed by it. In the middle of the halo the object itself is seen by direct vision, since the rays travel through the thin cloud along a straight line. The image is transmitted along broken, reflected lines and forms a circle. Inside and outside the halo one sees a dark colour because from there no reflection occurs to the observer; thus, that area is less white and appears as black due to contrast with the adjacent white. Moreover, because of the strength of the light of the luminous body that is seen inside the halo, it is as if no cloud exists there, for small, thin bodies are not visible in strong light; or if they are visible, they appear as black, such as a flame at daylight.105 The discussion of the halo as a weather-sign follows Aristotle. Furthermore, Ibn Sīnā says that more than one halo may occur when more than one suitable cloud is present. The halos in the lower clouds appear as larger because they are nearer to us. Some have reported that 103

Ibn Sīnā, aš-Šifā', Tab. 5 45,6-46,12. Both theories are mentioned by Ibn Suwār, Treatise on Meteorological Phenomena, see below p. 336. The latter one is Theophrastus' view, see above pp. 251-252. 105 Ibn Sinā, as-Šifa, Tab. 5 47,4-48,19. 104

they saw seven halos simultaneously, but this seems improbable.106 The diameter of the halo has been estimated as 45 degrees when it was measured off by the stars that were at the extremes of its diameter.107 Ibn Sīnā reports that he has seen a complete halo around the sun with the colours of the rainbow and says that a halo may become a rainbow if the cloud becomes denser and darker. He has seen the same phenomenon around the moon.108 As for the rainbow, Ibn Sīnā says that he has understood several aspects of it, but he admits that some aspects remained of which he did not know a true explanation. He is not satisfied by what the Peripatetics have said about it, for he has observed rainbows that appeared without a cloud being present. Also, what they say about the colours is stupid and untrue. According to Ibn Sīnā, a rainbow is formed by reflection not in a cloud, but in moist air which contains many small transparent watery particles, like a spray (rašš). For reflection to occur it is necessary that behind this moist air there is a dark coloured body; this may be a cloud, or something else. It is like glass that becomes a mirror if one side is covered. If the background of the moist air is a cloud, one might easily think that the reflection occurs in the cloud. Ibn Sīnā confirms his theory by observations of his own. He reports that he has seen a rainbow near a mountain; the upper parts appeared in a cloud surrounding that mountain, but the lower parts appeared in the air with the mountain as a background. He also saw a rainbow in cloudless weather in front of a mountain, while the air was moist. Furthermore, he saw two rainbows, partly against a cloud, partly against a mountain. Also, when he was on a high mountain he saw a rainbow on a cloud below; when he descended and almost arrived in the cloud, it disappeared.109 A further confirmation of the view that light is reflected from moist air containing something like a spray is that rainbow colours appear if water is sprinkled in the sun by a watermill. They also appear around a lamp in a bath and we see them around the sun when we rise in the morning with watery eyes. They disappear when we rub our eyes. They appear on the wall of a bath if the sun is reflected against the moistness there. Also, when the oars of a ship sailing on the sea sprinkle water, these colours are said to be formed.110 106 -pjjjs w a s reported by Aratus, according to Ibn Suwār, Treatise on Meteorological Phenomena, see below p. 330,18-19. 107 The text has: 45 stadiums (/'sfāiD; a stadium is a measure of length, like the bowshot used by Pseudo-Olympiodorus, see above p. 268. 108 Ibn Sînā, as-Sifa, Tab. 5 48,19-50,7. 109 According to Fakr ad-Din ar-Rāzī, who reports from the Si fa, the rainbow he saw here was a complete circle. This is not in Ibn Sinä's text of the Si fa, unless al-gamām in 53,1 is read as at-lamàm. See Fakr ad-Din, al-Mabähit_ II 183.

Ibn Sīnā gives the reason why the rainbow is a semicircle when the sun is at the horizon and less than a semicircle when it rises above the horizon in the same way as Aristotle, without further geometrical clarification. Furthermore, he tries to explain the Aristotelian phenomenon that the radius of the bow is smallest when it is a semicircle and becomes larger when the bow becomes smaller than a semicircle, i.e. when the sun rises higher. He says that when the sun is at the horizon, the rainbow is perpendicular to the plane of the horizon, whereas it makes an oblique angle with it when the sun rises.111 As for the colours, Ibn Sīnā says that maybe they are caused by the fact that the place where they are formed is far from the light source, so that darkness is mixed with the light. Apart from that, he says that he does not know how the three colours of the rainbow are formed and why they are ordered as they are. The theory that they arise because two clouds at different positions give rise to reflections with two different colours, while the third colour is formed by mixture of these two, cannot be correct, because one cloud is sufficient for a rainbow to be formed; moreover, even in homogeneous air a rainbow with three colours may be formed. Another theory is that the highest band is closest to the sun and therefore reflection to our vision from that band is strongest; thus, the colour of that band is a bright red (humra nâsi'a). The lowest band is furthest from the sun; thus, there one sees redness mixed with blackness, that is, purple (urjuwānī). The green (karrātJ) colour takes an intermediate position between brightness and darkness and is composed of the two others. This theory is not correct because, in fact, the colours gradually change from red to purple and it is nonsense to say that there is a neat separation between the colours. Also, there is no difference between various parts of the vapour in their ability to reflect the sunrays. For whether you approach or move away, or move upward or descend, the colours move with you: apparently all parts of the vapour are equally suitable to produce all colours. Moreover, a composition of red and purple will not produce green because it is not related to these colours. In fact, green is formed from a mixture of yellow and black. Ibn Sīnā concludes that the Peripatetics have brought forward nothing concerning the colours that he could understand; maybe others can understand it. He suggests that the cause of the different colours may not be in the mirror, but in the eye of the observer. He concludes his account of the rainbow by saying that his conjectures on this subject are too uncertain to be written

110 111

Ibn Sīnā, aŠ-Šifā', Tab. 5 50,8-53,6. ibid. 53,9-54,4.

down in his book and that further explanation must be sought with others.112 The discussion of mock suns (šumaysà) and rods (nayzak) mostly follows Aristotle. We again found several passages that are similar in content to passages from Pseudo-Olympiodorus. Also, some passages seem to have been derived from Ibn Suwār.

9. School of Ibn Sīnā The short account of Bahmanyār contains several passages from the Šifā'.113 Bahmanyār discusses the blue colour of the sky (not in Ibn Sīnā), saying that it arises as a mixture of black and white: black being the colour of the celestial body, which is transparent and therefore invisible, white being the colour of the visible particles of exhalation that rise in the air. The blue colour has a purpose, since it is the colour that is most suitable for our vision.114 We have seen above (p. 85) that Abū 1-Barakāt considers the halo, the rainbow and similar phenomena to belong to the same class as shooting stars and comets: they are light effects that are formed and retained by celestial forces. When these forces work in clouds, a halo or rainbow may formed by means of the light of the sun and the moon. Fakr ad-DIn ar-Rāzī precedes the account of visual effects in clouds by seven premisses. (1) If a ray of light is reflected, the angles of the incident and reflected rays with the reflecting surface are equal. This is illustrated with a figure. (2) The same holds for visual rays; the course of the rays is just the reverse of the case of rays of light. The visual rays are imaginary, but this makes no difference for the results. (3) Small mirrors do not reflect the shape of a body, but only its colour.115 (4) If a mirror is coloured the reflected colour is between the colour of the mirror and that of the luminous body. (5) The images are not imprinted in the mirror, for we see them moving when we move. (6) If the medium through which we observe a body is transparent, then we see the body as it really is, not as an image; if we see it as an image, the medium is not transparent and we do not see what is behind 112

Ibn Sīnā, aš-Šifà, Tab. 5 53,6-8 and 54,4-56,2. Bahmanyār, at-Tahsïl 712,7-8 and 712,13-713,5 are similar in text to Ibn Sînā, aš-Šifā', Tab. 5 51,11-13 and 47,17-48,4, respectively . 1 4 Bahmanyār, at-Tahsïl 714,4-8. Compare the account of al-Kindi above pp. 274-275. 115 Fakr ad-Din, al-Mabāhii II 177,4-9 is similar in text to Ibn Sīnā, aš-Šifā', Tab. 5 443-9. 113

it. (7) If different parts of the mirror have the same position in relation to the observed body and the observer, then the reflected image is seen as a circle.116 The text on the halo and rainbow is an almost complete reproduction of Ibn Sînâ's text on this subject; the order of the sections is different. We do not give a complete enumeration of corresponding phrases; some examples are in the following table: Fakr ad-Din

Ibn Sīnā

Fakr ad-Din

Ibn Sīnā

al-Mabāhit_ II

aš-Šifā', Tab. 5

al-Mabàhit_ II

aš-ŠÍfa,

178,9-179,4

47,6-48,4

181,6-19

50,18-51,16

Tab. 5

179,5-12

48,11-19

182,10-21

53,9-18

179,13-19

48,5-9

183,2-10

5248-53,5

Fakr ad-Din's account of the mock suns has no correspondence in Ibn Sīnā or any other author. He mentions three causes for mock suns. (1) A thick cloud with smooth (reflecting) parts has been formed near the sun; the sunlight is reflected in this cloud. (2) This cloud acts as a mirror that is so large that it reflects both shape and colour. (3) Sticky exhalation rises from the earth and is formed in a round shape, which is the shape that is naturally adopted by moist bodies in air. When this exhalation reaches the sphere of fire, it is ignited. If the material is dense, the ignition remains for days or even months. This view has been opposed by some people who argued as follows: The material that rises from the earth is either thin or dense. If it is thin, then either it extends from the earth to the fire as long as the phenomenon lasts—as a lamp remains in contact with the oil as long as it burns—or the contact with the earth is broken. Both cases are impossible. In the former case the material rises from one place on earth and in the upper atmosphere it moves along with the celestial motion; therefore it cannot remain connected with the earth. Moreover, the ignition will reach the rising exhalation before it has risen to that upper place. Furthermore, rising thin exhalation spreads to the left and right and does not necessarily adopt a circular shape. If the exhalation does not remain in contact with the earth, the ignition will be quickly extinguished, such as in the case of shooting stars. If the exhalation is dense, then it does not spread to the left and right, it may adopt a circular shape and the ignition may last for a period of time. However, in that case, the material will not remain in the atmosphere, but fall on 116

Faly ad-Din, al-MabāhiL II 178,3-6 is similar in text to Ibn Sīnā, aš-Šifā', 45,19-463.

Tab. 5

the earth due to its weight. Fakr ad-DIn answers this critique by stating that dry and moist exhalation are always mixed. It is quite possible that the mixture is such that the light (airy and fiery) parts dominate, so that it does not fall, whereas it is still dense enough for the ignition to remain for some period of time. Then Fakr ad-DIn says that when his opponent heard this answer, he claimed that such phenomena occur due to influence of the heavens or spiritual forces.117 Fakr ad-DIn responds to this, saying that illness and health are sometimes attributed to earthy causes, sometimes to influences of the heaven or effects of the soul. An attribution to one of these kinds of cause does not invalidate an attribution to another kind: the different explanations complement one another.118 The section on rods corresponds to Ibn Sînâ's text.119

10. Ibn

al-Haytam

Ibn al-Haytam has extensively studied the refraction of light in the seventh Book of his Optics and in his Treatise on the Burning Sphere. In this Treatise he studied the course of rays of light through a sphere made of glass that refracts incident rays of the sun to certain (burning) points behind the sphere. This treatise is also quoted by Kamā1 ad-Dīn a1-Fārīsī in an appendix of his revision of the Optics of Ibn alHaytam.120 Despite his knowledge of refraction, Ibn al-Haytam does not explain the halo and the rainbow by refraction, but he adheres to the Aristotelian explanation by reflection. His treatise on the rainbow and the halo is extant the above-mentioned appendix of Kamā1 ad-Din's revision of the Optics, which has been translated by Wiedemann.121 We 117

Fakr ad-Din means Abū 1-Barakāt al-Bagdadi, who mentions the same objections as quoted here by Fakr ad-Din in connection with an 'extraordinary' phenomenon (the appearance of a big, weak star with a tail). Abû 1-Barakāt considers all light effects in the sky as being due to celestial forces; see above pp. 83-85. 118 Fakr ad-DIn, al-Mabāhit_ II 184,16-186,15. 119 Fakr ad-Dīn, al-Mabāhit II 182,17-183,1 is similar in text to Ibn Sīnà, aš-Šifā', Tab. 5 56,5-15. 120 See Wiedemann 1910 for a German translation of Kamäl ad-Din's version of Ibn al-Haytarn's Treatise on the Burning Sphere. For a survey of the contents of Kamā1 ad-Din's revision and its appendices, entitled Tanqih al-manāzir li-dawi al-absàr wa-l-basa'ir, see Wiedemann 1912, Zu Ibn al-Haytam's Optik 1-11. These papers by Wiedemann are now partly superseded by the edition and translation by R. Rashed of the Treatise on the Burning Sphere both in Ibn al-Haytam's original version and in the version of Kamäl ad-Din. He also edited and translated relevant parts from Book VII of Ibn al-Haytam's Optics. See Rashed, Géométrie et dioptrique au Xe siècle ch. 2 and pp. 83-133. 121

Wiedemann 1914; see also Würschmidt; see further Boyer 80-82.

refer to Wiedemann's paper for the details of Ibn al-Haytam's account. The explanation of the halo is Aristotelian insofar as the halo is considered to occur by reflection of visual rays against particles of a cloud towards the luminous object. However, Aristotle could not explain why reflection only occurs in points of a circle, not in all points of the cloud, and he did not clarify how the reflecting particles were situated and how the law of equal angles was taken into account. These problems are solved in Ibn al-Haytam's account. He says that reflection occurs in a concave cloud. The centre Β of the sphere which is formed by this concave cloud and the light source A are the extreme ends of a line that forms the axis of the halo. Β is at the concave side and A at the convex side of the cloud. The point of vision C is on the same axis, at the concave side. Reflection of visual rays from C towards A occurs against particles that lie along a radius of the cloud, i.e. along a line from Β perpendicular to the surface of the cloud, such that the law of equal angles holds. If one considers the matter in a plane through the axis AB, then there is one point H on the surface of the cloud, such that reflection occurs in that point in the way as has been described.122 Ibn al-Haytam proves that indeed there is one such a point Η and describes the way how to find that point if the radius of the cloud, and points A and Β and C are given. If the plane ABH is rotated around the axis AB, Η describes a circle, and the relative positions of A, B, C and Η remain the same; thus, all points of that circle are also points of reflection and that circle is seen as the halo. We see that by specifying the form of the cloud and the position of the reflecting particles and by using the law of reflection Ibn al-Haytam was able to show that the halo must be a ring, and that reflection does not occur in all points of the cloud. Ibn al-Haytam's explanation of the rainbow 123 is Aristotelian insofar as he says that it occurs by reflection of the sunrays against planes of moist air (particles of a cloud), that together form a spherical surface. However, his geometrical account is different. He does not use an observer in the centre of a hemisphere on which the sun and the cloud are situated, so that they have the same distance to the observer. Moreover, he uses the law of equal angles in order to determine the points of reflection, not some undetermimed proportion of distances. Ibn al-Haytam says that the rainbow is formed, if the centre C of the spherical surface formed by the reflecting cloud is on the line between 122 In fact, there is one such a point at one side of the axis, and another one, symmetrically situated, at the other side. See Rashed 1970 for a study of the explanation of the rainbow by Ibn al-Haytam and Kamā1 ad-Din a1-Fārisī.

the eye D of the observer and the luminous object A. Suppose the line ACD intersects the cloud at B. The distance of the observer to the luminous object is much larger than the distance of the eye to the cloud. If the reflecting circle and the positions of A and D in relation to the circle are known, then in a plane through the line ACD at one side of ACD there is one point L on the reflecting cloud such that AL is reflected as LD according to the law of equal angles. If ALD is rotated around AD, L describes a circle, all points of which are points of reflection and may be visible as points of the rainbow. The centre of that circle is on the axis AB; it is the centre of the rainbow. If the luminous object is at the horizon, then also Β is at the horizon. Then the rainbow is visible as a semicircle. The rainbow is smaller than a semicircle, if Β is below the horizon—and consequently the sun A is above it—and the centre of the bow is between D and B, or when Β is above the horizon and the centre of the bow is between D and C. The rainbow is larger than a semicircle, if Β is above the horizon and the centre of the bow is between D and B, or when Β is below the horizon and the centre of the bow is between D and C. The different colours arise because the light of the sun penetrates into the cloud to a smaller or larger extent. If it is reflected against a layer on the surface, the reflected colour is almost the pure inciding colour. If it is reflected against a deeper layer, the colour becomes mixed with darkness because the light travels through dense matter. Kamā1 ad-DIn criticizes this theory of Ibn al-Haytam pointing out that, according to this theory, one cannot explain that the secondary rainbow is weaker and has the reverse order of colours.

11. Ibn Rusd Ibn Rusd in his Short Commentary starts his account of the halo, the rainbow, mock suns and rods ('asan, pi. 'isiy) with the following introduction: These phenomena are just images; they arise when a light source is present and the observer has a special position in relation to it. They have the properties that are characteristic for images: they change place when the observer changes place, and approach and recede when the observer approaches and recedes. We consider these things from a physical point of view, using the results of mathematics. We know that things have a different colour, size and distance, if we observe them through dense, transparent bodies that are present between the observer and the object. Such bodies also influence vision when they are opposite the object, with the observer in between, such

as when we see stars reflected in water. Images may also be caused by weakness of sight: someone with a weak sight may see images and things that are not really existing (laysa 'alā kunhihā). The effect of a weak sight seeing through thin air is the same as that for a strong sight seeing through dense air.124 For someone with a weak sight, air acts as a mirror. From optics we learn that vision is influenced by dense, transparent bodies by means of reflection (inikās) and refraction (iriitāf). 'Real' vision occurs by direct rays of light; vision of images occurs by rays that are reflected and refracted against transparent, dense bodies, such as water and moist air, into which rays penetrate; such bodies have no special colour. Ancient physicists thought that sight occurs by visual rays that are emitted by the eyes and the causes of the various phenomena used to be given in optics by means of these rays. One may also give these causes using rays of light that are emitted from the luminous body. It makes no difference for the result which of these theories is used, but we shall use the theory of rays of light, as it has been shown in De Anima that vision does not occur by visual rays.125 Ibn Rusd's account of the halo contains several phrases that are similar to Ibn al-Haytam's text on the halo (above p. 286). He says that the halo is a circular phenomenon around the moon and some stars, or—more seldom—around the sun. It occurs when a cloud is situated between us and the luminous body. Thus, it must be caused by reflection or refraction. Its colour must be a mixture of the colour of the luminous object and the cloud. To give the halo its circular form, the parts of the cloud against which reflection occurs must be continuous and be in one smooth surface, which may be flat, concave of convex. Its most appropriate form is concave when it is seen from the side of the observer. The position of the cloud must be such that our sight C, the light source A and the centre Β of the sphere which is formed by this concave part of the cloud are on one straight line, with the light source and the centre of the cloud at the extreme ends—the light source being at the convex side of cloud—and our sight in between, at the concave side of the cloud. Ibn Rušd describes the reflection that causes the halo in the same way as Ibn al-Haytam: a ray of light from A is reflected according to the law of equal angles in a point Η of the surface FHD of the cloud, against a surface in Η that is perpendicular to FHD, or in other words, against particles that are situated along the radius BH. If Η is rotated along the axis AB, then

124 125

A similar statement is in Pseudo-Olympiodorus, see above p. 267. Ibn Rusd, Short Commentary 59,3-61,11.

the rotated points are also points of reflection, and a circle results that is seen as the halo.126 If more than one halo occurs, then they are formed by reflection against different surfaces. The halos that are higher are said to be smaller because they are at a larger distance.127 The light source is seen in the middle of the halo such as it really is, by means of direct rays. The reason is that rays which fall on the surface at a right angle pass straight through it, for such rays are so strong that they disperse the cloud. For the same reason, i.e. the strength of the rays, no halo is formed around the sun.128 Ibn Rušd describes the rainbow in the usual way, as a phenomenon that is seen opposite the sun when dense, transparent clouds are present. When days are short, it may be seen at any time of the day; when days are long, it is only seen if the sun is near the horizon. It is always a semicircle or less. It has three colours: the largest band is light-red (ahmar aš-šuqra); then follows leek-green (akdar karrātJ); the smallest band is musk-red (ahmar miskī). Sometimes yellow is seen between the first two colours. At most two rainbows appear at the same time. The inner one has the colours such as we just mentioned; the colours of the outer one are weaker and in reversed order.129 The explanation of the rainbow, as given by Ibn Rusd, especially the geometrical part, shows that he is dependent on Ibn al-Haytam, just as in the case of the halo. He says that the cause of the rainbow must be reflection of the sunrays against the cloud to the observer. The cloud must have a specific form and specific properties, and the sun, the cloud and the observer must be in a special relative position. The form of the cloud must be a concave sphere, for if rays are to be reflected to one point, the reflecting surface must be spherical and concave. The cloud must consist of small spherical particles that act as mirrors in which the colour, not the form of a body is reflected. This occurs when the cloud is about to change into water (rain) and consists of a fine spray. Ibn Rusd quotes Ibn Sīnā who says (see above p. 281) that it is not the cloud itself that is the mirror for the rainbow, but moist air; the cloud has the same function as the coloured body that is behind the glass in a mirror. Ibn Sīnā says that an indication for this is that the rainbow is visible in mountainous areas, without clouds being present. 126

Ibn Rusd, Short Commentary 61,14-64,6. See Pseudo-Olympiodorus, above p. 269. 128 Ibn Rusd, Short Commentary 64,15-65,5. The view that rays incident along a line that is perpendicular on a surface are stronger than obliqely incident rays is mentioned by Ibn al-Haytam in Book I of his Optics, see K. al-Manāzir 1,6 §24; he gives an explanation in Book VII, see Lindberg 1968 25-29 and Lindberg 1976 75. 129 Ibn Rušd, Short Commentary 65,7-66,2.

Then the mountains perform the function of background, instead of a dense cloud. Ibn Rušd states that, if Ibn Sīnā is right, then it is not impossible for a cloud to be sometimes like an iron mirror, sometimes like a glass mirror. An indication that a rainbow may occur in transparent air is that it arises when one sprinkles water opposite the sun, or when water is sprinkled by oars from the sea. Ibn Rusd relates that he once saw a rainbow with turbid colours in a large cloud of dust that was swirled up by the marching army he was in.130 The relative position of the cloud, the sun and the observer must be such that—sun and observer both being on the concave side of the cloud—the centre C of the cloud and the observer D are on a ray that is emitted from the luminous body A perpendicular to the cloud. Suppose this ray falls on the cloud in B. Then D must be between C and B. If this is the case, then a ray from A may be reflected according to the law of equal angles in a point L of the cloud towards D. If we consider matters in the plane ADL, then for a given position of the cloud and of A and D there is only one point of reflection L possible at one side of AB. If we rotate AL around the axis AB, L describes a circle, all points of which are points of reflection. Part of this circle is seen as the rainbow. The question arises why we see the rainbow as a semicircle or less; the laws of optics do not prevent a rainbow from being larger than a semicircle or a complete circle, as appears from what precedes. In order to explain this we have to take into account the position of the horizon. Again considering the matter in the plane ADL, the plane of the horizon intersects this plane according to a line through D. This line intersects the cloud at F. If the sun is at the horizon, then DF coincides with the axis of the bow AB. In that case we see a semicircle—if the cloud extends until the horizon—for the centre of the bow falls on DF and the plane in which the bow lies intersects the plane of the horizon according to a diameter of the bow. Thus, one half of the bow is above the horizon. The centre of the bow may fall between C and D (the point of observation), or in D, or between D and B. If the sun is above the horizon, we have the following three cases (again, the centre of the bow lies on the axis AB): (a) If the angle ADL is right, then the centre of the bow coincides with the point of observation D, and we see the rainbow as a semicircle. (b) If the angle ADL is acute, then the centre of the bow falls between C and D, above the horizon, and more than a semicircle is visible above the horizon. If the sun is sufficiently high, it is not impossible for a complete circle to be visible, (c) If the angle ADL is obtuse, then 130

Ibn Rusd, Short Commentary

66,2-68,9.

the centre of the bow falls between D and B, under the horizon, and the rainbow appears as less than a semicircle. Aristotle claims that only the latter case occurs. If the cloud is homogeneous, then the position of its centre is fixed, and only one of the three cases is possible; in fact, only the latter case occurs, because AC is large in relation to CD. Indeed, the distance of the sun to the centre of the cloud is much larger than that of the observer to that centre.131 In the explanation of the colours the following premisses are used: Clear mirrors reflect the colours as they are; if the mirror is not clear, the reflected colour is a mixture of the inciding colour and the colour of the mirror. Also, if a mirror is far away, the reflected colour becomes darker than it really is. The colours of the rainbow arise due to reflection of the colour of the sunrays (white) against the mirror; the rays become darker, either due to admixture of the colour of the mirror, or due to the distance the rays have to cover to the observer, or due to both effects. In this way the colours light red (a'sqar) and purple (urjuwānī) arise; they differ in this respect that in purple more black is mixed with the white of the sunrays than in light red. Yellow is formed by mixture of white with a little black and green by mixture of yellow with black.132 The order of these colours is explained as follows: In the inner rainbow the outer band is closest to the sun; also, it receives more sunrays than the other bands because of its larger surface; therefore that band is light red. The other bands are further from the sun and receive less rays: they become green and purple. Ibn Rusd says that this is the explanation of the order of the colours such as it appears from the books of the commentators that he possesses and that Ibn Sīnā has criticized this explanation of the Peripatetics, including Aristotle. Ibn Sînâ's opinion is quoted, viz. that green cannot be intermediate between light red and purple in the sense of more and less.133 Ibn Rusd defends Aristotle against this criticism by saying that, in fact, Aristotle's opinion is not the same as what was described above as the opinion of the commentators. Either the account of the commentators has been misrepresented by a wrong translation or they did not convey Aris-

131

Ibn Rusd, Short Commentary 68,10-72,17. The last argument is not valid. ibid. 73,19-74,18. 133 That is, if red and purple differ due a different quantity of sunlight that falls on the bands and a different distance to the sun, and if we have a quantity of sunlight and a distance to the sun that have values that are intermediate between those for red and purple, then the result will not be green, as is assumed in the explanation of the commentators. Ibn Sīnā says that green is not related to red and purple and is not formed when red and purple are mixed; it is formed by mixing yellow and black, see above p. 282. 132

totle's meaning correctly. Aristotle said that green is intermediate between red and purple; he means that red and purple are contraries, between which the intermediate is not formed by a transition from one contrary to the other in the sense of more and less, but by mixing the contraries. Then green is formed by mixing the yellow that exists in light red and the black that is in purple. That there is yellow in light red is indicated by the fact that this colour sometimes appears in the bow between light red and green.134 The order of the colours of the secondary rainbow is the reverse of that in the primary bow, because the inner band of the secondary rainbow is closest to our vision and the other bands are further away. Thus in this case the distance to our vision determines the colours, not the distance to the sun, such as in the primary rainbow. In fact, every point of reflection has two contrary properties: if it is closer to the sun, it is further from our sight and vice versa.135 Ibn Rusd terminates his account by saying that he never saw mock suns and rods and that he will not properly discuss these phenomena. In his Middle Commentary Ibn Rušd follows Ibn al-Bitriq's text, but deviates from it in some details; in general, the result is more comprehensible and more in agreement with Aristotle. He often inserts his own explanatory remarks between the phrases of Ibn al-Bitriq and also adds whole sections that are not in Ibn al-Bitrlq. Some of these features have already been dealt with in the account of Ibn al-Bitriq's text. The halo, says Ibn Rusd, is formed when moist exhalation, from which clouds are formed, rises from the earth, increases and condenses in the air; when the light of the sun or the moon is reflected (inkasara) to our sight against that vapour, in such a way that all rays are reflected at equal angles with the reflecting surface, then it appears in that cloud as a circle.136 In the next section (not in Ibn al-Bitriq) Ibn Rusd says that what was explained above is only a general and remote cause of the halo. The more specific and proximate causes must be sought in the science of optics. There it is explained by means of which of the three ways of vision this phenomenon is observed: direct vision (istiqāma), reflection (inkisār) or refraction (in'itäf),137 what its position is and which body transfers the image. The star itself is seen by direct vision. The halo is 134

Ibn Rusd, Short Commentary 74,19-76,18. ibid. 76,18-77,9. 136 Ibn Rusd, Middle Commentary 140,16-141,2. 137 We note that the Latin versions in the Iuntas edition of the Short and Middle Commentary give reflexio as translation of in'ikäs and inkisār and refractio as translation of in'itäf, except that in the Middle Commentary the translations of in'itäf and inkisār are incorrectly given as, respectively, reflexio and refradio. 135

seen by reflection, not refraction. What reflects the light has the form of a circle. The triangles formed by the rays going from the luminous object to the reflecting body and by the reflected rays from the reflecting body to our sight have a common base, viz. the line that connects our sight with the luminous object and passes through the centre of that circle. The reflection occurs in such a way that the reflected rays form equal angles with the plane of that circle and that the just mentioned triangles are perpendicular to that plane. These conditions determine the position of the halo. Also, if the mirrors from which reflection occurs are small, only colour is reflected, not the form. Therefore the halo is the light of the moon, not its form, and we see its light in a place different from the place of the moon itself. We see a circle because not one, but many reflected rays arrive at our sight. Furthermore, if we consider the cloud as spherical—which is its natural form—then our sight C, the centre of the luminous object A and the centre of the cloud B138 are on one straight line, with our sight C between A and B. The points on the cloud where reflection occurs (i.e. where the halo is seen) are in a plane perpendicular to AB that intersects AB at a point within the sphere of the cloud. The reason is that the luminous object is far from us and the cloud is close to us. Reflection occurs with equal angles (i.e. incident and reflected rays form equal angles with the reflecting plane). These matters have been explained by Ibn al-Haytam.139 Then Ibn Rusd presents a digression, which, according to al-'Alawi, could be a correction,140 because it contradicts in a certain respect the preceding section. Ibn Rusd says that it is not correct to combine the methods of optics and those of natural science, such as Ibn al-Haytam did and also he himself did in the Short Commentary,141 The methods of both disciplines are different, for natural science gives causes that are clear by themselves, whereas optics gives causes that are remote and only accidental (not essential) for the natural objects that are studied in natural science. Therefore Aristotle did not give the causes derived from optics. We must not interpret this as that he was unable to give them or that he omitted something he should have said in this 138

The letters are added by us to facilitate the discussion. They correspond to the letters used in the account of th© Short Commentary. 139 Ibn Rusd, Middle Commentary 141,10-143,18. 140 ibid. 144n3 and 145n4. 141 In the Short Commentary Ibn Rusd announces that he considers the phenomena under discussion from the point of view of natural science, using the results of mathematics (optics); indeed, Ibn Rusd explains the halo with the methods of optics, in the same way as Ibn al-Haytam, see above pp. 288-289. See also Sabra 1976 6 for Ibn Rušd's critique of Ibn al-Haytam concerning this subject.

discipline (of natural science), for he was endowed with human perfection. What he easily understood could be difficult for others to understand. Therefore the commentators often expressed doubts on things he said, the correctness of which became apparent a long time later. Ibn Rusd continues to sing the praises of Aristotle and blames Ibn Bājja who said that Aristotle's words need completion,142 but he admits that there are things that neither Ibn Bājja nor he himself have understood. This digression must be a consequence of the fact that Ibn al-Bitrlq gives nothing of Aristotle's mathematical explanations of the halo and rainbow. Ibn Rusd declares that such explanations do not belong to the discipline in which these phenomena are treated. Aristotle justly omitted them and his omitting them does not diminish his perfection.143 After this digression Ibn Rusd continues with Ibn al-Bitriq's text. The corruption of the section on the halo as a weather-sign is emended in Ibn Rusd's reading (see above p. 262n40). Also, the statement and its explanation that the halo occurs less often around the sun is given in a correct reading (see above p. 263). Of course, this does not prove that Ibn Rusd's copy of Ibn al-Bitriq's text was better than the ones we have: he may have improved the text on the basis of Alexander. In what follows the account of the rainbow and the rods are given. Again, the text of Ibn al-Bitriq is followed, but one passage has changed place.144 As we have seen above (pp. 263-264) Ibn Rusd's description of the colours of the rainbow and of the process of reflection that causes the rainbow and the rods is better than that of Ibn al-Bitriq. Ibn Rušd inserts a section containing a mathematical description of how the rainbow is formed by reflection. He says that the rainbow is formed when the centre of the luminous body A, the centre of the sphere of the cloud C and the point of sight D are on one line, with C between A and D. Then the triangles formed by the incident and reflected rays have a common base AD and are all equal. C is between A and D in such a way that the angles of the incident and reflected rays with the radius of the cloud to the point of reflection are equal. Ibn Rusd says that this consideration is taken from optics and does not belong to this discipline (of natural science).145 Next, Ibn Rusd continues with Ibn al-Bitriq's text. His copy (and also

142

Ibn Bājja, Commentary on the Meteorology, see below 404,21-22. Ibn Rusd, Middle Commentary 144,1-146,12. 144 If the section on rods ('amūd or 'asan) in 148,15-149,7 is moved before its sequel 164,8 ff. the order of Ibn al-Bitriq is restored. 145 Ibn Rusd, Middle Commentary 152,8-20. A similar explanation is in the Short Commentary and is inspired by Ibn al-Haytam. 143

Ibn Tibbon's) apparently contained additions that are not found in the extant copies, as we saw above p. 264. Following Ibn al-Bitriq, it is stated that reflection occurs from water and (densified) air if they are calm. Reflection may also occur when someone's sight is weak; then he sees his image walking before him in the air, because for him the air acts as a mirror.146 A third way of reflection occurs when a colour is weak, so that it cannot move the air and is not able to penetrate into it; then it is reflected due to its weakness. These three ways of reflection all play a part in the explanation of the colours of the rainbow: the colours differ in distance to our sight, they differ in intensity due to different distances to the light source and different size of the bow, i.e. in a larger band of the bow the light is more intense than in a smaller one.147 After these preliminaries Ibn Rusd explains the colours of the rainbow. At first, Ibn Bitrlq is followed when he says that the cloud, before it starts to rain, forms a spray of fine drops. They form a smooth mirror for the sunrays and colours appear in them that are transferred to the air and then to our sight.148 The colours that appear in the cloud differ in accordance with differences in the cloud. If the cloud is black, then the colour that shines in it is dark red; this occurs when water is dominant in the cloud. If the cloud is white and close to us, then white is the dominating colour. The wine-red (light red) colour is formed in the smooth part of the cloud containing a spray of drops, and that is the outer band of the rainbow. The purple colour is formed in the part of the cloud that is black and also contains a spray; that is the inner band of the rainbow. The wine-colour is also seen in fire that rises from moist wood; this indicates that this colour comes about when light is mixed with dark, moist exhalation; another indication is that the sun looks red like smouldering coal or darker at its rising because of the exhalation that is present at the horizon at that time. The white colour in the rainbow is formed because the sun shines in the spray. From the mixture of this white colour with black the green and yellow colours arise that are seen as remaining a short time in the rainbow between light-red and green. This white colour quickly disappears because the spray is dissolved by the sun. If this did not occur, the colours would remain a long time and the bow would appear as a complete circle. These colours do not appear in the halo that forms around the sun, moon or a lamp, because the mirror in which rainbow 146

What follows next is not in Ibn al-Bitriq. Ibn Rusd, Middle Commentary 153,1-154,2. 148 What follows is more extensive than in Ibn al-Bitriq. Also Ibn Tibbon has a more extensive text, that he finds very confused, see above p. 265. 147

colours are formed is not just moist vapour, but a spray in a cloud; such a spray does not remain long in the neighbourhood of the sun, otherwise the halo would appear with the colours of the rainbow.149 Ibn Rušd tries to present in this section what he found in Ibn al-Bitriq and in Alexander and make one treatise of it. The result is rather confused; the source of this confusion is Ibn al-Bitriq's text corresponding to 373b33-374b20 which is muddled and incomplete. In this section Aristotle explains the difference between the formation of the halo, which is due to reflection from bright mist near the sun, and the rainbow, which is due to reflection against a spray of dark water drops in a cloud opposite the sun. Such a spray does not remain long if it is near the sun, therefore no halo with rainbow colours (= a rainbow which is a complete circle) can be formed. When the spray is opposite the sun, it remains and then a rainbow is formed. According to Ibn al-Bitriq's text the white colour, which is in fact the colour of the halo, appears in the rainbow due to reflection from the spray that is quickly dissolved. Ibn Rusd adopts from Ibn al-Bitriq this white colour in the rainbow and maintains (from Alexander) that the colours red and purple are also due to the spray. He does not mention that these colours do not dissolve quickly because that spray is opposite the sun. Next, Ibn Rusd follows Ibn al-Bitriq's text on rainbows that occur around lamps, rainbows that arise when water is sprinkled in a place opposite the sun or when water is struck by oars, and rainbow colours that are seen in a cloud reflected in water.150 Ibn al-Bitriq's version again enumerates the colours of the rainbow: wine-red, green, a mixed colour and white, i.e. yellow (see above p. 265). Instead of this, Ibn Rušd says that the colours are wine-red, green and purple and that between red and green sometimes yellow is seen.151 The next section, on the explanation of the different colours, is not found in Ibn al-Bitrlq. It follows the account of the Short Commentary, but is not completely similar to it. The outer band of the rainbow is light red because it receives more sunlight than the others, as it is nearest to the sun and has the largest surface. The inner, purple band is furthest from the sun and has the smallest surface; therefore, it receives less sunlight than the others and has the darkest colour. The green colour has an intermediate position. This does not mean, as Ibn Sīnā thought, that if purple and light red arise by mixture of sunlight with more or less blackness, green arises by such a mixture with an intermediate quantity of blackness. Green arises by a mixture of white 149 150 151

Ibn Rusd, Middle Commentary 154,4-156,20. ibid. 157,2-158,8. ibid. 158,10-13.

and black. This white is the colour that arises due to reflection from the spray; if it is mixed with the dark colour of the cloud, green is formed. Ibn Rusd says that this explanation agrees with Aristotle when he says that white is formed by reflection from the spray and that green is formed by mixture.152 Furthermore, Ibn Rusd says (not in the Short Commentary) that it is difficult to explain the difference between the colours if the cloud is assumed to be homogeneous. It is easier to explain when the cloud is assumed to be inhomogeneous, but then the problem is how it is to be explained that the order of the colours is always the same. This is a problem, especially if we also want to explain the order of colours in the secundary rainbow, which is contrary to that of the primary one. Ibn Rusd does not give a unambiguous solution. He says that also shadow may play a part in the formation of the colours, especially green, which may arise from a mixing of the dark shadow and the white spray.153 Then follows a discussion of the question why the rainbow appears as half a circle. Ibn Rusd says, with Ibn al-Bitriq, that the sunrays that shine on the cloud form a semicircle only, and adds: if the sun is at the horizon. Next follows a mathematical explanation along the lines of the Short Commentary and of Ibn al-Haytam. In the mathematical description given above (pp. 290-291) A, C and D are on one line; suppose this line intersects the sphere of the cloud in B. The circle of the rainbow has its centre also on this line AB, between D and B. If the sun is at the horizon, the line AB lies in the plane of the horizon and just one half of the circle of the rainbow is above that plane. If the sun is above the horizon, then DB, including the centre of the circle of the rainbow, is under the horizon; less than half a circle of the rainbow is visible above the horizon. Ibn Rusd then mentions that from a mathematical point of view three cases are possible. (1) The case set forth above, with the centre of the bow between Β and D. (2) The centre of the bow coincides with D. (3) The centre of the bow is between D and C. If in the latter case the sun is above the horizon, it is possible to see the rainbow as a full circle. Why does this not occur? The angle at which reflection takes place is a fixed angle, as we observe that the colours always appear in the same way. The reflection also occurs at a fixed

152

Ibn Rusd, Middle Commentary 158,15-159,14. Note here one of the results of Ibn al-Bitriq's introduction of a white colour in the rainbow, due to his misinterpretation of Aristotle's text, see above p. 265. This phrase is also an indication for an emendation in Ibn al-Bitriq's text, see above p. 265n55. 153 ibid. 159,14-160,7.

distance from the sun, for Aristotle said that it does not occur when the cloud is close to the sun. Thus, only one of the three mentioned cases actually occurs, and that is the first one. Ibn Rušd reports that one of his friends once saw a rainbow that was a complete circle. This apparently seldom occurs and must be due to some accidental circumstance in the mirror that causes the reflection. Ibn Rušd did not find any mathematical argument in Aristotle (Ibn al-Bitriq) and justifies this by repeating what he said in the account of the halo (see above pp. 293-294), viz. that Aristotle's method is that of natural science and one should not mingle this with the method of mathematics.154 The concluding remarks on why the rainbow is never seen in the south and the discussion of rods mainly follow Ibn al-Bitrlq. Ibn Rusd's treatises on the halo and the rainbow are in general clearer and more comprehensible than the text of Ibn al-Bitriq. He used the works of Alexander, Ibn al-Haytam and Ibn Sīnā to gain a better understanding of what according to him was Aristotle's doctrine on these phenomena. Some influence from Pseudo-Olympiodorus may be found in the Short Commentary. See also below pp. 302n7 and 309n28 from which it appears that words from Pseudo-Olympiodorus are used in the Short Commentary, whereas in the Middle Commentary words from Ibn al-Bitrlq are preferred. Sometimes it is possible to improve Ibn al-Bitriq's text using Ibn Rušd's commentaries; on the other hand, sometimes Ibn Rušd is still mislead by the misunderstandings that made Ibn al-Bitriq's text corrupt and confused. For instance, he did not arrive at an unambiguous explanation of the colours of the rainbow. He defends Aristotle against Ibn Sînâ's criticism of the Aristotelian explanation of the colours. Red and purple are explained in the Aristotelian way; green is not intermediate between these colours, but is, according to the Short Commentary, a mixture between the yellow that is in red and the black that is in purple; according to the Middle Commentary, it is a mixture between white that arises by reflection against a spray (a misunderstanding originating from the distorted text of Ibn al-Bitriq) and black that is the colour of the cloud.

12. Later authors on the rainbow After Ibn Sīnā, Ibn al-Haytam and Ibn Rusd the rainbow has been treated by such writers as a1-Qarāfī (t ±1285) and Kamā1 ad-Dīn al-Fàrisî (t ±1320). Al-Qarâfî's remarks on the rainbow have been 154

Ibn Rušd, Middle Commentary 160,15-163,9.

translated by Wiedemann.155 His treatise is mostly based on Ibn Sīnā. His explanation of the colours seems to be original: the cloud contains layers of vapour with different density and reflection against these layers gives different colours. Kamā1 ad-Din made a significant progress in the explanation of the rainbow. The result is known from an account in an appendix of his commentary on the Optics of Ibn al-Haytam. It has been translated by Wiedemann. 156 Kamā1 ad-Dīn made an experimental study of the paths of rays of light through a transparent sphere (of glass or water), more extensively than Ibn al-Haytam had done. He considered such a sphere to be a model for the drops in a cloud that are responsible for the formation of the rainbow. He explained the rainbow as arising from sunrays that are successively subject to refraction when they enter the drop, reflection against the concave inner surface, and again refraction when they leave the drop. The secundary bow is formed when two reflections occur in the drop. Kamā1 ad-DIn's explanation of the colours is based on the Aristotelian principle that they are arise due to weakening of the light. This weakening is less strong for the rays that fall on the drop at a small angle with the normal. It becomes stronger when the rays fall on it more obliquely.157 More or less simultaneously with and independently from Kamā1 ad-Dīn the same explanation was given by Dietrich (Theodoric) of Freiberg (t ±1310).158 He knew the Optics of Ibn al-Haytam and works of Ibn Sīnā and Ibn Rusd. Like Kamā1 ad-Dīn he experimentally investigated the paths of rays of light through transparent spheres and explained the rainbow in the same way as Kamā1 ad-Din. Again, the colours arise by weakening of light that depends on the angle of the incident rays with the drops. Dietrich adopts the colour theory of Ibn Rusd, as expounded in his paraphrase on De sensu et sensibilibus.159 He 155

Wiedemann 1913 458-60; see also Sayili 1940 and Boyer 125-126. Wiedemann 1910; see also Boyer 127-129. See Rashed 1970 for a study of the explanation of the rainbow by Ibn al-Haytam and Kamâl ad-Din a1-Fārisī. Rashed shows that the claim of Wiedemann, which is repeated by Boyer and others, that Kamā1 ad-Dīn derived his explanation of the rainbow from his teacher Qutb ad-Din aš-Sīrāzī, is unfounded. 156

157 158

Wiedemann 1912, Zur Optik von Kamāl ad-Din 166. Dietrich von Freiberg, Ueber den Regenbogen und die durch Strahlen

erzeugten

Eindrücke, a good account of Dietrich's theory is given in Crombie 233-289; see also Krebs, Meister Dietrich and Boyer 110-124. 159 See Krebs 33-39 and Ibn Rusd, Paraphrase of Aristotle's De sensu et sensibilibus 14B-H. Ibn Rusd's theory of colours is a version of the Aristotelian doctrine that what is observed is a mixture of the colour of the light with the colour of the reflecting object or the medium through which it passes. He says that light with its various degrees of brightness is mixed with the medium with its various degrees of transparency. This results in the different colours, that are all between white and black.

posits that the rainbow has four colours: red, yellow, green and purple. They arise from a combination of one of the two formal (active) principles bright and dark (as properties of the rays) with one of the two material (passive) principles transparent and opaque (as properties of the refracting medium). The correct paths of rays in raindrops that are responsible for the formation of the rainbow were found in the 14th century by Kamäl ad-Dīn and Dietrich von Freiberg. The quantitative calculation that explains the radius of the rainbow and an explanation of the colours would be given by Descartes and Newton. The complete theory of the rainbow was established in the 19th century.

EXHALATIONS WITHIN THE EARTH; BOOK IV OF THE METEOROLOGY

L Aristotle The last part of 111,6 is an introduction to an account of what is formed by the exhalations within the earth: minerals (ορυκτά, lit.: what is quarried) and metals (μεταλλευτά, lit.: what is mined). The dry exhalation by its heat causes the formation of minerals such as realgar, ochre, ruddle (red ochre), sulphur, cinnabar and unmeltable stones.1 Such minerals are like coloured dust or like stones formed out of such material. The moist exhalation is the cause of metals like iron, copper and gold; these are meltable or malleable substances. They are formed when moist exhalation is enclosed, for instance within stones; the dryness of the stone compresses and solidifies the moist exhalation. Thus, they are water in a sense, since their matter might have become water, but they were solidified before this could occur. They are all (except gold) affected by fire and they have an admixture of earth, because they still contain dry exhalation. Each kind of these bodies should be discussed separately. What follows in Book IV is not the discussion that is announced here and a book on minerals and metals is not known to have been written by Aristotle. We know that Theophrastus wrote treatises on these subjects: about what are here called minerals in the extant treatise De lapidibus and about metals in his (lost) Περί των μεταλλευομένων. Furthermore, he wrote about petrifications, such as fossils, corals and petrified plants, and about salts in his (lost) books Περί των άπολελιθωμένων and Περί άλών νίτρου στυπτηρίας. 2 Alexander and Olympiodorus mention that Aristotle did not write a book on minerals or metals, but that Theophrastus wrote a treatise on metals.3 Hunayn ibn Ishāq, Pseudo-Olympiodorus and Ibn Rušd (Short Commentary) did not discuss this section of Book III. 1

The dry exhalation is the effective cause here; the material cause must be earth. Steinmetz 1964 12 and chapters 3, 6, 7 and 8. 3 Alexander, in Meteor. 178,13-15; Olympiodorus in Meteor. 266,33-36. The quotations of Theophrastus by Alexander and Olympiodorus are included in Theophrastus 1992, 19932 vol. 1 366 no. 197A. 2

2. Ibn al-Bitrlq Ibn al-Bitrlq mentions as substances that are formed within the earth by dry exhalation: unmeltable stones like smaragd (zabarjad) 4 and ruddle (magra). He divides such substances into coloured5 dust and stones like marcasite (marqašītâ). Substances formed by moist vapour 6 within the earth are divided into those that are meltable, like copper, gold and silver, and those that are malleable (yu'arradu fi l-darb — lit: are extended by being beaten out),7 like iron.

3. Ibn Sīnā Ibn Sînâ's fifth chapter of the first treatise of the fifth section of the TabViyyāt from his Kitāb aš-Šifā', the treatise on 'geological' matters, deals with minerals (ma'diniyya; this includes metals).8 The Latin translation of this chapter forms the third of three chapters that were often added to Latin versions of Aristotle's Meteorology under the title De Mineralibus. This third chapter was entitled De quatuor speciebus corporum mineralium. The Arabic text has been translated into English by Holmyard and we refer to this translation for details on this subject. A partial Italian translation was made by Baffioni.9 Ibn Sīnā divides the minerals into four kinds: stones (hajar), meltable substances (da'iba), sulphurs (kibrlt) and salts (milh).10 He arrives at this division by first dividing the minerals into those that are strong in substance and composition and those that have a weak substance. The strong minerals are divided into those that are malleable (yantariqu) 4 Ibn Tibbon's translation, Otot ha-Shamayim 111,288 and Ibn Rusd, Middle Commentary 165,12 have: arsenic sulphide; apparently in their copy they read zirnīk, 5 'Coloured' is Ibn Tibbon's translation (Otot ha-Shamayim 111,289), in agreement with Aristotle 378a25; Ibn al-Bitriq 98,8 has: coagulated (ma'qid); Ibn Rušds Middle Commentary 165,13-14 says that the dust is soft and not coagulated, whereas substances like marcasite are coagulated. 6 Ibn al-Bitriq incorrectly has 'dry exhalation' (wahaj) instead of 'moist vapour'. Ibn Rušds Middle Commentary and Ibn Tibbon have the correct reading. 7 Ibn Rusd, Middle Commentary 165,16 has mutatarriq. The concept is discussed in Book IV. There the following words are used for malleable: Aristotle 386b18: ελατός; Ibn al-Bitriq 119,10 and Ibn Rusd, Middle Commentary 203,4: yariqqu wa-yansatihu\ Pseudo-Olympiodorus 182,14: mutaraqqiq\ Ibn Rusd, Short Commentary 97,4: mutaraqqiq wa-huwa muntariq. 8 Ibn Sīnāi as-Sifa, Tab. 5 20,4-23,15. 9 Baffioni 1980; see also Haschmi. 10 See Wiedemann 1911 for other divisions and enumerations of the minerals by authors such as Abü Bakr Muhammad Zakarīyā' ar-Rāzī ( t 925) (Kitāb al-Asràr), at-TugrāÌ (1" 1121) (Kitāb al-Jawhar an-nadir fi sinà'at al-iksir), ad-Dimašqī ( t 1327) (Nujcbat ad-dahr) and al-K.wärizmi (//. 980) ( M a f ā t i h al-ulūm).

(metals)—among them is quicksilver (zaybaq or zi'baq)—and those that are not (stones). The weak minerals are divided into those that are salty and easily soluble in moisture, such as alum (šabb), vitriol (zāj), sal ammoniac (nūšādir) and green vitriol (qalqand), and those that are oily and not easily soluble in moisture, such as sulphur (kibrit) and arsenic sulphide (zirnik). Malleable substances are meltable; their matter is water mixed with earth in such a firm way that they cannot be separated from one another. Their watery matter has been solidified by cold after having been matured by heat. Among these substances some are still 'lively' (hayy) and not solidified, because they are oily; therefore they are malleable.11 Stony substances are formed when watery substance is solidified not by cold alone, but rather by dryness that turns the watery substance into earthy substance. They are not malleable because they do not contain an oily moistness, and most of them are not meltable because they were solidified by dryness. Sal ammoniac is special among the salts, because it is more fiery than earthy. It consists of water with fiery hot smoke and has been solidified by dryness. The sulphurs have been formed when their moistness was leavened (takammara) with earthy and airy parts by means of heat; then they became oily and were solidified by cold. The vitriols are composed of a salty substance with sulphurous and stony parts and they have the power of some meltable bodies (metals). Mercury is like water that is very firmly mixed with sulphurous earth. It is solidified by vapours of sulphur (i.e. converted into sulphides). Mercury seems to be the essential element in all meltable substances (metals), for when they melt, they turn into mercury. It is clear that melted tin is mercury; when the other metals melt, the mercury appears red because of the severe heat that is necessary for them to melt. If mercury is mixed with sulphur, it solidifies and becomes one of the metals. If the mercury is pure and is mixed with pure sulphur that is not combustive, the result is silver (fidda). If the sulphur is still better and has a fiery, non-combustive power, then gold (dahab) is produced. If the sulphur is impure and has a combustive power, the result is copper (nuhās). If the mercury and the sulphur are both impure, then iron (hadid) is produced. Tin (rasas qala'ï) arises when the mercury is good, but the sulphur is bad, and when both are not well mixed. Lead (ānuk) arises when the mercury is impure, heavy

11 According to the Najāt meltable substances are malleable because they contain much moistness that does not become completely solidified because it partly turns into oil; see Ibn Sīnā, Kitāb an-Najāt 157,12-13. This is also the interpretation of the 'school of Ibn Sini', see below. Apparently they interpreted these phrases from the Sifa in this way.

and clayey and the sulphur is bad and weak and therefore the solidification is not perfect. The alchemists try to solidify mercury with sulphur; the results of their efforts may have a resemblance to the natural products, but they are not truly identical to them, as art always falls short of nature. The alchemists are only able to dye substances, so that they resemble silver or gold, but they cannot change their essential nature. It is not possible to split up the mixtures that form the metals and combine them into new ones, because the specific differences of the various metals are unknown; the properties by which we distinguish between them are merely accidental ones. Splitting up the mixture that forms a metal cannot be done by melting, because melting preserves the union. It is clear that this treatise is not based on some supposed sequel on Book III by Aristotle, as announced at the end of that Book. It does not derive from Book IV either, although there are some common features, such as that bodies which solidify by cold are melted by heat and those that solidify by heat (or dryness) are not meltable (see IV,5-7 and 10). There is no clear influence from Theophrastus either, except that one may point, with Steinmetz, to the fact that Theophrastus also distinguishes metals, stones, salts and 'earths', counting among the latter arsenic sulphide.12

4. School of Ibn Sīnā The text of Bahmanyār on minerals is mostly that of the Šifā'.13 He mentions wood as a malleable substance of weak composition (not in Ibn Sīnā) and says that malleable substances are malleable because they contain something like an oily substance that does not solidify. The account of Abù 1-Barakāt on minerals is rather different from that of Ibn Sīnā. He says that the powers of heating and cooling, that generate substances by mixing, work in order that each species is preserved through individuals that succeed each other. These powers by chance produce effects from the mixing of smoky and vapourous exhalation. Above the earth they produce the meteorological phenomena that have been discussed. Within the earth they produce substances by mixing the exhalations with earthy, watery and fiery parts, each in its own specific place, like mercury, sulphur, salt and metals. Whenever part of a such a substance is removed, new parts are formed at the 12

Steinmetz 1964 99 and 322. Bahmanyär, at-Tahsïl 718,9-720,6 is similar in text to Ibn Sīnā, 20,8-21,15. 13

aš-Šifā',

Tab. 5

same site. Other kinds of substances are also made in specific places of the earth, such as the material of seeds and roots of trees and plants. If something of such a substance is removed, new material is formed, also in other places. Roots, seeds and fruits may be transferred and sowed and planted in other places than where they were formed. As for the minerals, their generating force is not in the substance itself, but in the site where they are formed. Gold is not produced by gold, such as a tree is produced by a tree. The latter kind has a generating power in each individual, such as animals have. The generating power for minerals exists in the earth, insofar as the soil has the suitable constitution and is fit to preserve the formed substances, like mountains, depths and soil between stones that protect them. Minerals are formed in such places just as animals are formed in wombs. The formation and solidification of such mixtures either takes a long time—then they remain for a long time—or it takes a short period—then they do not remain for long.14 Mercury exists in sites spread like dew; it may be extracted from its site or it filters into wells from which it may be scooped like water. Gold and silver also exist mixed with soil or as large or small veins. Some minerals are malleable and meltable by fire; their principal material is moist, flexible, of firm composition and oily, so that the parts are not easily separated from each other, such as in other watery substances. Other minerals are meltable, but not malleable, like glass and rock-crystal, because they contain few oily parts and more earthy roughness. Again other minerals are neither meltable nor malleable; they are dry and their composition is weaker. Abū 1-Barakāt discusses some further properties of minerals like transparency (šafāf), being breakable (munkasir) and being capable of fragmentation (munfattit). These properties depend on the admixture of earthy, watery, airy and fiery parts.15 Next, Abū 1-Barakāt enumerates the minerals (ma'diniyyāt}, he does not have the systematic division that was given by Ibn Sīnā, although this division is still recognizable in his enumeration. Abū 1-Barakāt mentions stones, which are firm, capable of fragmentation, combustible, not meltable and not malleable; meltable substances, which may be malleable or not; substances that are inflammable by fire, like sulphur and those that are not affected by flames; substances that are soluble in water like salt, and substances that are not soluble, like pebbles; substances that consist of a weak material and have a feeble composi-

14 15

Abū 1-Barakāt, al-Miitabar ibid. 228,19-229,16.

II 227,7-228,19.

tion, like glass and substances that have a strong material; the latter ones are either malleable (iron, gold) or breakable and not malleable, like corundum and rock-crystal. People say that mercury is the essential element in the malleable substances and that these substances are formed from it. They have not thought very well, because mercury flees from fire, its matter remains together and keeps its own nature when it is mixed, so how could it be present in metals like gold and silver? If it were true, then we should have to find gold and silver in the sites of mercury, but we never found them there. Abū 1-Barakāt discusses how the properties of substances like sal ammoniac, vitriol, sulphur and mercury depend on the element that is dominating in their composition and how the metals arise from mercury and sulphur, according to those who think that such is the way how metals are formed; this more or less corresponds to the account in the Šifā\16 Abū 1-Barakāt agrees with Ibn Sînâ's view that the claim of the alchemists to produce the metals from mercury is not valid, because we do not know the composition of these metals. If they say that the metals are different because of a different degree of purity of the mercury and sulphur that are mixed, they do not say what this means: mercury is mercury, so what is less pure mercury?17 The final chapter of Abū 1-Barakāt's account of the meteorological phenomena is devoted to a refutation of astrology. He says that this discipline, just as alchemy, has no foundation in natural science. Fakr ad-Dīn follows Ibn Sînâ's division of minerals, with a few differences. He classifies mercury under the non-malleable substances of firm composition, together with stones such as corundum (yāqūt}, moistness dominates in mercury, whereas stones are dry.18 Furthermore, he enumerates the seven malleable substances (metals - al-ajsād as-sab'a) that are often mentioned in cosmographical and alchemistic literature: gold, silver, tin, iron, copper, lead and kārasīnī.19 Ibn Sīnā does not mention the latter substance. Fakr ad-Din says that these substances have as common properties that they are meltable, firm and malleable. They are different in these qualities from other substances, like lime and stone, which are unmeltable, pitch and wax, which are not firm, but melt and give off fumes, and glass and enamel, which are not malleable. Iron is meltable by means of a special process. Fakr 16 Abü 1-Barakāt, al-Mu'tabar II 230,13-231,18 is similar in content to Ibn Sīnā, as-Sifa, Tab. 5 21,1-22,15. 17 Abü 1-Barakāt, al-Mu'tabar II 229,17-231,18. 18 Fakr ad-Din, al-MabāhiL II 210,21-211,6 is mostly similar in text to Ibn Sīnā, aš-Šifa, Tab. 5 20,4-10. 19 Kārasīnī is probably some copper alloy; its exact nature has not been established. According to Wiedemann it is a kind of bronze, see Wiedemann 1905 404.

ad-DTn describes this process, in which iron filings are mixed with one fourth of pulverized red arsenic sulphide. The mixture is heated a whole night in a jar coated with clay in a hot oven. Then one sixth of natron and one third of oil are added, and the mixture is filtered off into another jar. Then sal ammoniac and pulverized glass kneaded with oil are added. This is melted as many times as you like.20 Next, Fakr ad-Din Next gives a further description of the metals, mostly along the lines of the Šifā'. He says that the metals consist of water mixed with earth in such a firm way that they cannot be separated from one another. They undergo a concoction, so that the moistness becomes oily. When this mixture is solidified by cold before the moistness disappears, then it becomes malleable due to the oily moistness. If no moistness remains, the result is not malleable like corundum and glass.21 The account of the way mercury is formed and how the metals are formed from mercury and sulphur follows more or less the Šifā', but some details are different. Fakr ad-Din says that silver is formed when the mercury and sulphur are pure and their concoction is complete and when moreover the sulphur is white; gold is formed if the sulphur is red and has a light, non-combustive dyeing power. Kārasīnī is formed when the mercury and sulphur are pure and the sulphur has a dyeing power, and when before the completion of the concoction the cold solidifies the mixture. Copper is formed when the mercury is pure, but the sulphur is impure and has a combustive power. Tin is formed when the sulphur is impure and is not very strongly mixed with the mercury. Iron is formed when the mercury and the sulphur are both impure and the sulphur is combustive. Lead is formed when the mercury and the sulphur are impure and their mixture is weak.22 The account of the other substances (stones, salts and sulphurs) follows the text of the Sifâ'P Finally, Fakr ad-DIn discusses the claim of the alchemists that metals may be produced by their techniques. He gives Ibn Sînâ's account in which the artificial manifacture of metals is declared impossible, because we do not know their essential specific differences; how could 20

Faly ad-DIn, al-Mabàhil II 211,8-212,4. He gives here a typical description of an alchemical process. Cf. Abu Bakr ar-Râzi's description of the process of tasmi' (bringing a substance into a state resembling wax) of iron, see Ruska 164. 21 Fakr ad-Din, al-Mabàhil II 212,6-11 is mostly similar in content to Ibn Sīnā, ai-Šifā', Tab. 5 20,12-14. 22 Fakr ad-Din, al-Mabàhil II 212,16-214,2 is mostly similar in content to Ibn Sīnā, aš-Šifā', Tab. 5 21,10-22,12. 23 Fakr ad-Din, al-Mabāhit II 214,4-16 is similar in text to Ibn Sīnā, aš-Šifa, Tab. 5 20,15-21,8.

we destroy or produce something of which we do not know these differences? 24 This is the first argument against the possibiliy of alchemy (1). Fakr ad-Dīn enumerates further objections against the possibility of alchemy that have been raised by philosophers. (2) Nature has produced the metals from certain elements, brought together in certain quantities, possessing qualities to a specific degree and acting on one another for a certain time. All these data are unknown to us, so how could we make or destroy these bodies? (3) If the dyeing substance is more resistant in fire than the substance to be dyed, then the latter will perish; if the substance to be dyed is more resistant, then the dyeing substance will perish. If both are equally resistant, they are of the same kind and one cannot say that the one is dyeing and the other dyed or vice versa. (4) If artificial gold were the same as natural gold, then what is artificially made would be the same as what exists by nature. This is impossible, for we have found no similarity between such things, and moreover, if what exists by nature could also exist by art, it would follow that what exists by art would also exist by nature. Then there would be a sword or bed by nature. (5) The metals have natural sites where they are formed and exist, just as wombs in which animals are formed. If metals could be formed in another way, it would be as if animals could be generated outside wombs. (6) The specific differences between these metals are unknown, so we can neither produce nor destroy them. If we would know the specific difference of each kind of thing, we would be able to make a donkey from a dog.25 Fakr ad-Din answers these objections by stating that we observe that antidotes have specific effects; we either have established the forms that constitute their essence and are the cause of these effects or we have not established them. In the latter case we say that the effect is due to the mixture. Similarly, we can say that the yellowness and firmness of gold are due to its mixture, not to its form; then the only differences that specify gold are yellowness and firmness and these are known and can be produced or destroyed. If we have established the form of the antidote, we will say that this form necessarily entails the effects. In the case of gold we know that its form entails that it is meltable, malleable, yellow and firm. Thus the answer to the first objection is that if we do not exactly know the form of a substance, we may still know the accidents that are appropriate and those that are not appropriate for it; we know that if an inappropriate accident increases, the form will perish. For instance, heat is not appropriate for 24

Fakr ad-Dīn, al-Mabāhil II 214,18-215,2 is an excerpt from Ibn Sīnā, Tab. 5 22,17-23,9. 25 Fakr ad-Din, al-Mabāhil II 215,3-216,4.

as-Šifa,

the form of water. Even if we do not exactly know the essence of water, we know how to destroy it, viz. by heating, and how to produce it, viz. by cooling air. The second objection is refuted by the discipline of medicine.26 The answer to the third objection is that if the dyeing and the dyed substance are equally resistant in fire, their essence is not necessarily the same: things that are different may have common properties. The answer to the fourth objection is that what exists by nature may also be produced articifially, for instance fire struck by a flint, wind produced by a fan and bellows. Furthermore, if natural things may be produced artificially, this does not imply that artificial things must exist by nature. The answer on the fifth objection is that transforming copper into silver is not the same as generating the essence of a thing; it is rather like the cure of a patient: copper has the same essence as silver, but it has some diseases. The sixth objection is anwered in the same way as the first one.27

5. Ibn Rusd — Middle

Commentary

Ibn Rušd's Middle Commentary is mainly Ibn al-Bitrlq's text with some additional remarks. Dry exhalation within the earth causes the formation of minerals (ma'diniyya) that are capable of fragmentation (yatafattatu) and breaking (yankasiru) 28 and are not meltable, such as arsenic sulphide and ruddle. There are two kinds of such substances: those that are like dust—they are soft and not coagulated (gayr mun'aqid)—and those that are coagulated, like marcasite. Moist exhalation within the earth is also is the cause of two kinds of substances: those that are meltable by fire, like copper and gold, and those that are malleable (mutatarriq), like iron. What is malleable is also meltable, but not all meltable substances are malleable.29 After some remarks on the formation of these substances that follow Ibn al-Bitrlq, Ibn Rusd concludes his commentary by saying that each of these substances will be discussed in the Book of Minerals. Before

26

The meaning of this becomes clear from the answer on the fifth objection, see

below. 27

Faly ad-DIn, al-Mabāhii II 216,5-218,1. These concepts are discussed in Book IV. Breakable: Aristotle 386a9: KOttOdlCîÔÇ; Ibn al-Bitriq 117,11 and Pseudo-Olympiodorus 181,2 and Ibn Rusd, Middle Commentary 201,10 and Short Commentary 98,8: munkasir. Capable of fragmentation: Aristotle 386a9: θραυσίόζ; Ibn al-Bitrlq 118,1 and Ibn Rusd, Middle Commentary 201,11: munfarik\ Pseudo-Olympiodorus 181,2 and Ibn Rusd, Short Commentary 98,9: munfattit. 29 Ibn Rusd, Middle Commentary 165,7-166,2. 28

that their common properties will be set forth in the fourth Book of the Meteorology.30

6. Book IV of the

Meteorology

Book IV of the Meteorology, is clearly not a sequel on Books I-III. The two exhalations that reigned in Books I-III are absent in Book IV. Most modern commentators tend to the opinion that it is a separate treatise by Aristotle.31 According to Strohm, it was written before De Generatione et Corruptione, maybe as a preliminary study for it.32 Alexander says that Book IV deals with subjects that do not belong to the Meteorology and that it is better to regard it as a continuation of De Generatione et Corruptione. The subject of the latter book is the four basic principles (heat, cold, moisture, dryness) and the elements that arise by combination of two of these principles, an active and a passive one. In Book IV Aristotle explains how the active principles work and how the passive ones are affected and what results from the influences of the four principles on each other.33 Olympiodorus is aware that the connection of Book IV to the preceding books is problematic, but he rejects Alexander's opinion. Olympiodorus distinguishes, with Aristotle, between homoiomerous and anhomoiomerous substances, all being composed of the four elements. Another distinction is that between inorganic and organic bodies. Homoiomerous inorganic substances are things like minerals, stones and metals; homoiomerous organic substances are tissues in animals like flesh, bone, etc. and in plants, like wood and bark leaves. Anhomoiomerous bodies are made up from homoiomerous substances; they are organs such as head, hand and foot (388a13 ff.). There are no inorganic anhomoimerous bodies, unless one would like to classify artifically made things (chair, table) under this heading. Olympiodorus arrives at the following classification: Aristotle treats the inorganic homoiomerous bodies at the end of Book III and in his book On Metals.34 Then, 30

Ibn Rusd, Middle Commentary 167,4-6. See for instance Düring 1944 17-19; see Baffioni 1981 for a survey of the discussion on the authenticity of Book IV and its place among Aristotle's works and her own contribution to it. 32 Strohm 1984 216-219. 33 Alexander, in Meteor. 179,3-11. 34 This passage is included in Theophrastus 1992, 19932 vol. 1 368 no. 197C, although Theophrastus' name is not mentioned here. It is as if Olympiodorus ascribes the work On Metals to Aristotle. However, in his commentary on the final part of Book III he says that, as far as he knows, Aristotle did not write a book on metals, but Theophrastus did. See in Meteor. 266,35-36, above p. 301. 31

Book IV discusses the homoiomerous bodies in general (inorganic and organic). The discussion of the organic homoiomerous bodies will follow in Historia Animalium, that of the anhomoiomerous (organic) bodies in De Partibus Animalium. When Alexander, says Olympiodorus, makes Book IV follow upon De Generatione et Corruptione, this would cause a break in the discussion of the elements, for De Generation et Corruptione and Meteorology I-III both treat the elements, whereas Meteorology IV deals with what is composed of them.35 Philoponus thinks that Book IV is the treatise on minerals and metals that was announced at the end of Book III.36 Ibn al-Bitriq in his version of Aristotle's enumeration of his works in 1,1 says that the subjects to be treated after the Meteorology are animals and metals.37 When Pseudo-Olympiodorus enumerates the contents of Aristotle's books, he says that De Gener atione et Corruptione treats the bodies that are subject to generation and corruption, whereas the more specific phenomena that arise from them above the earth are treated in the first three Books of the Meteorology. Then, the fourth Book deals with anorganic bodies and homoiomerous organic bodies.38 Between the sections of the Kitāb aš-Sifa' in which Ibn Sīnā treats the subjects of Aristotle's De Gener atione et Corruptione and Meteorology he inserts a section entitled O n action and passion" (or: O n affecting and being affected" - Fi l-afāl wa-1-infi'ālāt). He says that after having discussed the principles of coming to be and passing away, he intends to treat the effects of the primary qualities (hot / cold, dry / moist). After that he will treat the different classes of things, starting with the meteorological phenomena and the minerals, followed by the soul, the plants and the animals. This section on action and passion—the fourth section of the Tabtiyyāt—deals with the sea and its saltness and with the subjects from Book IV of the Meteorology. Apparently Ibn Sīnā considered these subjects to connect with those from De Generatione et Corruptione, but he does not give an explicit opinion on the order of the subjects to be treated. Ibn Bājja, contrary to Alexander and Ibn Rusd, considers Book IV to be a continuation of Books I-III: it is a general account of the phenomena in the earth, as announced at the end of Book III. The specific account of the minerals will follow in the Book of Minerals (see above p. 59). 35 36 37 38

Olympiodorus, in Meteor. 5,24-6,30 and 272,5-274,2. Philoponus, in Meteor. 3,14-16. Ibn al-Bitriq, Meteorology 12,5. Pseudo-Olympiodorus, Tafsir 83,3-20.

Ibn Rusd says, following Alexander, that Book IV deals with the homoiomerous bodies and their general properties, as a continuation of what is said in De Generatione et Corruptione about how composed bodies are formed. What follows upon Book IV in the order of Aristotle's works is the account of each kind of (composed) substance in particular, starting with the simplest ones, those which are closest to the elements, sc. the minerals. Ibn Rusd continues his enumeration of Aristotle's works with the book on plants, the books on animals, etc.39 An Italian translation of Ibn al-Bitriq's version of Book IV and of Pseudo-Olympiodorus' paraphrase of this Book was made by Baffioni.40

39

Ibn Rušd, Short Commentary pp. 309-310. 40 Baffioni 1980.

3,18-4,5; also Middle

Commentary

167,4-6, see above

SUPPLEMENT 1

IBN SUWÄR IBN AL-KAMMĀR TREATISE ON METEOROLOGICAL PHENOMENA

INTRODUCTION We have used the following manuscripts for our edition: H: Hyderabad, Andhra Pradesh Governmental Oriental Manuscripts Library and Research Institute, Arabic Manuscripts no. 371 40b-48b, 1614 / 1023 A.H. R1: Rampur 3468/4 9b-26b, 18th century. T: Teheran, Majlis 3211/6 14 ff., 18th century. R: Rampur 1027 156b-161a, 18th century. R 1 is identical to H, except in two places, where the transcriber has omitted one line. He probably has copied from H, because in H the same words occur below one another in two successive lines in these places. R is very incomplete: it is only an extract from the text in H and T. The three manuscripts Η, Τ and R have a few common features, such as a lacuna of about a quarter of a line in the same place and a space left empty for a figure that was not drawn. They must have been copied from a common source. They differ from each another in such a way that it could not be established how they might be related. We took Η and Τ as the basis of our edition. Sigla Besides the letters referring to the manuscripts mentioned above, the following signs are used: In the text: <....> [....] t .... t * **

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IBN SUWÀR - TREATISE ON METEOROLOGICAL PHENOMENA

In the name of God, the Merciful and Compassionate; to Him we turn for what is noble. This is a treatise by Abū 1-Kayr Hasan ibn Suwār on imagined phenomena in the atmosphere caused by watery vapour, sc. the halo, the rainbow, mock suns and rods (qudbān).

Abū 1-Kayr Hasan ibn Suwār says: All things which have causes and principles are well and properly known (only) after having gained knowledge of their principles and causes; the halo, the rainbow and atmospheric phenomena such as rods and mock suns are things which have causes and principles; therefore we first have to mention their causes and principles, if we want to get certain knowledge. As some of their causes are proximate and others distant, we decided to start with mentioning the distant causes, which are comparable with the proximate causes, in order that this study, which we intend to undertake in a most wise manner and by clarifying what we are turned to, becomes more important and excellent. We shall not remove ourself in this to such an extent that we are taken from the path suitable for a study of natural science. If we go beyond this1 in clarification and comprehension, we shall not undertake such a detailed examination as is fairly required, but we shall confine ourself to what we think is sufficient to reach our aim, with God's granted success and guidance.

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We say that it has been explained in the book On Generation and Corruption that all things subject to generation and corruption consist of four elements: fire, air, water and earth. (Furthermore, it has been explained) that all these elements change into one another, any arbitrary part in any other, and that during the process in which a part changes from one element into another, before it has become that towards which it is changing, that part is a third thing, different from that from which it is changing and from that towards which it is changing; that (third thing) is what we call exhalation. Therefore one says that the elements change into one another by means of exhalation, i.e. by first becoming exhalation. For instance, if a part of earth changes into fire, it first changes into a kind of exhalation, which is called smoky exhalation. If a part of water changes into air it first becomes an exhalation that is called watery exhalation (vapour). All phenonema occurring in the atmosphere arise from these two kinds of exhalation, the smoky one and the watery one. In this treatise we intend to discuss from the atmospheric phenomena the halo, the rainbow and the phenomena known as mock suns and rods: therefore we must state from which of both types of exhalation they arise. We say that they originate from the watery exhalation. This is proved from the fact that they only occur if clouds are present and at a time when there is moistness in the air; thus, they arise from watery exhalation. Some of the phenomena originating from watery exhalation are really existing, as can be seen, such as clouds, rain, hail, snow, dew and such kind of things; others are not really existing, as we see, but are an image imagined by vision, such as in a mirror a form is imagined that is not existing as it is perceived. Vision perceives an

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image in a mirror, which is like the mirrors in which one sees a face different from what it is; what is seen in those mirrors is different from what it is. Therefore we should investigate whether the phenomena which we intend to discuss, sc. the halo, the rainbow, the mock suns and the rods, are things which really exist or are imagined. I say that from the words of Aristotle, Nikolaus and Olympiodorus it appears that they believe that these phenomena are something imaginary which occurs due to reflection of the visual power against a cloud towards the luminous object; they differ according to the different kinds of deflection (inkisār) of sight and the places from which this deflection occurs. If the cloud is situated between the luminous object and the observer and sight is reflected (in'akasa) against them from all sides in the same way towards the luminous object, then a halo occurs. If the cloud is situated opposite the luminous object and the observer is between them on a straight line and sight is deflected from the cloud to the luminous object, then a rainbow occurs. If the cloud is situated besides the luminous object and sight is reflected towards the luminous object, then mock suns and rods occur. This is the opinion of these people concerning the occurrence of these phenomena. From the discussion of Alexander it follows that the form of the halo is existing and that its colour is something imagined, so he thinks that these phenomena are imagined.2 He says that the proof that it is an image which appears to sight by means of a cloud, such as an image appearing in a mirror, is that if the halo were a really existing object, it sometimes would have to exist without the moon being directly overhead, because of the swiftness of the moon; but it does never exist without the moon being directly overhead, therefore it is not something

2

We suspect a corruption of the text here; this phrase does not make sense and in what follows it is shown that the halo, its form as well as its colour are imagined. The whole paragraph presents several problems: According to Olympiodorus (in Meteor. 210,17-18), Alexander's opinion was that the form (of the rainbow) is something imagined, whereas its colours are really existing. However, in the text of Alexander's commentary on Aristotle's Meteorology, Alexander follows Aristotle, stating that the halo (its form and its colours) is something imagined. The arguments given in the next phrases are neither in Aristotle nor Alexander. The second argument is given by Olympiodorus (in Meteor. 210,34-37), and the first one resembles an argument by Olympiodorus (in Meteor. 210,23-28), but it is different.

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existing. Furthermore, if the halo were something existing, it would not be an exact circle, for nature is unable to create a form which is exactly a circle, as if it were a mathematical form; the halo is an exact circle, as will be explained later, therefore it is not something existing. Furthermore, if the halo were something existing, it would not always display the same regularity in its occurrence—mostly under the moon, and less frequently under the sun—since material things do not always display the same regularity; the halo always displays the same regularity; therefore it is not something existing. That the rainbow is something imagined appears from the fact that when it is present, and the sun is concealed by a cloud passing between the rainbow and the sun, the rainbow disappears; if that cloud has passed the area between the rainbow and the sun, then the rainbow appears again. This is a clear proof for the fact that the appearance of the rainbow in a cloud is like the appearance of a thing in a mirror.3 Furthermore, what makes this opinion stronger and gives support to it, is that the rainbow is seen outside the cloud, not in the cloud itself; this occurs, according to what Ptolemy says, when the cloud is thick and very dense.4 Alexander relates5 that a Platonic philosopher says that the proof that the rainbow is something imagined arising from the reflection of sight, is that in whichever direction we move when the rainbow is appearing, we see it (moving) with us. If we move to the right we see it (moving) to the right and if we move to the left we see it (moving) to the left; this is specific for things which are imagined. If the rainbow were something existing by itself, it would not change place when we are changing place, and it would (seem to move) contrary to our motion, so that when we move to the right it would (seem to) go to the left, and when we move to the left it would (seem to) go to the right.

3

This argument is also in Olympiodorus, in Meteor. 21031-33. Perhaps Ptolemy has discussed the rainbow in the lost Book I of his Optics, Boyer 62. 5 Alexander, in Meteor. 152,1-14. 4

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Furthermore, when we approach the rainbow a certain distance, it approaches us the same distance. If the distance between us and the rainbow is, for instance, thousand cubits and we move hundred cubits into its direction, then this results in a distance of eight hundred cubits between us and the rainbow. This is a specific property of imagined things which arise in mirrors, i.e. when an observer approaches his image in a mirror, then his image approaches him the same distance as he has approached towards it. The situation for the rainbow is similar; therefore it must be something imagined. This statement is confirmed by the rainbow that occurs around lamps on winter days when there is moistness in the air. It occurs to people having moistness in their eyes or having a weak sight that they see around lamps circles with dispersed colours (alwānuhā mufarraja). This occurs because the smoke which rises from the lamp becomes something like a mirror and impedes sight from viewing the luminous object directly; (the light of the lamp) is reflected from the mirror, i.e. from the smoke rising from the lamp to the luminous object on all sides. The result is that one imagines something like a circle, in which there is dispersion (tafrîj) (of the colours); when one turns away from, or approaches towards the luminous object those circles are not visible anymore. Similarly, when we look at the sun and we look at it very sharply, and then close our eyes, we see colours of the rainbow after having closed our eyes: we first see a red colour, then green, then purple, and then we no longer see anything.6 Therefore, if it is possible for us to see something like the appearance of the rainbow as an imagination which is not founded in reality, there is nothing which

6

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prevents the possibility of a rainbow to arise from a cloud. If this is possible and it has become clear from its specific properties that it is something imagined, then we have proven that it is something imagined. This holds in a similar way for the phenomena known as mock suns and rods. Thus far we extend our discussion of the fact that these phenomema are imagined. Some commentators have put forward refutations against people who intended to explain that the halo is not an image, but something really existing by itself. Then the fallacy of this became clear and we shall leave off these refutations as we do not like to lengthen (the discussion); the question amounts to what we have said, namely that these phenomena are images, not things really existing by themselves, like clouds, hail, rain and such kind of things, which are atmospheric phenomena existing by themselves. When it has become clear that these phenomena are things imagined in the way in which we imagine things in a mirror and that things which are seen and imagined in this way occur either by reflection (irìikās) or by refraction (iriitāf)—by the expression 'by reflection' I refer to what is seen by means of a thick, smooth mirror and by the expression 'by refraction' I refer to what is seen through moist things, such as things we see in water—then one should investigate by which of these ways vision of these phenomena occurs, by reflection or refraction. We say: Alexander says that the halo arises by reflection and the rainbow by refraction.7 Olympiodorus does not agree with this statement of his, and says that he is mistaken and that the truth is that these phenomena are seen by means of reflection, not refraction. The

7 Olympiodorus says (in Meteor. 210,15-17) that this was Alexander's opinion; however in the text of Alexander's commentary on Aristotle's Meteorology, Alexander follows Aristotle stating that both the halo and the rainbow arise by reflection. See above p. 321n1. Alexander does mention the opinion ('of others') that the halo arises by refraction, but he refutes it (in Meteor. 143,9-14).

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proof of this is8 that if the halo and the rainbow were formed by refraction, it would necessarily follow that the luminous object, for example the moon or the sun, would be seen larger than it really is, because things which are seen by means of refraction are seen with a size which is larger than their real size; however, when a halo or a rainbow occurs, one does not see the luminous object larger than it is seen when they do not occur; therefore they do not occur by way of refraction. If they do not occur by refraction, then they must occur by reflection. Furthermore, 9 if the rainbow occurred by refraction, it would necessarily follow that the clouds would be situated between the observer and the sun while the rainbow appears; one of the conditions for a thing to be seen by means of refraction is that the moist things are between the observer and what is observed; this is not the case for the rainbow, since for the rainbow to appear it is necessary that the observer is located between the clouds and the luminous object; threfore the rainbow does not arise by refraction. The same situation occurs for the mock suns and the rods. Thus, it has become clear that all these phenomena occur by reflection of vision from watery vapour to the luminous object and it has been made clear that this is a common feature for all of them. We now start to investigate each of these phenomena separately. We begin with the halo, for it deserves to come first because of the excellence of its form—for it has the form of a circle—and the unity of its colour, and because its being discussed first is useful for what has to be explained about the rainbow. What has to be investigated concerning the halo is the question of its being and the question of the causes of its properties. The investigation of its being deals with the question why it appears as a circle. The investigation of the causes of its properties

8 9

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deals with the question why it is mostly a (complete) circle and sometimes part of a circle; in the latter case it is either more than half a circle, or less than half a circle, but never exactly half a circle. Furthermore, (this investigation deals with the question) why it appears around the sun, the moon and the luminous stars, while its occurrence under the moon is most frequent, under the sun less frequent and under one of the luminous stars very seldom. (It also deals with) the reason why it appears more often at night than during the day and why it always occurs near the meridian circle and not at one of the horizons, and (with the question) under which condition it is a sign of rain, of fair weather and of wind and the reason for this, and why it occurs in places near the earth, not high above it, and why one sees in its middle something verging on white and what it is and what the circle is adjacent to it, which is black. These are the questions concerning the halo that are investigated by Aristotle.10 There are other questions concerning the halo which may be investigated: what is the reason that we see the halo as a circle while the moon is not full of light but has a gap? We have seen it as a circle while the moon was less than half full. Also, why do we see many halos and how do they arise? Aratus says that at a certain time he has seen seven halos. And when there is more than one halo, why do they not have one and the same size, but different sizes, in such a way that the one which is nearer to us is larger than the next ones?11 These are the questions concerning the halo which should be investigated. Before

10 This enumeration of properties of the halo is also—in a somewhat different form—in Olympiodorus, in Meteor. 217,27-218,17 and Pseudo-Olympiodorus, Tafsir 147,4-14. 11 The question of multiple halos is in Pseudo-Olympiodorus, Tafsir 147,10-12, not in Aristotle nor Olympiodorus.

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presenting the discussion of each of these questions we must first examine why it is called 'halo'. We say that the term for halo in Greek and Syriac is derived from 'being circular' and 'surrounding'. The term in Greek is halös and in Syriac hūgetā, they derived the term for the halo from the meaning existing in it, which is 'being circular'—that is the most proper meaning—and from 'surrounding'—i.e. because it surrounds the luminous object on all sides. Let us now start the investigation of its being, which is the first investigation from the questions we have enumerated. We say that it has been explained above that the halo comes into being by reflection of sight from clouds towards the luminous object which is situated right above the cloud. Therefore the cloud which is under the luminous object when the halo has been formed must be smooth and shining, and must consist of very small parts with equal colour, i.e. they must have one colour, tending towards white, and with different positions, in such a way that the parts close to us are higher than the parts farther from us, so that the lines drawn between our sight and those parts are equal. The things that must exist in a cloud for a halo to appear are: smoothness, so that sight may be reflected against it; thickness, lest sight penetrates into it, is dispersed and not reflected from it; transparence, so that sight becomes stronger when it is reflected, not weaker; smallness of parts, so that it accepts the colour of the luminous object, not its form; equality of colour in whiteness, lest different colours arise like red, green and violet as in the case of

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the rainbow; a different position of its parts, so that all lines between sight and the cloud and those which are reflected from these lines towards the luminous objects are equal, for in this way a circle is formed.12 If the cloud has these properties, the luminous object is right above it and sight is under it, then a halo appears, for under these circumstances two cones are formed, which have equal sides and angles, the top of one of them being sight and of the other the luminous object, and which have the cloud as their (common) base. This base must be a circle. This becomes clear when we imagine ourselves a straight line from the point of sight towards the luminous object, and then imagine that lines are drawn from the point of sight towards the cloud and that each of them is reflected towards the luminous object. Then many triangles arise having equal bases, all being one and the same, namely the straight line we have imagined from the point of sight to the luminous object, and having as sides the lines which were drawn from sight to the cloud and from the cloud to the luminous object. These lines are all equal, i.e. the lines from sight to the cloud are equal to one another, and those from the cloud to the luminous object are equal to one another. If this is the case, the line passing through the tops of the triangles, these tops being at the cloud, must be a circle. Therefore the halo is a circle, and this is the mathematical proof for it.

12 The discussion of the cloud properties in this paragraph resembles Olympiodorus, in Meteor. 215,15-216,7 and 219,10-16. The properties smooth, thick and transparant are also mentioned in Pseudo-Olympiodorus, Tafsir 146,13-15.

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The physical proof, given by Theophrastus, runs as follows: He says that the light of the luminous object causes a wave in a thin cloud. This is the kind of motion as occurs in water when a stone is thrown into it; then a circular motion arises round the stone. Furthermore, the rays from the moon make the air opposite the moon thinner, and thus the cloud opposite the moon, in which the halo comes into being, becomes thinner; the thinner the part opposite the luminous object becomes, the thicker the parts around it become; these thicker parts are seen as a circle (halo). This is comparable to (what happens when) someone blows with a tube on dust which is under it; then he cleans the place on which he is blowing and the dust which is swept away gathers in the places surrounding the clean places and forms a circle. This is what Theophrastus says on this matter.13 Alexander relates that the ancients thought that the cloud is a spherical body with depth, and that if the light of a luminous body situated right above it falls on it, something like a circle is formed because the light makes a cut in the sphere, and every cut in a sphere is a circle. This is what Alexander says about it, and these are the things which make it necessary for the halo to be a circle.14 The cause that it is most often a circle and more seldom part of a circle, is that the cloud in which the halo appears is wholly in one state of thickness, smoothness, transparency, partition (into small parts) and colour, as we have said above.15 Because it is wholly in one state, it most often occurs that sight is reflected from all sides towards the

13 The text of this paragraph is almost identical to that occurring in Ibn Suwâr's translation of Theophrastus' Meteorology. See Daiber's edition 1992 ch. 14,1-13 and above p. 251. This is not found in the text of Alexander's commentary on Aristotle's Meteorology . However, compare Alexander, in Meteor. 143,10-12, where he mentions those who suppose the halo to arise by refraction, not reflection. He says: "they suppose the cloud to be spherical and hollow; then the star above it is seen as a circle as (its light) is dispersed in it." 15 See p. 333.

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luminous object in the same way, and therefore in most cases a circle arises, and more seldom part of a circle; part of a circle arises when sight is reflected from some sides and not from other sides, when some parts of the cloud are different from others. That parts of the cloud are different occurs more seldom. The reason why the halo, although it may be part of a circle, is never exactly half a circle, is that the (number of) reflected lines is never exactly equal to the (number of) lines which are not reflected, but one of them is more, so that exactly half a circle is never formed.16 The reason why a halo occurs under a luminous object is that for a circle to appear it is necessary that sight is reflected against a cloud from all sides; this can only occur when it is situated under the luminous object. The reason why it mostly occurs under the moon and more seldom under the sun is the heating power of the sun. Because of this heating power the substance in which the halo is formed is dissolved more than it is dissolved by the moon. Therefore it occurs under the moon more often than under the sun. The reason that it occurs under the luminous stars very seldom is the weakness of their light, their distance, and the small quantity of vapour which is gathered under it; therefore it is almost impossible for a halo to arise under them, for the distance and the small size of the mirror against which sight could be reflected makes it impossible for sight to be reflected, or if it is reflected, it is weak because of the great

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distance. Furthermore, since the light is weak, it is not possible that in the gathered vapour the same thing occurs as what is brought about by the moon, sc. a wave-like motion in the vapour. Thus, for the halo to appear under the stars the vapour must be thin, so that it can be influenced by the light of the stars in the same way as the gathered vapour under the moon is influenced. It very seldom occurs that such vapour is gathered under the stars and therefore the occurrence of the halo under the stars is very seldom. This is what I think of these things.17 The reason why the halo occurs at night more often than during the day is that the vapour in which the halo is formed exists at night more often than during the day since the force of the coldness at night is stronger than during the day; and (also) because the halo occurs less often during the day, as we have explained, due to the sun's power of heating and its dissolving the cloud which is suitable for the appearance of the halo; therefore it will occur more often at night. The reason why it occurs near the meridian circle is that it cannot appear at the horizon, according to Olympiodorus, because of its size, for its size is such that it cannot come into being at the horizon without being partly concealed by the horizon.18 I think that the cause of this is that, as has been explained, for a halo to come into being the luminous object must be right above the cloud, not opposite it, and the observer must be under it, not opposite it; in short, one should note that when a halo appears, the luminous object, the centre of the halo and the observer are on one line, and this can occur in this way only if the luminous object is near the meridian line; since the cloud must be under it, the halo can only appear in this position. As for the question under which condition it is a sign of rain, fair weather or wind, we say the following: It is a sign of rain if it is stable and black. It is a sign of fair weather if it is gradually vanishing and

17

The halo around the stars is discussed—somewhat differently—by Pseudo-Olympiodorus, Tafsir 147,21-148,2, not by Olympiodorus. 18 Olympiodorus, in Meteor. 230,5-9. Olympiodorus adds that the diameter of the halo is 40 degrees. See also Pseudo-Olympiodorus, Tafsir 149,15-18; he adds that the diameter is 45 degrees.

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becoming weaker. It is a sign of wind if it is breaking up; the wind is coming from the direction where the breaking up occurs: if it occurs in the north, a northern wind arises and if it occurs in the south, the wind will be from the south, and similarly for the other directions. As for it being a sign of rain when it is stable and black, the cause for this is that its being stable and black is a sign that the coldness has become strong, that it has made the cloud more dense so that rain will be falling from it and that it will turn the cloud into water. As for it being a sign for fair weather when it is vanishing and becoming weaker, its vanishing is a sign that the heat has become stronger and causes the cloud to disappear and to vanish. As for it being a sign for wind, the halo exists on account of the cloud and the motion of the wind starts from above, as has been explained in the discussion of the winds; therefore it is first the cloud which is influenced by the motion of the wind. Thus, when the halo breaks up, we know that its breaking up is caused by the motion of the cloud in to which the halo appears, and that its motion occurs because it is moved by the wind. Consequently, when the halo is breaking up, we know that a wind is blowing, and it is blowing from the direction where the breaking up occurs, because it is there that the wind first hits (the cloud). The reason that the halo more specially occurs in places near the earth is that since the wind comes from above, its motion above is stronger than its motion below. The substance in which the halo comes into being does not withstand the wind, and therefore the halo occurs in places in which there is not much potentiality for wind; thus, it mostly occurs in places near the earth, more seldom in higher places. It is also possible that the reason is that the places near the earth are far

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from the moving air, as the motion of the air impedes its appearance and therefore it appears in places near the earth because these are far away from the moving air. This is what he says about it. As for the reason that one sees in its middle something like a void and that the adjacent circle is black, Alexander says19 that the reason is that when the rays of the luminous object fall on the cloud, the spot of the cloud on which they fall is dissolved by their intense heat, so that in its middle a void arises—it appears from this that Alexander thought that the middle is a real void—and that the colour (of the adjacent circle) is imagined (to be black) because the light of the luminous object surrounds the cloud, while the cloud impedes the colour of the luminous object to be seen clearly. Olympiodorus says20 that the reason is that the axis of the cone is the smallest connecting line between the observer and the luminous object, so that sight sees the luminous object from close by and pierces through the cloud because of its strength, and thus, this place is seen as strongly lit. The other lines which can be thought to go from the observer to the luminous object do not run straightly to it, but arrive at it after reflection from the cloud; then the distance is long and one does not see the luminous object in this place strongly lit, such as we see it at the axis. Therefore there is something like a void inside the halo. The adjacent circle is black because of its (the halo's) whiteness, since if white is on a spot next to something else which is less white, then what is less white is seen as black. The reason that the halo (also) exists when the moon is not full is that the thing (cloud) from which the halo arises is the same, and the

19 Alexander, in Meteor. 142,28-1433 and 143,35-144,2. He says that the cloud in the middle of the halo becomes thinner because that part of the cloud is dissolved by the rays of the luminous object, whereas the parts surrounding the middle remain as they are, and from these parts reflection takes place. He does not use the word 'void'. 20 Olympiodorus, in Meteor. 222,29-223,3 and 227,1-5. Pseudo-Olympiodorus, Tafsir .151,2-10 gives an account (without mentioning names) of both theories, that of Alexander (or Theophrastus) and Olympiodorus, see above p. 269.

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rays of the moon fall on the cloud, also when it is half-moon; then a wave-like motion occurs, such as occurs when a stone is thrown in water, as Theophrastus has said.21 Furthermore, if the moon's light falls on the cloud and the cloud is spherical in form, deep and transparent, it produces a cut, and any cut produced in a sphere gives rise to a circle, as we have said above.22 The reason that many halos may come into being is that there may be many clouds situated one above another, each of them being suitable to generate a halo. Thus, many halos appear, in accordance with the clouds in which it is possible for a halo to be generated. The reason why the nearer one is seen larger than the one further away is that the nearer to the observer some visible object is, the larger it is seen: we see what is near at a larger angle than what is far, and what we see at a larger angle, we see as larger. We may observe something similar in mirrors we have, namely that what we see in it is larger when the mirror is close, and smaller when it is far.23 These are the questions which we have included in the investigation on the halo and thus far goes our simple discussion of it; we know that we shall give a more thorough discussion in the third chapter of this treatise, when we give a commentary on what Aristotle has said about it. We shall now start the discussion of the rainbow, and we shall follow the same method as we have done in the discussion of the halo, to wit simpleness and explanation to the extent which is suitable for our purpose in this chapter. We say that the rainbow is not a complete circle, but part of a circle, either half a circle, or less than half a circle,

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See p. 337. id. 23 A similar account of multiple halos is also in Pseudo-Olympiodorus, 150,9-12 and 20-24, not in Olympiodorus. 22

Tafsir

348

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because it can never be larger than half a circle. If the sun is at one of the horizons, the rainbow is half a circle and if the sun has risen above the horizon it is less than half a circle; if the part is less than half a circle, the (radius of) that circle is larger than when it the part is half a circle. The rainbow mostly has three colours. The first one, which is the outer circle, adjacent to the sky, is red, as the colour of land;24 the second one, which is under it, is green; the third one, which is the one under the green circle and which is adjacent to the earth, is purple. This colour, i.e. purple, is a colour resembling the blood of the shellfish25 known as purple murex,26 because with its blood the purple clothes are dyed worn by the rulers of Byzantium; it is close to the colour violet

24

lawn al-barr, Pseudo-Olympiodorus calls the red colour date-red (busri), see his 152,13. Somewhere barr and busr must have been confused. The Arabic word here is saratān bahrl. The word saratān occurs in the Arabic version of De Partibus Animalium 683b28, where it is the translation of KOdptCtVOÇ (lobster) (see the edition by R. Kruk), one of the four subclasses of the class of Crustacea. The purple murex is a kind of saratān, according to Ibn Suwâr's text; this is not correct: the murex is a kind of shellfish, belonging to the class of Testacea (see e.g. PA 661a21). Pseudo-Olympiodorus says (152,13-16) that purple resembles the colour of the blood of the marine animal from which purple dye is made. This is also mentioned by Alexander, in Meteor. 161,8: the colour is similar to the colour of the blood of the Tafsir

πορφύρα της θαλασσίας. 26

Murex or purpura: the Arabie word used here is halazūn ; in the Arabic version PA this animal is designated by an Arabic transcription of the Greek πορφυρά (see the edition by R. Kruk, passim). The Arabic halazūn occurs in PA 661a22, where the corresponding Greek word is κογχυλίον, which designates a more general class of animals, and is translated there as 'shellfish'. From PA 679b14 ff. we learn that purpuras, trumpet shells (κήρυκες) and sea snails (κόχλΟί.) belong to the class of spiral shells (στρομβωδη), which in turn belong to the shellfish. The Arabic halazūn also occurs in De Generatione Animalium (see the edition of Brugman and Drossaart Lulofs, passim). The corresponding Greek word here is οστρεον, which may also be translated as 'shellfish', or more specifically as 'oyster', depending on the context. Thus, halazin, that often designates the class of shellfish, is used here as the name for an animal species belonging to this class, the purple murex. Furthermore, we remark that κογχυλίος designates the colour 'purple', and that Aristotle says that οστρεοί are used by painters (i.e. to manufacture their purple paint) (Historia Animalium 548a12), and that όστρείον means 'purple dye' as well as 'oyster'. Thus, also in Greek these words may designate a class as well as a species.

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which we have. I said 'mostly' because sometimes another colour appears between red and green, something like yellow. The colours of the rainbow are unique for it and nothing like them exists for anything made by handicraft, since the colours manufactured by handicraft are a mixture of black and white, whereas the colours of the rainbow do not arise by mixture in the same way as the colours made by handicraft. Therefore painters cannot make such colours. Sometimes two rainbows appear simultaneously; each of them has the same three colours as we have mentioned (to exist) for one rainbow, but the position of the colours of the outer rainbow is just the opposite of the position of the colours of the inner rainbow. The outer circle (of the outer rainbow), which is adjacent to the sky, is purple, the next one in the direction of the earth is green, and the next one is dark red (ahmar aswad); the outer circle of the inner rainbow, which is adjacent to the sky, is dark red, the next one in the direction of the earth is green, and the next one is purple. Thus, the red circles in the two rainbows are placed next to one another; they are followed by green in each of the rainbows; then follows purple. In this way the position of the colours of the two rainbows may be described whenever they appear together. Apart from the fact that the position of the colours of the outer rainbow is the reverse of the position of the colours of the inner rainbow, they are also less clear than the colours of the inner rainbow.

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The rainbow appears in winter, during the short days, at any time of the day; in summer it does not appear at any time of the day, for it cannot appear at midday. In winter, during the short days, it may appear at any time of the day, also at midday. These are the things which necessarily belong to the rainbow and which we observe in it; we shall first discuss each of these things and we shall explain and make clear its causes. We start with the discussion of the colours of the rainbow, for it is the first thing which strikes the senses, and the soul strongly desires to get to know their cause. We say: we have explained that the rainbow arises due to reflection of sight from the cloud towards the luminous object which is situated opposite it. As for how this occurs and in which way, it is well known that sight is reflected from a smooth object, such as a mirror, and what is present before it is seen in the mirror, and what is not present before it is not seen in the mirror. Water is a smooth body while it is being formed in the cloud and when it has almost actually become water and (also) after it has actually become water. Therefore sight is reflected from it, and the visible objects which are present before it are seen in it, and those which are not present before it are not seen in it. This is confirmed and witnessed by prominent people, for we see our image in water that exists here with us; (we see the image of) the stars and all heavenly bodies too when we look at it, because our sight is reflected and falls on it, so that we see these things in it. In some mirrors one sees the form and colour of a thing, in others one does not see the form, but the colour only. (The latter ones) are mirrors which are extremely small, so that reflected sight cannot encompass the visible object. If the reflected sight does encompass

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the visible object, then one sees its form and colour in the mirror. This means that when the dimension of the water is large, it is possible to see the form in it; when the water is divided into very small particles and sight is reflected against it, it does not encompass the visible object, so that one sees its colour (only), not its form. This is shown by the drops falling from the oars when a ship is sailing; then we see, when there are stars opposite, the light of the stars, but we do not see their forms. Also, when we stand between the sun and a space in which we sprinkle water with our hand, the colour of the sun appears in those drops, and we do not see its form. If it is as we have explained, then it is also clear that when the vapour in a cloud turns into water and changes into the smallest possible drops—which stay there and do not fall because of their smallness and their being too weak to traverse the air, so that they remain as they are—and when the cloud is opposite the luminous object, sight is reflected towards the luminous object from each of these drops which act as a mirror. Then one can only see an image of the colour because of the smallness of those mirrors, and one does not see an image of the form. Because these mirrors are situated beside one another and each of them is opposite the luminous object, not reflecting its form, one sees the totality of this as something continuous. Since these mirrors are so near to one another, the colours seen in each of them give the colour of the whole. In this way vision of the rainbow occurs due to reflection of sight from such a cloud, if it is located opposite the luminous object, while sight is between them. If the situation is different, no rainbow will arise.

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We shall explain the cause of the different colours of the rainbow after some preliminary remarks. Firstly, all colours between black and white arise from a mixing of white and black. When something that is luminous, radiates and emits light, and generally something white is observed through something black or when (its light) is mixed with black, the result will be the other colours. If the luminous object is prevailing, then one sees red, if not, then one sees green or purple; it is more dominating in green and less dominating in purple. The truth of our remark is confirmed by the fire we have. For if we light the fire in moist wood, and much smoke rises from it, we see it red; if we light it in dry wood, while not much smoke rises from it, we see it as whitish; if fire burns in very black coal, we see it through the blackness as greenish. The second preliminary is that sight is too weak to grasp what is far away, so that it fails to grasp it. If it is located opposite (what it is looking at), sight errs27 in seeing its form, its roughness, its place and its size. This is the reason why we see a large mountain from far away and think that it is small or why we see stars being small-sized, whereas they are many times larger than we see; then we err in their size. We see them as flat, whereas they are spherical; then we err in their form. We see them as if they were all located in the plane of the sphere of the fixed stars, whereas there is a large distance between the moon and that sphere, so we err in the place. This is also the reason why we see the earth, which is much jagged, from far away as if it

27 The idea that sight 'errs' when the visual rays are weak is taken from Olympiodorus, see in Meteor. 236,31-32, where he uses the word ΟίΠΟαη. The expression is used passim in what follows. See also above pp. 258-259.

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were smooth, so we err in its roughness; due to the distance the holes in the earth are hidden. Therefore we think that forms which are jagged, are not jagged and circular. These are the ways in which sight may err if it is far from the visible object and located opposite it.28 If (vision) is not opposite the object of sight, then it grasps the object of sight either by means of reflection, or by means of refraction. If it grasps it by means of reflection and it errs in its colours due to the distance, then it sees black as more intense black and white as less radiant. An example is that we see the clouds in water, when we look at them, as blacker, and that we see the luminous stars in it as less radiant. If vision grasps the object by means of refraction, vision errs in its size, so that its seen as larger than its real size; (it also errs) in its colour, so that it is seen as less black when it is black, and as more intense white when it is white. This is the second preliminary remark.29 The third preliminary is that the colour black is equivalent to absence of vision, for if we do not see the sun or some (other) radiating object, we think that we see something black. <When the object of vision is far away, vision is weak, so that we see it as black>;30 when the object of vision is closer to our sight, we see the visible object nearer to white.31 Therefore painters paint the parts of the body which are protruding, such as the nose and breasts, with a white colour, and the parts of the body which are deep-set, such as the eyes, with a black colour, because white causes a strong motion of vision, and black a weak motion, and what is near moves (vision)

28 Olympiodorus says that objects far away seem darker, less jagged and smaller, see in Meteor. 236,7-9. This paragraph is more similar to Pseudo-Olympiodorus, Tafsir 159,11-160,1, where it is said that vision is deceived in the size, jaggedness, roughness, sphericity and relative distance of distant objects. 29 This paragraph corresponds to Pseudo-Olympiodorus, Tafsir 160,1-8. 30 The phrase between < > is a conjecture. The text in the MSS is corrupt here. 31 The MSS have 'black' (as-sawād), but see note to the Arabic text.

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32

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vision of the third is (still) weaker than of the second. Therefore one errs a little in (viewing) the first ring; one errs more in (viewing) the second ring than in the first ring, and in the third ring one errs more than in the second one, if error (in vision) follows the weakness of vision. Therefore the first ring is imagined as black, the second as blacker than the first, and the third as blacker than the second. Vision also receives light from the luminous object; this light is much in the first ring because it is the largest one; it is less in the second ring because that ring is smaller, and in the third ring it is still less because that ring is smaller than the second one. If sight sees the luminous object through these (rings), it sees the first ring as red because it has a small amount of black and a large amount of white; it sees the third and last ring as purple because it has a large amount of black and a small amount of white; it sees the middle ring as green because it has an intermediate position between the two (extremes), between a small and a large amout of black, and a small and a large amount of white. This is the cause of the different colours of the rainbow.33 When there are two rainbows, then we see that both have the same colours, but the positions of the colours are inverted, i.e. the outer ring of the inner rainbow is red, and the inner ring of the outer rainbow is red; the adjacent rings in both rainbows are green; then comes the purple ring, as we said above. The cause of the colours is the same as was mentioned for the single rainbow, i.e. mixture of white, which is the (colour of the) luminous object, with the black colour of the cloud. The cause of the inversion of position of the colours is that there is a gap between the two rainbows—the first and the second one—which has no colour in it. The cause that we do not see a colour in it is either that the sun is straightly opposite it and dissolves (the cloud) by the power of its heat, or because the axis of the cone connected with the eye falls on that place and due to the strength of sight in that place sight does not err; therefore it does not see a colour in that gap. Both the inner ring of the outer rainbow and the outer ring of the inner rainbow are seen as red, because they are close to the spot where

33 The explanation of the colours in this paragraph follows Aristotle and Olympiodorus, but in its stressing the role of error in sight it is similar to Pseudo-Olympiodorus, Tafsir 156,1-13. Ibn Suwār adds an explanation that uses rays of light from the sun to our vision.

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vision does not err, so that vision errs in these places to a small extent only. If error occurs to a small extent, the cloud is seen as less black than in the place where vision errs to a large extent. Vision receives from the luminous object here the largest quantity (of rays) of all three rings, because these two rings are larger than the others. Therefore these rings are seen as red; the two rings next to them are seen as green because they have the same distance to the place where vision does not err. The next two rings are purple because they are furthest away from the place where vision does not err. This is the cause of the inversion of position of the colours of the two rainbows, according to what Olympiodorus has brought forward.34 We have performed a more complete investigation on this subject in the third chapter of this treatise, when we comment upon Aristotle's discussion. The yellow ring, which is sometimes seen between the red and green rings, does not arise by means of reflection—next to the red we see the green colour. The border between 35 a yellow colour. The cause of this is that if white is located next to black, it is seen as whiter. The red ring has something of white in it and the green one is verging on black; therefore the edge of red is seen as whiter due to its vicinity to green. Yellow is whiter than red; therefore the edge of the red ring, close to the green, is seen as yellow. Why one always sees this and what its reason is, and why it is not seen by means of reflection, and how this is proved, we shall discuss in the third chapter of this treatise when we give a commentary on Aristotle's

34 See Olympiodorus, in Meteor 238,20-239,13 for this theory that the axis of the cone of visual rays falls in the gap between both rainbows, and that vision is at its strongest in that direction, whereas is becomes weaker above and below that central direction. Olympiodorus ascribes this theory to his teacher Ammonius. The same theory is expounded in Pseudo-Olympiodorus, Tafsir 157,14-25 and the paragraph resembles most to that passsage of Pseudo-Olymopidorus. 35 The words between < > are a conjecture; the MSS have a lacuna here.

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words. We shall also mention the cause of the fact that the outer rainbow is less luminous and that there are never three rainbows simultaneously. We have performed the discussion we intended to give in this chapter about the colours of the rainbow. As for the question why the rainbow is half a circle when the sun is precisely at the horizon and why it is less than half a circle when the sun rises above the horizon, while the rainbow can never be more than half a circle, Aristotle explains this with a geometrical proof, in which there is some obscureness; we have mentioned it in the third chapter of this treatise and we bring forward what Theophrastus has said about it. As we do not want to give a repeating of it, disliking to lenghten (the discussion), we refrain from it (here) and give a discussion which is convincing and easy to grasp, as follows: We say: let us imagine that the hemisphere of the cloud is on the circle of the horizon, and that one half of this (hemi)sphere is visible, with the centre of its curvature located on the line of the earth, in such a way that the concave surface of this (hemi)sphere is facing east and what is above the earth is the surface of a quarter sphere; (let us further imagine) that the luminous object has risen from the east and that its centre is exactly at the eastern horizon, that a line goes from the centre of the sun, at point C, straight to the pole of the cloud-sphere, at point B, that sight is located on that line at point D, between Β and C, and that lines are going from sight at point D to the surface of the hemisphere of the cloud, which is spherical, has depth,

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and is opposite the luminous object. All these lines are equal, and they are reflected backwards from the surface of the cloud-sphere towards the luminous object. They all come together at point C on the line BC. All this together means that two cones arise, the one inside the other, with the same axis, sc. the line BC; one of them, namely the inner one, has sight as its top, at point D, and the other has the luminous object as its top, at point C; the cloud is the base of both cones. As their axes are lying in (the plane of) the two horizons, it necessarily follows that what is visible of their base is half a circle, because the line going through the tops of the triangles, which lie on the hemisphere of the cloud, must enclose a segment of a circle which has as its centre a point on the line BC; that is the centre of the circle along which the rainbow appears; let us call this point L. Then the discussion on this figure comes down to the same as what was said about the halo. This makes clear what we have said, namely that when the luminous body is at the horizon, the rainbow is half a circle. It also makes clear that if the luminous body rises above the horizon, the rainbow is less than half a circle, for when point C rises above the eastern horizon, the centre of the circle of the rainbow, located at the western horizon, descends; if this centre descends, then what is visible of the circle of the rainbow above the horizon is less than half a circle. The more point C, i.e. the luminous body, rises above the horizon, the more the centre of the circle of the rainbow descends towards the horizon and the

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whole is seen going down towards the horizon, until it disappears; then no rainbow is seen at all. Therefore no rainbow appears at midday in summer, because then the sun is very high above the horizon and the centre of the circle of the rainbow has descended an amount equal to the height of the sun, so that no rainbow is seen at that time. In winter, when the days are short, it does appear at midday, because then the sun is not so high, so that the centre of the circle of the rainbow does not descend to such an extent as to make the whole rainbow invisible. This also explains what we have said, namely that the rainbow cannot be larger than half a circle, because this could occur only if the sun were under the eastern horizon; then the sun is under the earth, and when the sun is under the earth, vision is not reflected to it; therefore the rainbow cannot be larger than half a circle. This is the proof taken from mathematics presented in an account which is satisfactory; the complete proof is the one we have included in the third chapter of this treatise. The proof which should be given by the physicist is as follows: We say that it is evident that at night the rainbow cannot exist caused by the sun. If it cannot exist at night caused by the sun, then it must exist caused by it during the day, when the sun has risen; when the sun has risen its centre must be visible, either at the horizon itself, or above the horizon. If it is at the horizon, the rainbow is seen as equal to half a circle, and if it is above the horizon, it is seen as less than half a circle. If the sun is at the horizon itself, it is at the extreme of a diameter of the rainbow cloud, and it is extremely far from it; if the sun is above

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the horizon, it is not at the extreme of a diameter of the rainbow cloud and not extremely far from it. Thus, the reason that the rainbow is seen as half a circle and as less than half a circle is the distance of the sun to the rainbow cloud which is differing. The varying size of the rainbow is due to the fact that the rainbow cloud receives the colours, i.e. the light of the sun; when the sun is far from the cloud, the cloud receives more light from it than when it is close to the cloud, because of the intensity of its light . . . .36 This is (also) the reason of the differing quantity of light of the moon, namely the differing distance of the sun to it: the further the sun recedes from it, the more light it receives; the closer the sun approaches it, the less light it receives. It has become clear what we have said, namely that the rainbow cloud receives the colours, i.e. the light of the sun and that it receives more light when the sun is far from it than when it is close to it. The sun is at its furthest when its centre is at the horizon; it follows that the rainbow cloud receives most light when the centre of the sun is at the horizon itself. When the sun is not at the horizon itself, the cloud does not receive its light to the largest extent because of the sun's proximity to the rainbow cloud when it is above the horizon. When the centre of the sun is at the horizon itself, the rainbow is half a circle, so that it is lit at its most; it follows that it is less than half a circle when the sun is above the horizon. An indication of the fact that

36

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when the centre of the sun is at the horizon itself, the rainbow is half a circle, is that when the centre of the sun is at the horizon itself, a quarter of the sphere of the sun is facing the rainbow cloud. Thus, the rainbow, which we may imagine as forming the separation between the quarter of the sphere of the sun which is facing the rainbow cloud, and the quarter which is not facing it, but is facing the opposite side, ending at the straight imaginary line passing through the centre of the sun, which is like one of its diameters, must be half a circle. If this quarter of the sun, which is a semicircle, is opposite the rainbow cloud, which is like a mirror, a form arises which is half a circle of the rainbow cloud, since what is opposite it is half a circle. This is the reason that the rainbow is half a circle when the centre of the sun is at the horizon, at the extreme of a diameter of the rainbow cloud. The reason that it is never more than half a circle is that the sun cannot be further from the rainbow cloud than the above-mentioned distance, i.e. (the distance it has when) it is at the horizon itself at the extreme of the diameter. If the sun were further away it would be under the horizon and there would be a rainbow at night caused by the sun, which is impossible. Therefore it is impossible for the rainbow to be larger than half a circle. Furthermore, if it were possible for the sun to rise above the horizon and at the same time to be at the real extreme of the diameter, rising, as it were, along a straight line, it is not possible for the rainbow to be more than half a circle. For (even) if a part of the sun with a circumference of more than half a circle were

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facing the cloud, this would be impossible, i.e. it is impossible for the rainbow to be larger than half a circle, because it cannot keep its form and its distance (to the sun) simultaneously. The cause of the different forms of the rainbow has become clear, and the reason has been explained why it is half a circle and less than half a circle and that it cannot be larger than half a circle, in the way in which a physicist should discuss it. Why it is part of a larger circle when it is less than half a circle, we have explained in the third part of this treatise. Why the rainbow is seen in winter during the short days at any time of the day, whereas in summer it is not seen at midday, we have already indicated when we discussed the form of the rainbow. The explanation is what we have said, i.e. that in summer at midday the sun is near our zenith. If this is the case, and the rainbow occurs at the extreme of the diameter from the sun, then the centre of the rainbow is, when the sun is close to our zenith, in a point opposite our zenith, so that it is close to the spot opposite the sole of our feet; therefore the rainbow does not appear. In winter during the short days the sun is far from our zenith, so that the rainbow is not hidden. This is discussed more extensively in the third chapter of this treatise. The phenomena known as rods occur when the cloud has different colours and vision is reflected from it to the luminous object. Some commentators have said that it occurs by reflection of vision from

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such a cloud to another cloud on which the light of the sun is shining.37 The phenomena known as mock suns occur due to reflection of vision towards the sun from a smooth, thick, evenly coloured cloud. We have discussed both these phenomena and the causes of their properties at length in the third chapter of this treatise, and we do not want to repeat it here, from a dislike to lengthen (the discussion) about what has no sequel and because it can easily be taken from the place we mentioned. We have performed the discussion we intended to give in this chapter of this treatise about these phenomena. Next, we shall present in the second chapter the text of Aristotle's discussion, after having translated it from Syriac into Arabic in a way which reveals an understanding and preserving of the meaning, with the help of God and his will and perfect granted success.

37 This theory is probably the one mentioned by Olympiodorus as originating from Ammonius, although the light of the sun is not playing a part in it; see in Meteor. 263,20-264,26; also mentioned by Pseudo-Olympiodorus, Tafsir 162,12-14.

SUPPLEMENT 2

IBN BÀJJA COMMENTARY ON THE

METEOROLOGY

INTRODUCTION Ibn Bâjja's Commentary on the Meteorology is contained in two manuscripts which shall be referred to as the Oxford MS (Bodleian Library, Pococke 206) and the Berlin MS (Royal Library, Wetzstein 87, Ahlwardt's Catalogue no. 5060). The latter MS was considered lost since the end of the Second World War, but in 1988 it was rediscovered by Endress in the Jagellonian Library in Krakow, where it is deposited at present. These manuscripts are a collection of several works of Ibn Bājja; some of them (among which the Commentary on the Meteorology) occur in both manuscripts, others only in one of them. We have given a description of both manuscripts in our Aristotle's Physics and its reception in the Arabic world} and we refer to it for further details. Here we only remark that the Oxford MS is a copy, transcribed at an unknown date from another copy dated 1152; the Berlin MS was transcribed in 1271. From a comparison of both manucripts we can only conclude that they are not dependent on one another. However, the common lacunas in both manuscripts (see below pp. 403, 423, 435 and 449) are an indication that they are derived from a common origin. A similar indication occurs in the copies of the text of Ibn Bâjja's Commentary on the Physics.2 Sigla In the text: <....> [....] t.... t * **

to be added to be omitted corruption lacuna

In the notes: Ο Oxford MS; O1 and O2: text in the margin of Ο in the same and in a different hand, respectively. Β Berlin MS; B1 and B2: text in the margin of Β in the same and in in a different hand, respectively. + added by omitted by * conjecture 1 2

Lettinck 1994 677-679. ibid. 678.

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The first treatise of the Meteorology, commented upon by Abü Bakr Muhammad Ibn Yahyā, may God have mercy on him.

It has been explained in the preceding books that the concave sphere of the moon does not surround one body, but four, and that these bodies move with opposite rectilinear motions because they have by nature opposite places: above is the place of fire, below is the place of earth, and between them are two bodies moving with opposite motions. It has also been explained that these bodies have natures, not souls; therefore they each have one extreme of each opposite.1 One subject may have opposite motions at different times in two ways: either both motions are not the natural motions for that subject, e.g. moving forward and backward of a stone, and being beaten or not of gold, or they are motions for what has a soul, for what has a soul moves with opposite motions and has the opposites, as far as it is something having a soul. What has a nature has always one extreme of the opposites (by nature), and it has the other extreme by force. In some of those subjects the existence of the other extreme is not even possible, such as in fire; there the opposite extreme does not exist, neither by nature nor by force. In the other elements the other extreme, opposite to the one they have by nature, exists by force.

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The bodies which by nature have simple motions are simple, as has been explained, and those which have composite motions are composite, as has been explained. For suppose the simple body is A, the simple motion is B, the composite body is C and the composite motion is D. I say that A cannot have the motion D. The proof is (as follows:) Β has an opposite; let it be called E; then nothing which has by nature Β has E. A has by nature B, so A does not have E. As D is composite, it is composed of Β and E. Nothing of Ε belongs to A by nature, therefore motion D does not belong to A by nature. A has either Β or D or E; it does not have D by nature, nor E, thus it has Β only, and everything which has Β is A. It may be explained in a similar way that everything which has D is C. Thus, every simple body moves with a simple motion, and everything that moves with a simple motion is simple. Motion follows place, so how can a motion be composite if the place is not composite, and how can a place be composite? We shall investigate this. We say that motion may be composite in various ways: (A) A body moves with two opposite motions at two different times; then it has a twofold motion and it is called composite; this is what follows from the preceding proof. (B) Motion may be called composite in another way, sc. when something has two opposite motions at the same time, the one dominating and the other dominated; this may occur in

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two ways: (1) When a homogeneous body by nature moves down, such as a stone, and moves up by force; then the meaning of 'composite' is here: 'forcing and being forced'; this meaning is rarely used. (2) (Motion) may be composite in another way, sc. when a body is composed of opposite elements, one of them dominating; then it moves with the motion of the dominating element, and the dominating element forces the weaker one—this is different from the previously mentioned motion, because for that motion the mover is outside from what is in motion. This case may occur in two ways: (a) Both parts are mixed; this is more appropriate2 insofar as that it is not a forced motion, and more distinctive;3 (b) Both parts keep their boundaries, such as air in a copper sphere; (if it is immersed in water), then the air forces the copper and moves it up towards the surface of the water. Someone might ask about a copper bowl, when it is floating on water: how can it be floating? We say that there is something in this body which dominates: the solidity of the copper dominates the air and keeps the air inside; the lightness of the air dominates the heaviness of the copper in water. Thus, there are two forces in both (copper and air), a dominating force and a dominated one: the solidity of the copper is dominating and its heaviness is being dominated; the lightness of the air is dominating and the force of the air related to the solidity of the copper is being dominated; one should investigate what it is. Place is one in species in two ways: (1) (As) the extreme terms of the opposition (sc. the oppostion formed by above / below); for the absolute above is one in species and simple, and the same holds for the absolute below. (2) The places between these extremes are composed of the extremes; they are one in species if their relation to one of the extremes is one in species. Place is one in number if it has a relation to one of the extremes which is one in number. This composition exists in

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being; it is not a composition related to the bodies, but another kind of composition; therefore, each of these places is simple. If there were only one kind of composition, then air and water would not be simple, or the composite bodies would be simple. Place is called simple in the first two ways. Every simple body has a simple place, for if its place were not simple, its motion could not be simple. Therefore, if a body is moving to a simple place, and then stops before reaching it, it must be composed of two or more bodies, in which opposite forces exist, one of them dominating, which is the one by which it moves. An example is the copper sphere (immersed in water); what primarily dominates (the copper) is the air in the sphere, which causes the sphere (to move to a place) adjacent to the air; then this (effect) increases, and more of the sphere appears, until most of its surface appears. We have give a survey of how this occurs in the fourth Book of De CaeloThe place of the sphere is composed, for the surface which is its place (makān) is in a place (mawdu). This follows from the study of composed things in the discipline of (the study of) surfaces as surfaces. As these composite bodies are existing, (know that) every composite body consists of four simple bodies; the composition is either by means of juxtaposition or by mixing. How one gets one body by means of juxtaposition will be explained in the Book of Animals.5 How one body is formed by mixing has been surveyed in De Generatione et Corruptione.6 There it is explained that what is simple perishes to what is simple, not to something composite. What is composite eventually

4 Aristotle, De Caelo II 4, esp. 311b10 ff., where it is stated that a composite body in which air dominates over earth and water rises to the surface of water, but does not rise in air. A commentary on De Caelo by Ibn Bājja is not extant. 5 Ibn Bâjja's commentary on Aristotle's De Animalibus is preserved in both the Oxford and the Berlin manuscript. See the paper by R. Kruk: "Ibn Bâjja's Commentary on Aristotle's De Animalibus". 6 Aristotle, De Generatione et Corruptione 327a30 ff.

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perishes to something simple. There it is also explained how mixing, action and passion occur. It is also explained that the principles of mixing are the two contrarieties of affection (being influenced). One of them is active, sc. heat and coldness, and by the other the body is acted upon, i.e. dryness and wetness.7 The simple bodies as such are not elements; they are elements only insofar as they are parts of composite bodies. Composition belongs to them in the mixing, especially the mixing occurring when a thing comes to be by means of these four forces (hot / cold, dry / wet), not the other forces belonging to simple bodies by which there are what they are, sc. weight and lightness. These four are the forms through which the elements are what they are. In the above-mentioned book {De Generatione et Corruptione) mixing is investigated in general, as it is related to the elements absolutely. Each particular mixing is related to each particular kind of composite body. Composite bodies, as one may observe, are either homoiomerous (composed of one kind of parts) or not-homoiomerous (composed of different kinds of parts); each not-homoiomerous body is composed of homoiomerous parts. We start to investigate the homoiomerous bodies, for the elements are homoiomerous, but in another way. The homoiomerous bodies are either by nature parts of some composite body, like bone and blood and such kind of things, or they exist by themselves, like gold and silver and similar things. We first investigate the latter ones and the conditions of the former which are common to the latter. We investigate what is specific to the former in the Book of Animals. As these (homoiomerous bodies) are bodies, they necessarily are in a place. It has been explained in De Generatione et Corruptione8 that the

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the elements seldom occur separately, but that they are (mostly) mingled, in two ways, either by juxtaposition or mixing, and that the kinds of the three elements (sc. the elements other than earth) all occur in the earth. In air and water they seldom occur, as we observe, because air and water are such that the other elements, sc. fire and earth, move in them. The earth is not such that they move in it, for that in which they move should be easily affectable and divisible. Therefore motion in air is easier than motion in water, because air may be quickly divided and is easily affectable, whereas water is difficult to divide and to affect in comparison to air. Heat continuously arrives at the earth from the celestial9 bodies and reaches, when it arrives, the air and water in the earth. Then bodies of different kinds of mixture arise, which all move upwards. What does not happen to find a way out (of the earth) stops where it is arrested by something, i.e. a compact (impenetrable) place. If it meets heat there, it is also moved: either it ripens, rots, matures or is burned, or it becomes solid. There are no names for the different kinds of solidification, such as there are for the kinds (of processes) occurring by heat. We shall investigate this at the end. If these bodies (exhalations) do not meet (heat), then they remain in the same condition, until what prevents them (from getting out of the earth) vanishes, or until they meet cavities. In the latter case earthquakes arise, and tremblings and noises which may be heard within the earth. If it escapes (from the

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earth), meets water and is prevented by the water to get out, and if the water does not dissolve it by its coldness, then the heat in the hot part remains inside, i.e. inside that smoke, and then water moss and such kind of water plants arise. If the coldness changes it into a kind of water, then salt water arises from it. We shall discuss the former phenomenon in the Book of Plants,10 and the latter when we undertake a further study of the statement. If it comes out into the air it rises. If the hot part escapes, it arrives in the place above; if it does not escape, it remains in some place in the air, as something prevents it (from escaping). It appears that there are different kinds of air surrounding the earth and the water. Closest to the earth and surrounded by the mountains is the thick air which does not move as a whole. What surpasses the tops of the high mountains escapes from the hold of the earth and moves in a circle; it is thinner, because the motion makes it thin and the thick exhalations do not reach it. As soon as they arrive they are made thin by the motion which is conferred to them by the motion of the sun. It has become clear that in the water there is one place and in the air there are two places; one is (the place of) the surrounding air which is close (to the earth), the other is far away. As these are places, there must be bodies which fill them and bodies must arise and exist in them. It has also become clear that the cause of all this is the heat caused by the sun that arrives (at the earth); the cause that things arise one after another is the inclined orb (ecliptic). As for the eccentricity (of the sun's orb), its effect cannot be separated or differentiated from that of the ecliptic in the inhabited regions; however, if it were possible to observe another part of the earth in which the sign of Sagittarius transits overhead, then for someone who observed (took) this as the inhabited region it would be possible, since he would be able to obtain

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(the measure) of the two forces and distinguish between them.11 Aristotle only discussed this one single cause (i.e. the inclination of the ecliptic) and failed to mention the eccentricity, just as he failed to mention the effects of the eccentricities of the orbs of the planets. Each orb rotates around its centre, so that what turns around it adopts a certain amount of its motion. It has been explained in mathematics that the eccentric orbs of the planets, not the deferents, all move, except (in the case of) the moon.12 We have found for all things an account which includes everything by which it exists, but for some things this is difficult and what we investigate now belongs to these (difficult things). For the principles we have are not sufficient, and maybe someone cannot find a satisfactory account for everything he says, especially for the higher things, because of the long time one needs, which may surpass one's power. Although we may not be able to do that (i.e. give such an account), there is no reason to give no account at all; therefore, let us give an account insofar as we are enabled to do it by the principles we have found. For we have now more principles from mathematical astronomy than in any preceding time and we shall mention those with which we have become acquainted. Aristotle failed to mention this at all because, according to him, there is no difference between the effect of the eccentricity and the inclination of the ecliptic and he did not find an existing effect which would be distinct from (an effect of) the sun's motion in the inclined orb. We have not found this either, but if we do not say more (add to it), by what can we say that we complete Aristotle's account?

11

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He says that all spheres, except the one that moves with the daily motion, are not-homoiomerous. Of the parts grasped by sense there is a part grasped by vision, and another part by an account. What is grasped by the senses (vision) are the stars; these are the parts which are luminous. The non-luminous parts are grasped by an account only; he discussed this in De Caelo}3 The stars are observable insofar as they have colours; they move the elements insofar as they are luminous or by some property related to it. We say that it is the sun which moves the elements in a way which is grasped by the senses, and the moon comes after it. Let us suppose for the moon an eccentric orb instead of an epicycle, according to what Hipparchus has laid down; he did for this case (of the moon) what Ptolemy did for the sun, for, according to him, this is possible.14 We say that the case may be settled in different ways. (1) The daily motion is considered as a separate motion and the eccentric motion causes approaching and receding from the zenith; by this variation in distance a difference in reflection occurs; this (is the case), if one does not consider the nearest and farthest distances, i.e. the perigee and the apogee to have a force, that is when the straight lines do not have a force in the orbs; then we find the centres and the poles having forces. (2) We may also say that by means of the daily motion the force of the stars arrives at all parts of the elements in the time of twenty-four hours (a day and a night); within that period there is the same reflection in the air in all areas of the earth and the water. Aristotle did not make the reflection which occurs in the air responsible for the

13 Aristotle only says that the stars are made of the same substance as the spheres in which they move. Their heat and light arise in the air by the friction that results from their motion, see De Caelo 11,7. 14 For the explanation of the sun's motion Hipparchus and Ptolemy assumed an eccentric orb. For the motion of the moon Hipparchus assumed an epicycle and he showed that this epicycle model is equivalent to a model with an eccentric (see Ptolemaeus, Almagest IV,5). Ptolemy's theory of the moon starts from Hipparchus' epicycle model (Almagest IV,5), but is more complicated (.Almagest V,2).

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formation of clouds and vapours, and we did not find it has a clearly (observable) moving force (either); if it has such a force, it is very weak and its influence is imperceptible. The force of the reflection is evident in water. In the sea it may be felt by the sense of touch, and it is observed in the large amount of vapour and in other effects of the heat. In countries with large rivers one sees a variety of trees and plants; they all occur in countries of the same latitude because of the reflection, although the reflection may be imperceptible because of the low position. As for the earth and its parts, the sense is sufficient (to grasp the effects of reflection), especially in countries with stones and stony mountains. By means of the motions which each star specifically has the elements move in the way they move. (3) We may still give a third account, which is the most suitable for us to express and to suppose, namely that the question is settled by (recognizing) both motions, each having a force. Similarly, the most suitable for us is to suppose that each star has this (effect) and that they are differing in distance (being close and far away) and in size (being small and large). Let us establish the matter in this way. In mathematics it has been explained that the centres are all close to the centre of the world, for the eccentricity is hardly getting outside the elements . . . .15 However, that has no order which is known and it does not play a role in most things, except in what occurs due to the sun. Its parts and the conditions of its parts differ, so that each kind in it is not similar to each kind in its parts. Let us make from causes of that a cause for this. By doing this the potentiality is not removed

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from the elements, but we say that if we were not to know this and its situation were not clear to us, then we would not find the order of motion and change; for what is connected to the forms of the elements and the animals is permanent; in general, what occurs from the action of the intellect, which is another form, is not a permanent action, but we find the arrangement in most things and most places. What we have said about this may suffice. Let us now state that the heat of the sun reaches the earth, this heat arising from the sun's motion and from the reflection; (the heat from) the motion of the sun reaches the whole (earth), and its interior is reached by the heat from the reflection to a certain extent. This is in agreement to what is observed: springs in hot countries are not extremely cold at all, except those which come from a big mountain; those are colder than the springs in cold and moderate countries, to such an extent that one cannot keep one's hand in it for a long time. Three verses of Arabic poetry talk about this, saying that the people (of that area) supported themselves with bets on it with foreigners who did not know it. This (heating) occurs by a kind of reversion (rujū'), for the reflection heats the air, then the air 'reverts' and heats the earth: thus, so both of them (earth and air) become a cause for the other, but from different points of view and in different conditions: the earth heats the air by reflection, and the air heats the earth by the warmth in it; thus, two motions arise by two processes. That reflection may have such an effect is clear from the burning mirrors. In mathematics it is explained how this occurs by the rays and this is observed in the case of parabolic mirrors. They are the most powerful burning mirrors and more powerful ones cannot exist. The

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cause of this is given in mathematics.16 Aristotle postpones the treatment of the effects of heat and coldness and the other contraries to the fourth Book of his Meteorology, and because we follow his arrange- ment, we (also) postpone our discussion on it, for that is the suitable place to discuss it. It has been explained that air and water are places by nature, because the generation and being of natural bodies is completed there. As for water, it is clear that there are three places in it: (1) (The place of) what is contiguous to the earth, i.e. the plants. (2) (The place of) what is contiguous to the air, such as the vegetations occurring on permanent water surfaces which do not move. (3) The place of the animals, i.e. the place where they move. Why we observe plants existing in the middle of the water at all, and also what is generated in the water, shall be discussed in the Book of Plants}1 The air has four places. The highest is for the comets and such kind of things. The second is for the clouds and the rains. The third is for dew and hoarfrost (jalid). The fourth is common to animals and plants; this place (has an intermediate position) between the third and the fourth. We shall discuss this in the Book of Animals. The earth is one place; it is common for the elements and the minerals. If a part of the earth has this power it is called ma'din (mine). The mineral bodies are generated in it, but no animals are generated in it nor any animate things. Briefly stated, animate bodies only exist at

16 Burning mirrors were a well-known subject of discussion in the Arab world. The Greek works on this subject, such as those of Diocles and Anthemius were available in Arabic. The work of Anthemius was used in the writings on burning mirrors by al-Kindi, 'Utārid ibn Muhammad al-Häsib, Abü Sa'd al-'Alâ' ibn Sahl and Ahmad ibn 'Īsā. Ibn al-Haytam wrote two treatises on burning mirrors, the one on spherical, the other on parabolical mirrors. See for further information and references Sabra 1989, vol. II XLII-XLV and Rashed 1990 and 1993. 17 Asin Palacios, Avempace Botánico 273, 275.

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the surface of the earth or in the water. Therefore Aristotle's investigation in this book is only concerned with these places (sc. earth and air), for in them exist these homoiomerous bodies which by nature are not parts of other bodies. He first gives the causes of what occurs in the first place of the air, then of what occurs in the second place, then the third, and then the fourth. After that he gives what occurs in the earthy place. What causes the phenomena (in the earth) are the same kinds of thing as what causes the phenomena in the air; he enumerates them together with what occurs in the air in order not to be long in his account and not to repeat many things. He gives an over-all account of that which specifically causes them in the treatise in which he mentions the earthy matters in general, i.e. the fourth Book; (this account) comprises the homoiomerous bodies. He leaves the account of what is specific for the minerals to the Book of Minerals. Thus, the special subject of this book is an account of the exhalations, their specific properties and their common properties as exhalation. In this respect they are like the simple elements. Among the the composite things they are the primary composites and the simplest composites. Therefore he describes in this book their return to the elements; one could say that in this book he discusses the simple composites and their return to the simple (elements). In this book he discusses the primary composition and its general properties. He leaves the account of the secondary composition to the books on minerals, plants and animals. The ancient people used to call this discipline the Science of Upper Phenomena ('ilm al-ātār al-'ulwiyya), because they intended to investigate the things which were observed in the upper place. They conceived the principles and acted accordingly and carried

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out this investigation, because in their time it was not yet achieved. Thus, this book contains the information on the general and specific properties of the exhalations and what is related to them. We say that the earth and the water are like one sphere and that they exist as such in relation to the whole world, as is explained in mathematics. They are mixed, and a part of the air is mixed with them. If heat reaches the earth, an exhalation arises from both (earth and water). The exhalation from the water moves upward separately. What moves upward from the earth is mixed and composite; a small part of it moves separately. The form of fire is preponderant in that in which the heat dominates most and that moves to the first (highest) place; if it is not dense (thick) at all, it becomes fire and arrives at a part of the simple place, which part becomes hotter. What has a certain degree of density remains as it is and is slowly affected upon until it is complete, as long as there does not come a strong motion to it, of course, or as long as it is apt to rest because it has reached the surroundings of one of centers of the eccentrics, until it gets burned and is not observed anymore. If it happens that things arrive there which are dissolved by an excessive motion, they are burned, and appear to our vision. This is not something which occurs necessarily, but it is possible; what is possible and what is existing are the same for permanent things, because what is possible must have existed and will exist.

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We must discuss the way how the causes are pursued, since this is a general consideration for what is in this book and what comes after it. We say that what exists is either necessary, or not necessary, or necessary as a whole, but not necessary in its parts. To the first category belongs the celestial body, as has been explained. What is not necessary in both ways is what exists under the sphere of the moon. This may be divided into the homoiomerous things and the composite things. In both of these sorts of things there is the species which may be (necessarily) existing as a whole, but generally there is (also) something which is possible. The elements belong to the things the species of which cannot be non-existing generally, but in the parts of which there exists possibility. To the composite things belong the animals which never cease to exist, such as man and in general the animals which sexually reproduce themselves18 their species can never get lost at any time, otherwise they would not reproduce themselves sexually. As for what is said that at a certain spot a man or something else would be generated spontaneously, as one says about Egypt, that is another story; there is nothing impossible in it. If that is the case, then it is possible that there something is generated spontaneously which is essentially geared to reproduce itself sexually and that what is geared to reproduce itself sexually may cease to exist, such as the silkworms19 and the locusts; these should be counted among what is not-necessary. What exists besides these is neither necessary as a whole, nor in its parts. The investigation into its differentiae is carried out in Metaphysics.20 18 Cf. the phrase after the next one, where tanàsul (sexual reproduction) is set against tawallud (spontaneous generation). See R. Kruk, art. tawallud in: The Encyclopaedia of Islam, 2nd edition, Leiden 1960 ff. 19 Ibn Sīnā says that silkworms usually reproduce themselves sexually, but sometimes come into being spontaneously; see the article mentioned in the previous note. 20 It is not clear to which passage Ibn Bājja refers. Differentiae of the not-necessary (the possible) are not found in Aristotle's Metaphysics. The possible and impossible as logical concepts are treated in Metaphysics Δ 5 1019b22-33; also in De Interpretatione 12, 13 and Analytica Priora I 13. As for the necessary, Aristotle mentions three kinds in Metaphysics Δ 5 1015a20-1015b15; two of them derive from the third one, which is defined as "that which cannot be otherwise". In Metaphysics Λ 6 1072b4-12 Aristotle explains that the first Unmoved Mover is necessary. It moves the other spheres, which are not-necessary, because they can be otherwise, sc. in place—not in substance, since they are not subject to generation and destruction. Ibn Sînâ's metaphysics also distinguishes between the necessary and the possible. The necessary is the Necessary Being, the source of all existence. The possible beings are divided into those that are possible by themselves, but are made necessary (in the sense that they cannot not exist) by the Necessary Being and those that are just possible. The former ones are the bodies, souls and intelligences of the celestial spheres; the latter ones are the bodies in the sublunar region which are subject to generation and destruction (see e.g. K. an-Najāt 224 ff., Le livre de science I 218-220.). Ibn Sînâ's division of the possible (contingent) beings might be what Ibn Bājja had in mind when he referred to the differentiae of the not-necessary. We must, however, also mention a1-Fārābī, as Ibn

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What is not-necessary in the first and in the second way has no existence as a genus, and they have no species, and therefore it cannot be reached by division (taqsirri). As has been said in Analytica (Posteriora), a definition is either formed by means of division, or by means of delimitation (tahdīd), or by means of demonstration (burhārì).21 These ways are not the ways used by Socrates, for these ways explain the existence of a predicate for a subject by means of something else. The relations of the parts of the syllogism belong to the relations of delimitation; then one considers the combined whole; if it is equal to something existing or to a subject, then the definition is composed of them. The (definition by) way of division is not possible in this case, for it can only be applied when something has a genus. 'Being' is said of these things in a homonymous way, because form and matter of the fifth body22 are different from those of the elements. Therefore division does not lead to the final definition, for one nature cannot form a genus. 'Body' is also said of the elements and of the animals in a primary and a secondary way. The (definition by) way of composition (tarkib) is not enumerated with this. As for the species, one gets informed about them and one gets to know how many there are, so that one can say that such and such a thing is disposed to exist, and such and such a thing cannot exist. Something similar holds for the (definition by) means of demonstration and in general the way which leads to the final species. What comprises them is the way of division by differentiae or by specific properties. This is not possible in what does not have a genus which comprises them. Someone might ask, however, if it is possible for something to exist when it is not disposed to exist. Our expression 'what is disposed to exist' and 'existing' are the same and there is no difference between them, since we do not discuss what exists insofar as it is in time; for what is disposed to exist will exist at a certain time, but some of it will

Bājja knew his work better than that of Ibn Sīnā (see Lettinck 2). He posits a necessary First Being, from which nine intelligences emanate, each of them producing one of the celestial spheres (see a1-Fārābī, The Perfect State chs. 1-9). 21 Aristotle, Anal. Post. 96b15 ff., 96a24 ff. and 94a1 ff. 22 I.e. the celestial spheres. For the use of this term see Daiber 1975 66-69.

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Lit.: a cone with equal surfaces.

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24

This means solving the not for any pair of lines, but a length of one unit, then it q, or smaller, just as is said in 25 See Aristotle Anal. Post.

set of equations χ . y = p; x + y = q. A solution exists, only if q > 2 Vp. If one assumes the smaller line ρ has follows that a solution exists if ρ is equal to one half of what follows. 94a2.

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and the cause together, but mostly that is not possible. What most often occurs in this discipline is what we have said, sc. the proof of cause. It is possible that we know something by sense and then also know its genus, such as in the case of the clouds: we know that it is a body. It may also occur that we do not know the genus of a thing, such as the rainbow, the comets and the rods. It may happen that for some of these things we can explain the existence of the genus by means of something connected with the subject; then we have a complete demonstration. It may also happen that we can explain it by means of something else, and in such cases this discipline gives this demonstration absolutely. An example is . . . .26 As for the explanation of the existence of a certain species of natural bodies which are unobserved at all, an absolute demonstration in this discipline is not possible; for if it were possible, it could only occur if the middle term were a goal or a form; only from these two the other things necessarily follow and the existence of the species would necessarily follow. The forms of the natural bodies do not exist absolutely, nor do they exist by account; therefore it is not possible, as Aristotle says, to define the flesh or the bone without motion. Thus, if the form is known, the matter is known and these two (determine) the final species and the goal. So we only know what they are when we have got knowledge about the world as a whole; then we know the order of that species in the world, and then one gets acquainted with the goal of that species; this only occurs after completion of natural science. Knowledge of the goals of the final species is a very difficult matter, and it is sometimes thought that it is impossible. Knowledge of the existence of a final species by means of proof is possible, for instance . . . .27

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These are the ways followed in natural science and we have to inspect each kind of the given disciplines and, on the whole, each kind of the observed phenomena, and in each kind we (have) to keep in mind the way which has to be followed in pursuing knowledge. This is our (way of) studying this science, in accordance with the order used by Aristotle in the eleventh Book of the Book of Animals?* There he mentions the ways to obtain knowledge of the animals; he classifies them and distinguishes what is necessary for each of them and for what subjects it is needed. The things which are observed in the first place, which is contiguous with the motion of the fifth body (i.e. the upper part of the atmosphere), are enumerated by Aristotle in the first Book of this book. We have not been informed and we have not observed that there occur other kinds of things than he has described. According to Aristotle's enumeration, there are seven kinds of such things: the Milky Way, the comets, the 'goats', the rods, the torches, the mock suns and the halo. Let us establish their causes. About all these and similar things one might ask whether they are bodies, or imagined phenomena in bodies. The first of them is the Milky Way. We should investigate whether it is a body or some reflection from a body. Various accounts have been reported of people who said something about it. Those who

28

Aristotle, Parts of Animals I 639a12-642b5.

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adhered to divine wisdom at that time said that it is a trace of milk from the suckling of some stars which flows from the nipple of a breast, or that it is a trace of the burning which has occurred, and they say that its name is derived. The first account is that of Homer, the second one is the account of the school of Pythagoras. This follows the opinions people had in that time concerning the stars. Those who studied it from a physical point of view had different opinions. Some said it is the trace of the course of the sun before it turned away from it. This was concurrent with the opinion of the general public in that time; (In fact,) that body cannot not be influenced at all. These were the opinions of the natural philosophers. Democritus thought that the light of the stars comes from the sun and that the shadow cone of the earth reaches as far as the sphere of the fixed stars, to an extent that is limited by its width; therefore it only takes a place of the sphere to the extent of its width; he thinks that in that place there are stars which are not reached by the light of the sun, but to which still a certain amount of light arrives, and those stars form the Milky Way. He does not specify which of the two shadow cones (he means), the cone which draws together (to its top) ('alā l-inkirāt)—that one does not reach (the fixed stars) at all—or the cone which spreads out (from its top) ('alā l-ittisā'). It is possible to discuss this, but Democritus has not done it because of his little experience in mathematics. The dark shadow cone does not reach the sphere of the sun, so a fortiori it does not reach the sphere of the fixed stars. As for the cone which spreads out, the power of the earth does not reach so far and does not cause motion (have an influence) at such a distance. If we suppose that such were the case, then it would necessarily be many times larger than the Milky Way. Furthermore, the

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Milky Way would be seen differently; it would be seen in different places (of the sky) and not (always) in the same condition, because of the difference in relation between the sun and the earth, since the sun moves along the ecliptic and along the altitude circles. Also, those who say that it is reflected light of the light of the sun—whether it is directly reflected from that part of the celestial sphere, or is reflected from the water to that part and then from there to our vision—all this entails a difference, a difference in its parts in themselves and a difference in the way they are observed. There is an account ascribed to Aristotle, according to what we have found that the commentators have said in the commentaries on this account, namely that in the celestial sphere there are many fixed stars which are close together. They draw smoke to themselves, as he found for the sun and the moon; for Mars, especially if it is in its perigee, this is very much in evidence. Then (the smoke) is ignited and bursts forth burning, and its trace becomes visible. Thus, the Milky Way comes to be in a way similar to the comets. If one accepts this theory, it is thought that it is seen differently in its parts, according to rising, setting, and intermediate positions, and according to different locations which are far apart in the north and the south. It necessarily follows that, for instance, the people of the arctic regions (ahl as-suq') see the luminous stars which are are under the tail of Aquila (at-tā'ir), which is in the middle of it (sc. of the Milky Way) on its northern edge in the northern countries, or near to it; and they see southern stars on its northern edge, in accordance what someone sees who is in the extreme south: he sees them in the extreme north in its interior.29 The difference in vision (parallax) of some of its southern parts is possible, because necessarily the furthest distance from the place of perception is less than the closest distance to the sphere of the moon. Then it is seen

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south of the ecliptic in the other direction. And the latitude of the moon is five (degrees).30 From all this it follows that the parallax must be more than two degrees;31 in general, if we posit it to be a certain body or an affection in a certain body that is below the sphere of the moon, it will always have a parallax. Furthermore, it is unreasonable to suppose that this body would be subject to generation and perishing when it is neither increasing nor decreasing in a certain interval of time, and when in certain countries like Ethiopia and Yemen it is not larger than in the Roman and neighbouring countries; for that is what would follow from this view. As for what we found in the copy which we have got, we did not gather from it this very same opinion, but another one, which is a supposition which does not necessarily entail a difference of vision (parallax), nor a difference of being. It shares with the above-mentioned opinion (the idea) that in the sphere (of the fixed stars) there are places in which there are many stars—as we found in our copy in the following formulation: "There are in the sphere in the place where the Milky Way is seen many small stars which are closely packed together, and big stars which are far apart. When their light shines from behind this inflamed place one sees there a elongated (patch of) light. These are fixed stars which almost touch one another."32 (He says this) after having said at the beginning of the paragraph that the essence of the Milky Way is as follows: "The pure fire which is close to the sphere is inflamed and luminous."33 We say that air surrounds the earth and water and fire must surround the air; thus, every star appears while it is seen in (through)

30 The (maximum) latitude of the moon is the angle between the moon's orb and the ecliptic. It is not clear why he brings in this quantity here. In the margin of the Oxford MS. the following remark is added here: "These degrees are all established in the books of Ptolemy." See Ptolemaeus, Almagest V,8 for the value of five degrees for the moon. 31 I.e., if the moon's parallax is two degrees, stars seen in the Milky Way must have a parallax greater than two, as the Milky Way is supposed to be something below the moon. 32 This is a quotation from Ibn al-Bitriq, Meteorology 25,11-26,2. 33 Again a quotation: Ibn al-Bitriq, Meteorology 25,10-11 and 25n11.

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the surfaces of these—in optics the deflection (iríikās) is explained which may occur to light rays—in accordance with the thickness of these. The refraction (iríitāf) is always from the thinner (medium) to the denser one in the direction of the vertical. The visible things get, in accordance with the different surfaces of the bodies through which they appear, different conditions, such as different size, distance, frequency of appearance and colour, such as we observe in the sun. For we often see the sun at its rising and setting as red and large, and sometimes, in extremely hot weather, as yellow. All these are accidents occurring to its apparition, which are acquired in the surface of this body. This is clearly shown when we are submersed (in water) opposite the sun and we look at it. Then we see it in a different condition from how we see it from air. The light of all stars is subject to refraction (irìitāf), which is a kind of deflection (iríikās}, that is a property belonging to it. The fire which is an element is pure; after that comes the smoke which is ignited, as Aristotle says, and which is as a boundary between fire and air, and which surrounds the air. Because these stars are so many and closely together, they form a continuous phenomenon in the surface of that body and we see it as such. In places where the stars are scattered or where they are less numerous, what is seen is more widespread or dispersed; because of the small quantity they do not shine brightly, as if they were extinguished and obscured stars, and they are even smaller. Therefore one sees (them as) something continuous and one sees the separate stars which are far apart as circular. The (present) investigation is about the perception of something in the sky which has a long-stretched form, whereas stars are not extended in such a way. That leads to the idea they are not stars and the question arises what they are.

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We have observed Jupiter being eclipsed by Mars, and then Mars appearing from under it in the ensuing conjunction in the year 500 A.H. When it touched it, or almost, one saw them having an elongated form, curved at both extremes which were seen in the circles which pass through the poles of the zodiac.34 The lines which extend along the surface of the ecliptic in the sequence of the zodiacal signs looked as if they were straight.35 Because the moon at that time was high, the case of this observed form was often not clear.36 It is evident that this can also happen in other surfaces which are under it. Therefore, if we were to posit that the body which is the source of the Milky Way is bounded by surfaces and elongated, such as a ring for instance, it would follow that it has a parallax, which is impossible for the Milky Way. If we were to posit it as something encompassing (the sky), with uniform parts, then, because it is incited by the fast motion (i.e. daily motion), it would be ignited and transparent, so that it would not reflect vision, such as vision is not reflected from the other stars either. We have said what the Milky Way is. Thus, anyone who thinks that it is something under the moon has said a certain truth, but included in it a certain falsehood, namely that he restricted it to (the region) below the moon. Anyone who said it is above the moon has said a truth, but included in it a falsehood, for he did not use the elements. The truth unites both statements. Therefore Alexander . . . ,37 The Milky Way is among the things that remain in the same condition. Therefore it is impossible that it belongs to what is subject to generation and perishing. Furthermore, as it is in its whole different in shape from what is in the heaven, an impossibility also follows, since in the heaven there is nothing that is not perfectly circular, either a ring or other shapes. Furthermore, in the heaven there are only things that exist permanently, like the sun, the moon and the other stars, or that exist in

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a regular way, like rétrogradation (rujū') and direct motion (istiqāma) and the motion of the planets. Therefore everything that shows an irregularity (in the heaven) requires the existence of uniform, regular motions, from which these irregularities will ensue. Then, when one investigates this, (one finds) that the irregularity exists because of a regularity which has regular causes, such as the second anomaly of the moon, for it brings forth how this irregularity is made regular; the same holds for the irregularities of the planets at their various distances to the sun. Let us now discuss the observable phenomena which come next, namely the comets, the 'goats', the rods, the mock suns and the torches. First we say that no other phenomena than these have been observed, and that none of them are observed to exist permanently or in a regular way or as having irregularities in a regular way; we shall explain this when we give a more detailed account. We say, as we already have said, that from the earth two bodies of exhalation move upwards because of the reason mentioned before; one of them is hot and dry, the other is hot and moist, and they become mixed. Each of the simple components is either thick or thin. The dry, thin exhalation is the fiery exhalation (wahaj) and the thick one is the smoke (dukān). The moist exhalations are both called vapour (bukār) and there is no name for its différents sorts. They all rise to the sky, as has been said. A part does not rise above the air which has as its farthest limits the tops of the mountains and from this six bodies and accidents arise, which we shall discuss when we give a more detailed account. A part arrives in that part of the air which surpasses the highest tops of the mountains, and what gets into that moving air must gradually become thinner. A part becomes air, which is wholly moist, and a part becomes fire, which is dry. The thick part of the dry exhalation remains stable in accordance with its thickness, and the thin part quickly dissolves. Therefore there may exist accidents for and from

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these things by which one perceives these thick exhalations and their shapes, motions, number, extents and position in relation to the parts of the world. One may ask about all these how they are and how they occur, and that is the cause according to the matter. The moving cause is the heat which arises because of the sun and the fire. One has once witnessed a large town which was completely burned; then, its people returned to it at the beginning of summer. When autumn came and the air got colder many thunderstorms descended on it incessantly, without respite, although they were weak. One had never seen this before, nor after it. This town was surrounded by the sea and many other waters, flowing as well as stagnant, and meadows; therefore much moist vapour moved upwards from it. When the vapour got cold and was squeezed out the thunderstorms poured down. Their existing on account of the two matters is necessary, but the generation of what arises from these matters is not necesssary. This (generation) occurs on account of the moving cause: when it happens to encounter them (i.e. the matters), then it (the phenomenon) is generated, if not, then it does not come into existence. As has been said in the Analytica Posteriory there are things which have the same efficient cause, but differ in matter. Water dissolves salt and it coagulates gypsum; fire melts gold and coagulates honey.38 Other things have common matter, but different moving causes, such as wood; a carpenter makes a chair or a bed out of it, and someone who is cooking turns it into ashes. Things may also be different in all their proximate causes: a garment has the weaver as proximate efficient cause and silk as proximate matter; a chair has wood as proximate matter and the carpenter as proximate efficient cause. The relations

38 This specific case, and the one mentioned in the next phrase are not mentioned in the Anal. Post. However, for the general case of different causes pertaining to the same thing, see Aristotle, Anal. Post. B16 98b25 ff.

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between the different causes of things have been enumerated in the Analytical9 and this has to be investigated here in this discipline. Then, firstly, we have to look for the form, what it is, for when we have found it we are able to find the other causes. First, we look for the genus of the thing, such as the Milky Way; we first investigate under which genus it falls and by which one it is described. We should proceed in this order when it is our aim to know its causes. Then we should look for the species we are searching to know, for when its genus is clear we start to look for what specifies it, i.e. its differentiae or such kind of things, even if we did not look for its genus first. By studying its genus we know what it is and that is the first thing we look for, and is the first desire of the logician. After that he wants (to know) the other things he desires which are enumerated in the Analytical these are the things that are used in the problems. We first ask what it is what we see, and we may ask what we mean, (for instance), by the word 'point'; when this has been explained, we ask about it specifically whether it exists; if it is true that it exists, then we can look for the answer what it is what we see. After that (we investigate) about what belongs to it; when we have found this, we have found that by which that thing exists while it is in it. After that we look for that which caused its existence and by which cause the form was connected with the matter. After that (we investigate) why it exists and what the goal of its existence is. These are the kinds of concepts (tasawwurāt}, the statement which is composed of all of them is the complete definition according to existence. Concerning what is known to exist one may first ask whether it has some quantitative or qualitative condition, or some other condition from the various categories which belong to its essence. These various categories and the

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relations of these desirable things to one another and to the rational power are investigated in other places. The things subject to investigation in this kind of natural science are the following; they all have the same 'where', for their place is the moving air; they are: the comets, the mock suns, the stars which appear together with the sun, the 'goats', the rods and the torches. We first say what is common to them. We say that they are not necessary and do not fall under any kind of necessity. That they do not have a permanent existence does not need to be explained. It is evident that they do not appear permanently. I say that they do not have a regular order either, for it was found by astronomical observation that they do not appear at regular times, and the times of their absence are not regular either. Many of them are destroyed without entering under the rays (of the sun), such as the star which appeared in the Byzantine countries when the sea destroyed many towns. It appeared when the sun was in the region of the winter-solstice and remained a few days; it dissolved in a star of Gemini without the sun having reached it and then disappeared. This, and similar things come to be and perish, and do not exist by necessity. Everything which is luminous and is not counted among the fixed stars and the five planets must be subject to generation and perishing. What occurs in this place (the upper atmosphere) specifically are luminous bodies which move with the motion of the universe, so that one might think that they are part of the fifth (element): they are seen in the same surface as the stars, their light is similar, and most of them are circular and circular surrounded by things, with the rods as the only exception. These attributes belong to them insofar as it belongs to them that they are observable. If they are called stars, then it is by homonymity, for their being has nothing in common with them. They have, however, one accident in common and they have different

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relations to it. The 'stars' we see are influences of the stars in the inflammable body: they are ignitions of a certain actual body. It has been explained before that they are moving with irregular motions. It is seen that they are moving for a certain period and then disappear vision; one may observe that they are extinguished and that they take different colours when they are extinguished. A star has appeared which moved and left behind it a rod; then some parts of the rod got a certain breadth, and the star spread out and got a different shape; then it changed from shining bright and yellow to the colour of fire, which got extinguished in some wetness while it got different colours, partly green with some yellow in it, partly looking as if it were smoke which arises at an ignition. The generation and perishing of these things do not require demonstration. The situation is also clear when it moves for a certain time with the motion of the universe, and then is dissolved before the sun reaches it and disappears. This was the case of the star which appeared in some Byzantine towns. As for that which disappears when the sun reaches it, some of those who have considered these matters have thought that it is a planet which is retrograding to the sun, and if it does not emerge from the other side (of the sun), then it has, when it goes away from the sun, approximately the same arc. Then the period from its disappearance to its reappearance must be equal to the time the sun takes to traverse twice that arc with its irregular motion; it is easy to calculate and determine the times of its disappearance and reappearance. Therefore they considered it to be a planet, but this did not force them to do so. They said that it is like Mercury which does not go far from the sun at all and hardly emerges from its rays, for Mercury appears in the morning and evening in Aquarius and Gemini, while it is removed

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(from the sun) about one part, i.e. one zodiacal sign. Those who say that it is one of the stars consist of two groups: the Pythagoreans consider it a sixth planet and Hippocrates and his school consider it one of the planets. The cause of the mistake of the Pythagoreans is their little experience in in mathematics. If this star were to be what they say it is, it would always appear in the evening and be within reach of the sun. If it were a planet, it would display an irregularity (anomaly) in motion, and if it is irregular its mean motion would be the same as the motion of the sun. If that were the case, it would reach quadrature (rabba'a) with the sun and be in opposition to it. This is clear for anyone who has studied mathematics. If that were the case, it must have a epicyclical orb or such a kind of thing and its greatest distance in the evening is that it is preceding the sun in the sequence of the signs of the zodiac. It should rise first in the evening, then arrive at its greatest visible distance and set in the evening. This is not what is seen, but it is seen all of a sudden. By 'all of a sudden' I do not refer to existence: it always exists and has its largest distance from the sun while it is stable, until it disappears under its rays. While it is moving, it dissolves, as is clear from the star at the appearance of which Byzantine towns were drowned. Furthermore, let us posit that it exists under the rays. Then, if it were to get at the other side of the middle, it would get a morning distance, a morning rise and a morning setting and that is not observed. Furthermore, it is generally seen far from the sun and it has appeared close to Capricorn, while the sun was at the end of Pisces or in that region, and it appeared in the North while the sun was far from it. We have assumed that it has a motion by which it does not appear in the morning; then its appearances in the evening must be regular, which is not the case. If this is investigated by

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means of mathematics it appears that this cannot occur. If someone were to say that it is one of the planets, whichever it may be, or that it is one of the planets specifically, then the explanation of the impossibility of his claim follows from what is observed, because one observes that its distance from the ecliptic is more than the distance of any of the planets and that it occurs in a region different from where the planets move. The planets are removed from the middle of the ecliptic not more than six degrees, and three when they are in the nodes and in the lowest part of the epicycle. For Venus and Mercury this is four degrees. This was the case also in the preceding time in which astronomical observations were made which have been mentioned to us. Venus is removed (from the ecliptic) this distance and the largest latitude of the heavy40 stars from the middle of the zodiac is less than this latitude. Mars has a latitude of . . . .,41 Jupiter has a latitude of . . . ,42 and Saturn has a latitude of . . . .43 It is not one of the fixed stars either, as all fixed stars are observed in a solar revolution (i.e. a year) and they are all enumerated, and not one of them exists always in that condition and they do not have the local motion which specifies each star, neither regular, nor irregular. These stars have appeared opposite the parts of the sphere we observe and we observe all of its stars. Thus, the comets are neiter planets, nor fixed stars, so they are no stars at all and they are not in the fifth body. Only what is observed is in the fifth body. If it were possible that there would be a star in the fifth body which is not observed, that would only be possible if its motion would surpass the motion of the sun.

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The heavy stars are the outer planets Mars, Jupiter and Saturn; they are called heavy because their motion in the zodiac is slower than the sun's motion. For the use of this expression, see Abù Ma'sar, The Abbreviation of the Introduction to Astrology 42 (ch. 3.14) and Sarnso, Islamic Astronomy and Medieval Spain 236. See for the latitudes of the planets Ptolemaeus, Almagest XIII,1-6. 41 Lacuna in the MSS. 42 id. 43 id.

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Fourth Book What occurs on account of the two simple exhalations, each of them separately and combined, that is to say (on account of) the two exhalations and their changing back into the elements, has been discussed by us.44 The manner in which we have discussed it there was that the exhalation was considered either as the substrate or as the efficient cause. The exhalation is a substrate in such things as rods, torches and the rainbow. It is an efficient cause in thunder and earthquakes. We have not discussed (the exhalations considered as) matter, except only for rain, snow, hail and the water of the sea. Exhalation is the matter for these things and motion (i.e. change) of that matter is generation (of these things). We only selected these (phenomena) from the rest, because they occur in the place of the air and on the surface of the earth. The (phenomena) for which both exhalations are matter are in the last place, sc. within the earth; what occurs in these things, their efficient causes and what is common to the homoiomerous bodies needs other principles. We have found there (i.e. in the preceding discussion) the proximate causes, namely the cold, the hot, the wet and the dry. Thus, the elements are the matter and these forms are the proximate efficient cause insofar as they are in their substrates; their substrates (are substrates) insofar as they exist in them. Local motion occurs in the air on account of the circular (= heavenly) bodies and is caused by the stars, their inclined orbs and the eccentricity of their centres; sometimes it causes certain states of parts of the earth, namely condensation and rarefaction; the cause is coldness and heat and both follow the motion of the stars in their orbs. A division (of things) which leads to the enumeration of the kinds of things without one kind being included in the other (min gayr tadākul) makes it necessary to repeat the account of the same thing several times; it is not possible to separate something

44

Of the phenomena caused by the exhalations only the Milky Way has been fully discussed in what precedes. The phenomena mentioned in what follows in this paragraph have not been discussed at all. This indicates that only a small part of Ibn Bâjja's work is extant.

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from its conditions and it is not possible to arrange it in its order, as Aristotle said in Book XI of the Book of Animals.45 Here we shall discuss what arises from them (sc. the exhalations) within the earth and the conditions which are common to them. What is specific for each kind of these phenomena will be the subject of the Book of Minerals, the Book of Plants and the Book of Animals. The principle of that investigation is a different one which is common to these three kinds. We say that what is generated arises from one element or from more than one. From one element another may be generated, such as from fire the other three may be generated, as is said in De Generatione et Corruptione,46 From two elements another (kind of body) may be generated, as is said in De Generatione et Corruptione·,41 this occurs when the combination is corrupted by the perishing of the powers of each of the elements, or the perishing of the power of one of them. If the limits perish and the powers actually remain, not in their pure form, but in the way that a composed, intermediate power comes into being as long as they remain mixed, then another being arises from them and another form. In that case many forms may arise, by various kinds of composition, various kinds of alteration, followed by various kinds of generation, especially if a mover of a different kind comes to it. Thus, there are many kinds of generations and there are many conditions in the elements. The elements do not exist with every condition, as has been explained in De Generatione et Corruptione,4* but they are elements only by these two contrarieties (hot / cold and dry / wet); the other powers are useless for the generation of elements. The extremes of one contrariety are active, those of the other one are passive: the mover moves by heat and coldness and what is moved is

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Aristotle, Parts of Animals I 639al5-30. Aristotle, De Generatione et Corruptione 11,4. ibid. 11,7 334b8. ibid. 11,2.

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affecred by wetness and dryness. It is a problem what it is that is affected and in what it is that it moves; the discussion and survey of both questions is something which is very useful in natural science. We say that it has been explained in what precedes this book that the body which is adjacent to the sphere of the moon moves with a circular motion, which is not its natural motion, but (it moves) because something else moves it, not by being continuous—for it is subject to division and separation—but by its parts. Insofar as it receives this motion, it has this motion by nature, for it belongs to its nature to receive the motion from the circular body; that is not contrary to its nature, as the circular motion is not contrary to the rectilinear motion, as has been explained. This has been outlined in De Caelo.*9 The difference between the circular motion for this body (i.e. the fire, the upper atmosphere) and for the fifth body is that the mover of the latter is one of the things by which it gets its essence (becomes a substance); thus, it moves by itself in the sense that its mover is part of it and it has the principle of its motion in it. The mover of the former, insofar as its rectilinear motion is concerned, is (also) one of the things by which it gets its essence and the principle of its motion is in it, but insofar as this (circular) motion is concerned, its mover is external, and it does not move because it is in its nature to move in that way. For if that were the case, then that would be its form, and it would be impossible for it to move in another way, unless contrary to its nature. It moves on account of its being adjacent to it, not by being pushed, for the sphere is neither rigid so that it could push, nor soft so that it could be pushed, but it moves due to the motion of the body which surrounds the fire and the other elements. It moves around its centre, for everything that moves (in a circle) does so; thus, each point on the

49

Aristotle, De Caelo 1,4.

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lines drawn from the centre to the circumference describes a circle parallel (to other described circles). 50 if that were the case, the point would be separate; thus, each moving thing needs something resting around which it moves.51 Also, when the centre of a moving circle moves, the circle moves with two kinds of motion. One of them is a motion by which it changes its place completely and it has this motion insofar as it is a moving part, as is said of the epicycle; the other is the motion which is specific for it, insofar as it occurs around its centre. This matter is studied in mathematics. The reason why the earth is at rest and how it does not take part of this motion is not that the point is not separate; that (circumstance) is a proof of the existence of rest in a part of the world. There is a reason for the fact that the earth is at rest and does not rotate (around an axis through its centre) in a circle; the place suitable for its discussion is De Caelo.51 Let us establish the matter according to what is observed and to what is given in the account and leave the investigation of the cause of this condition of rest to another place. For (one should note) that this (kind of) rest is different from the one studied in De Caelo. In De Caelo it is investigated whether the earth as a whole has a rectilinear motion. 53 Thus, (the earth) as a whole it has no motion at all, neither rectilinear, nor circular. Similarly, the fire as a whole has no rectilinear motion and the same holds for everything that is in its natural place; however, all the (elements) do have a rectilinear motion if parts are considered. The ancient natural philosophers have especially studied the

50

Some phrases seem to be missing here. The argument may be completed from Ibn Bâjja's Commentary on the Physics', see Lettinck 1994 296-297. 51 See also next paragraph. 52 Aristotle, De Caelo 11,14 296a25-34. 53 ibid. 11,14 296b27-297a3. The possibility that the earth rotates around an axis through its centre is also ruled out in De Caelo, see previous note.

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earth (and investigated) what kept it at rest and why it was at rest here, for they thought that every part of it was moving in the air. Also, he who thought that it was not circular and thought that it extended without limit, conceived a bearer for it, such as the Greeks talked about Atlas. Let us leave this study to its proper place. Thus, the fire moves in a circle with the motion we have mentioned. Because there are many moving things which have their centres here, the fire and the air have many motions. Because they do not move by themselves in their circular motion, they have motions in them which are opposite and which are cooperative; motions in them are sometimes oblique, sometimes around the centre; it may occur that parts of fire and air are separated from each other and that parts move towards each other, as (occurs in) the separation of waters. For one sometimes sees that the water in rivers, especially large ones, partly moves upwards and partly downwards. This may be observed in small rivers too when they are full of flowing water. It is observed in the sea, known by those who travel on the sea, (to the extent) that (the sea) was subject to a kind of separation. Thus, the fire as a whole moves with the daily circular motion and in its parts it is subject to other motions on account of the other spheres. We have often said that it has this motion in this way, and that it does not follow that the fire belongs to the fifth body; for the fifth body has a nature or a soul by which it moves, and mover and moved are in the (same) moving body such as the doctor who heals himself. The mover of this one, however, is external, such as the hand which moves the pen. This is the cause of the perishing of fire, for it

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only perishes by getting wet or cold, and this only occurs when it is at rest. Therefore it moves with these motions until it gets further away from the daily motion; the motion in it becomes weaker, it arrives in another place and becomes air. If it occurs that is gets even farther away, so that it comes to rest completely, it becomes water or earth. This account is more fitting for De Generatione et Corruptione,54 for it gives the cause of the continuous generation of elements from each other. Besides the fact that fire always moves in this way, it has also the power to cause motion. If one enumerates the moving powers one finds them to be hot or cold or causing growth or generation; (also) local motion is an effective power. The question should be investigated whether it is an effective power because it is in something not moving, so that it becomes effective accidentally, not essentially. It has, however, also a moving power by its parts, another one, which is this: it has a certain kind of extension which is specific for it, because every simple body has such a kind of specific extension around the centre of the world, for there is no centre but that one. We have discussed and demonstrated this, and also how its cause lies in circles which are concentric around one centre, and we have used in it what has been explained in geometry.55 The form and matter for this kind of extension are common, and (if it has this kind of magnitude) it does not have the other kind of magnitude, because a magnitude does not arise from what is not-magnitude, as we have discussed in the Physics, but this magnitude arises from what is not-this-magnitude.56 Not-this-magni-

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with a small thickness it may become a cone or a cylinder and the proportion between the diameter of its base to its height becomes different. Generally, one of its sizes becomes equal to the pores of that (resisting) body. Therefore, when a thunderbolt is extremely rarefied, it does not ignite wood, nor plants, whereas it melts copper because copper does not have pores; if it is more solid it does ignite wood. The first kind is called white thunderbolt." Earth is not as easily divisible; it can be divided only by something which overpowers it in an appreciable time. This is the essence of thickness and thinness. Someone could raise a question by saying that if that is the case, then fire must be moist, because it is something that moves in its parts and can take different shapes, so how could one say that it is dry? This is investigated in De Generatione et Corruptione,58 for there it is made clear how the elements fire and earth are called dry and there it is investigated. There does not exist a magnitude which is smaller than the earth, because if that were the case it would have a motion around its centre and it would have a motion around it with a smaller radius. There does not exist a magnitude which is larger than the fire either—by 'there' I mean: in this matter which accepts the straight lines (i.e. which is subject to rectilinear motion)—for if there were a magnitude larger than the fire, that body would be moving around the fire; then fire would move around something, and around fire would move something of its kind; then it would be possible that fire changed into it and fire would be mover and moved. Then it would move to it and become moist. Similarly, the earth would necessarily become moist, as moistness has previously been defined in that way. Then fire would not be the

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59 See Ibn Bäjja's commentaries on Book VIII of the Physics, 612, 615, 621.

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the power of my hand to move the pen, sc. that it causes motion insofar as it is in motion. The power of the fire to cause motion is a primary power, which is connected to the imperfect perfection;60 generally, its motion is a primary motion which moves something which is not a part. The deferent orb of the epicycle is a mover which is in motion, but that (the epicycle) is a part of it, as has been explained in mathematics. In Physics the subject of motion has been explained. Air has a power by which it causes motion, but it is not a primary power; water is the last (element) that moves. The daily motion is not a mover at all, although it exerts some influence, otherwise a finite body would have an infinite power. If every finite body has a finite power it will give a motion and a power to cause motion; when the power to cause motion ceases to exist, the motion ceases to exist and this motion in this body is not a mover at all. Then the body which moves by it is at rest, such as the earth. It is necessary that there is no body which moves by it, for it does not move at all. There is no other simple body at the centre of the world which is smaller than the earth, for if that were the case, it would have a position in relation to what is outside of it in one of two ways; either it could not have had another position by itself; then it has been given the best possible position, as occurs for the sphere insofar as it has a position because of the two poles; but for what is worst necessarily exists what does not exist for what is best, which is the fire, and from that many impossibilities follow; in general, it has been made clear that this is impossible, except for what eternally

60

/.e., motion, see Lettinck 1994 208.

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exists. (The other way is) that it is possible for it to be in another position; then it is possible that it moves, while we have said that it does not move. Thus, there is no body smaller than the earth. If there were a body larger than the fire, the motion of the fire would not be a primary mover and it would not be a primary motion acquired by nature, nor by force. For fire moves air by means of some sort of pushing and it does not give it motion in the same way as it has received motion. Look how being is divided among the bodies, in such a way that a body is not devoid of perfection. For rest is an absolute privation and motion is one of the two kinds of perfection, and in general being is: being given perfection. Therefore earth, as it is imperfect by itself and completely lacking perfection by nature, is given perfection, together with water and the part of air which is adjacent to earth, in the way of what is possible in it, when part of it gets moving by itself, such as the motion of air we may observe in the air, and the motion of water by the winds. This results in earth being put in motion by them; it was potentially (moving) and this motion of them causes (this motion of earth). Therefore it becomes clear what Aristotle often says, that fire is like form and earth is like matter,61 from two points of view, namely from the point of view of motion and causing motion, and from the point of view of magnitude; for the small is related to privation and the large is related to being, as the small is the way to privation and the large is the way to being and perfection. Let us return to our subject. We say that it has become clear that the elements all have one matter, and that in every part of each element there are necessarily potentialities for the other elements; this is evident by itself. So in this part of the earth there is a potentiality to

61

Aristotle, Physics

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become fire and to gain this kind of magnitude around the centre. The circles are not necessarily separated from their centre; if they were, then the account on them would be from the point of view of not being circles in natural bodies, but separate magnitudes. The geometrician posits equal circles around centres which are not on the same line, an infinite number of them; it is clear that this is impossible in natural things, unless there is an infinite number of worlds and it has been explained that this is impossible. Thus, in that magnitude in the earth a motion occurs, which is not a motion of increase and decrease. For in this (motion) the being changes completely, whereas in that (motion) the subjects keep the same definition and are called with a synonymous name. Everything which moves has a mover, so the earth will get a mover that moves it. That will be either something which is not of the same kind, which is the circular motion, or what is of the same kind, let us say the fire. Because the matter is the same, it is not by earth rather than by fire, as it is potentially the body of fire. Therefore, if it (fire) touches it, it extends it in the way it extends things and it moves its parts so that it makes from the small many. The power of the fire to extend something is the heat. I use 'extending' in a metaphorical sense because this way of causing motion has no name, and therefore the name of the things which resemble it most closely is transferred to it and it is called extending and rarefaction. Its opposite is compression. The name is transferred to both of these by way of something intermediate. It is necessary here, when we investigate and examine,

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that the matter itself exists, not an imagination of it or something that resembles it, so that the intellect considers the things which exist. Therefore something occurs for the earth that prevents it (from moving), because (the earth) is not something that is corruptible in its whole all at once; subsequently the motion occurs. During the time that the form of fire is in the matter, one says it is getting heated; because the body of fire has the same matter, it is potentially the form of earth and the mover towards it is the compression in the way we have said. The matter moves in all of them and the goal is all the forms. The form, such as the form of fire, is a goal, a form and an efficient cause for the perfection of that pure potentiality which is in the earth and it is a form insofar as that is a potentiality. But everything which is in motion has a mover, and that potentiality is connected to the form of the earth and it also has a potentiality and an efficient cause, as in the case of the form of fire. When the first perfection occurs, which is motion, the motion must have a subject while the earth is existing; the potentiality connected to its form is turning into perfection, so the earth is moving during all that time and it is the subject of the motion in order that the potentiality exists; the generation (of the motion) occurs at an instant. The kind of change occurring to the earth, which takes it from its form to the form of the fire is the heating (istihrār), for heating extends it, so that it gets that extension and it leaves completely the other magnitude which was in the matter. The power by which that which moves moves is the form of the fire and the air, which is the heat. The forms of fire and air occur in the definition of heat, such

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as nose occurs in the definition of snubnosedness, so that if nose is taken away from the account of snubnosedness, it is not snubnosedness anymore. Such is the case for heat. How we conceive it when we do not know this about it, that belongs to what occurs to the sense, for when the sense observes a hollowness in a nose it grasps snubnosedness. Suppose we do not know that it is a nose, but a certain body, then the sense has grasped snubnosedness. The intellect does not grasp snubnosedness, but it grasps it in the way the sense grasps it. When it has become clear to us, what this hollowness is, which is the primary efficient cause, then we know the potentiality by which we grasp that point, either by sense or by reason. Such things as grasping snubnosedness by sense are discussed in De Anima.62 Similarly, we know about heat and such things only what is grasped by sense such as it grasps it, and we know by sense that heat is something in a subject. We do not know it by reason until we have grasped it in the way of grasping that is specific for reason. The sense does not have a part in this grasping. Someone might say, however, that if that is the case, then heat is a homonymous name; for the form of fire and the form of air occur in their definition and these two are not called by a synonymous name, except that they are one genus. If that is the case, then the heat of the air must be intermediate and the same must hold for all kinds of heat and generally the contraries and then the air would be cold. What is implied by this problem does not necessarily follow from the account, but what follows is that 'heat' is used in a primary and secondary sense, and that it is nearer to synonymous words, such as 'being' and similar ones. Potentiality exists in them (in fire and air) in two manners of analogous ways and that is right, for it is observed in that way. The fact is, as Aristotle says, that

62

Aristotle, De Anima 431b13 ff.

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63

Aristotle, De Generatione

et Corruptione

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as it causes motion, and insofar as it is matter, not insofar as it is form. These two qualities are more entitled to be material than heat and coldness, whereas those are more entitled to be forms. Similarly, the influence to which bodies are subject occurs on account of these qualities, and they exert an influence on account of the other two powers, and this is right. They become similar by these, but moistness is like form, and dryness is like matter. Let us investigate how heat and coldness become similar. Here ends what is extant of this treatise.

BIBLIOGRAPHY AND BIBLIOGRAPHICAL ABBREVIATIONS Abū

1-Barakāt a1-Bagdādi, al-Mu'tabar = Kitàb al-Mu'tabar fï l-hikma, 3 vols., Hyderabad 1939. Abü Ma'sar, The Abbreviation of the Introduction to Astrology. Together with the Medieval Latin Translation of Abelard of Bath, ed. and transi. C. Burnett, K. Yamamoto, M. Yano, Leiden, New York, Köln 1994 (Islamic Philosophy, Theology and Science 15). Ahlwardt, W., Verzeichniss der arabischen Handschriften der Königlichen Bibliothek zu Berlin, 10 vols, Berlin 1887-99. al-'Alawi, J.A., Mu'allafät Ibn Bäjja, Casablanca, Beirut 1983. Alexander of Aphrodisias, in Meteor. = In Aristotelis Meteorologicorum libros commentaria, ed. M. Hayduck, Berlin 1899 (Commentaria in Aristotelem Graeca III 2). Aristotle, Opera ex recensione Immanuelis Bekkeri, 2 vols., Berlin 1831. , Meteorologicorum libri quattuor, ed. F.H. Fobes, Cambridge (Mass.) 1919. , The Arabic Version of Aristotle's Meteorology (Kitāb al-àtàr al-'ulwiyya li-Aristūtālīs), ed. C. Petraitis, Beyrouth 1967. , Aristotelis omnia quae extant Opera. Averrois Cordubensis in ea opera omnes, qui ad haec usque tempora pervenere, commentarii, 10 vols., Venetiis 1562, repr. Frankfurt am Main 1962. , Meteorologica, transi. E.W. Webster, in: The Works of Aristotle Translated into English, vol. Ill, Oxford 1923. , Meteorologica. With an English Translation by H.D.P. Lee, Cambridge (Mass.), London 1952 (Loeb Classical Library 397). , Meteorologie. Heber die Welt, transi. H. Strohm, Darmstadt 1970 (Aristoteles. Werke in deutscher Übersetzung, Bd. 12 I-II). , Generation of Animals. The Arabic Translation Commonly Ascribed to Yahyä ibn al-Bitriq, ed. J. Brugman and H. J. Drossaart Lulofs, Leiden 1971 (Publication of 'De Goeje Fund' XXIII). , The Arabic Version of Aristotle's Parts of Animals, ed. R. Kruk, Amsterdam 1979 (Aristoteles Semitico-Latinus 2). Badawï, A. (ed.), Commentaires sur Aristote perdus en grec et autres épitres, Beyrouth 1971. Baffioni, C, "La tradizione araba del IV libro dei 'Meteorologica' di Aristotele", in: Annali dell'Istituto Orientale di Napoli 40 (1980) fasc. 2 suppl. 23. , Il IV libro dei 'Meteorologica' di Aristotele, Napoli 1981. Bahmanyâr ibn al-Marzubân, at-Tahsïl = Kitàb at-Tahsïl, ed. M. Mutahhari, Teheran 1970. Bergsträsser, G., Neue meteorologische Fragmente des Theophrast (Sitzungsberichte der Heidelberger Akademie der Wissenschaften. Philol-hist. Klasse Jg. 1918 9. Abh.). Böhm, W„ Johannes Philoponos. Ausgewählte Schriften, München, Paderborn, Wien 1967. Boyer, C.A., The Rainbow; from Myth to Mathematics, 2nd ed., Princeton (N.J.) 1987. Brockelmann, GAL = Brockelmann, C., Geschichte der arabischen Litteratur, 2 vols, and 3 suppL vols, Leiden 1937-1949. Crombie, A.C., Robert Grosseteste and the Origins of Experimental Science, Oxford 1953. Daiber, H„ Ein Kompendium der aristotelischen Meteorologie in der Fassung des Hunayn ibn Ishäq (Jawämi' Abi Zayd Hunayn ibn Ishäq al-'Ibädi li-kitäb Aristūtàlis fï l-àtār al-'ulwiyya), Amsterdam, Oxford 1975 (Aristoteles Semitico-Latinus 1).

Daiber, H., "Nestorians of 9th Century Iraq as a Source of Greek, Syriac and Arabic. A Survey of Some Unexploited Sources", in: Aram 3:1-2 (1991) 45-52. , "The Meteorology of Theophrastus in Syriac and Arabic Translation", in: W.W. Fortenbaugh and D. Gutas (eds.), Theophrastus; His Psychological, Doxographical and Scientific Writings, New Brunswick and London 1992 166-293 (Rutgers University Studies in Classical Humanities vol. V). , Naturwissenschaft bei den Arabern im 10. Jahrhundert n. Chr.\ Briefe des Abu l-Fadl ibn al-'Amid (gest. 3601970) an 'Adudaddawla, Leiden, New York, Köln 1993 (Islamic Philosophy, Theology and Science 13). Dietrich von Freiberg, Ueber den Regenbogen und die durch Strahlen erzeugten Eindrücke, ed. J. Würschmidt, Münster 1914. ad-Dimašqī, Κ it ab Nukbat ad-dahr fi 'ajä'ib al-barr wa-l-bahr, ed. A.F.M. Mehren, Petersburg 1866. Düring, I, Aristotle's Chemical Treatise, Göteborg 1944. , Aristoteles. Darstellung und Interpretation seines Denkens, Heidelberg 1966 (Bibliothek der klassischen Altertumswissenschaften, N.F. 1. Reihe). Endress, G., review of C. Petraitis, The Arabic Version of Aristotle's Meteorology, in: Orient 23-24 (1974) 497-509. van Ess, J., Frühe mu'tazilitische Häresiographie. Zwei Werke des Nasi' al-Akbar, Beirut 1971 (Beiruter Texte und Studien 11). Fakr ad-Dīn ar-Rāzī, al-Mabähit_ = Kitāb al-mabähil al-mašriqiyya fi 'ilm al-ilāhiyya wa-t-tabi'iyyàt, 2 vols., Teheran 1966. Fakrī, M., Rasa il Ibn Bājja al-ilāhiyya, Beirut 1968. a1-Fārābī, The Perfect State. Abu Nasr al-Färäbi's Mabādi' ārā' ahl al-madina al-fādila, ed., transi, and comm. R. Walzer, Oxford 1985. Fontaine, R„ "Why is the sea salty? The discussion of salinity in Hebrew texts of the thirteenth century", in: Arabic Sciences and Philosophy 5 (1995) 195-218. , Otot ha-Shamayim; Samuel Ibn Tibbon's Hebrew Version of Aristotle's Meteorology, Leiden, New York, Köln 1995 (Aristoteles Semitico-Latinus 8). Gardet, L., Anawati, G., Introduction à la Théologie musulmane, Paris 1948. Gätje, H„ "Zur Farbenlehre in der muslimischen Philosophie", in: Der Islam, 43 (1967) 280-301. , Das Kapitel über das Begehren aus dem Mittleren Kommentar der Averroes zur Schrift über die Seele, Amsterdam, Oxford 1985 (Aristoteles Semitico-Latinus 3). Gilbert, O., Die meteorologischen Theorieen der griechischen Altertums, Leipzig 1907. Haschmi, M.Y., "Die geologischen und mineralogischen Kentnisse bei Ibn Sīnā", in: Zeitschrift der Deutschen Morgenländischen Gesellschaft 116 (1966) 44-59. Heath, T., Mathematics in Aristotle, Oxford 1949. Holmyard, E.J., Mandeville, D.C., Avicennae De Congelatione et Conglutinatione Lapidum, Being Sections of the Kitāb al-Shifā', Paris 1927. Horten, M., Wiedemann, Ε., "Avicennas Lehre vom Regenbogen nach seinem Werk al-Schifä", in: Meteorologische Zeitschrift 30 (1913) 533-544, repr. in: E. Wiedemann, Gesammelte Schriften II, 733-44. Hunayn, Jawämi' = see Daiber 1975. Ibn abi Usaybi'a, Kitäb 'Uyün al-anba fi tabaqät al-atibba, ed. A. Müller, 2 vols., Cairo 1882.' " Ibn al-'Amid (Abü l-Fadl Muhammad), see Daiber 1993. Ibn Bājja, Rasa il Ibn Bäjja al-ilähiyya, ed. M. Fakri, Beirut 1968. , Kitäb al-Kawn wa-l-fasäd (Avempace, Libro de la generaciôn y corrupciön), ed. and transi. J. Puig Montada, Madrid 1995. Ibn al-Bitriq, Meteor. = see Petraitis. Ibn al-Haytam, Kitäb al-Manäzir. Books / - / / / : On Direct Vision, ed. A.I. Sabra, Kuwait 1983. See also Sabra. Ibn Rusd, Middle Commentary = Talkis al-āt_ār al-'ulwiyya, ed. J.A. al-Alawi, Beirut 1994. Partial Latin translation in: Aristotle, Aristotelis omnia quae extant Opera, vol. V.

Ibn Rušd, Short Commentary = "Kitāb a1-ātār al-'ulwiyya", in: Rasâ'il Ibn Rusd, vol. 4, Hyderabad 1947. Another edition: Kitâb al-ât_âr al-'ulwiyya (Epitome Meteorologica), ed. S.F. Abü Wāfia, SA. 'Abd ar-Rāziq, Cairo 1994. Latin translation in: Aristotle, Aristotelis omnia quae extant Opera, vol. V. , Paraphrase of Aristotle's De sensu et sensibilibus. Latin translation in: Aristotle, Aristotelis omnia quae extant Opera, vol. VI/2. Ibn Sinā, aš-Šifā", Tab. 4 = Kitàb as-Šifā', at-Tabi'iyyàt 4: Fi l-af'àl wa-l-infi'àlât, ed. M. Qäsim, L Madkür, Cairo 1969. , as-Sifä', Tab. 5 = Kitàb aš-Sifà', at-Tabi'iyyàt 5: Al-ma'àdin wa-l-àtàr al-'ulwiyya, ed. A. Muntasir, S. Zâyid, A. Ismâ'ïl, I. Madkür, Cairo 1964. , an-Najàt = Kitâb an-Najàt, ed. Mû. al-Kurdï, Cairo 1938. , Livre des directions et remarques (Kitàb al-iiâràt wat-tanbihât) transi. A.-M. Goichon, Beirut, Paris 1951. , Le livre de science, transi. M. Achena, H. Massé, 2 vols, Paris 19862. , Kitàb al-Hidâya, ed. M. Abduh, Cairo 1974. , Avicennae De Congelatione et Conglutinatione Lapidum, see Holmyard. Ibn Tibbon, Otot ha-Shamayim, see Fontaine, Otot ha-Shamayim. al-Idrïsi, Description de l'Afrique et de l'Espagne, ed. and transi. P.A. Dozy, M.J. de Goeje, Leiden 1866, repr. Amsterdam 1969. Ikwān as-Safā', Rasâ'il, 2 vols, Cairo 1928. al—'Iraqi, M.A, al-Falsafa at-tabi'iyya 'inda Ibn Sinà, Cairo 1971. al-Kindi, Rasail = Rasâ'il al-Kindi al-falsafiyya, ed. M. Abü Rida, 2 vols, Cairo 1950, 1953. Krebs, Ε, Meister Dietrich, Münster 1906. Kruk, R„ "Ibn Bäjja's Commentary on Aristotle's De Animalibus", in: G. Endress and R. Kruk (eds.), The Ancient Tradition in Christian and Islamic Hellenism: Studies on the Transmission of Greek Philosophy and Sciences dedicated to H.J. Drossaart Lulofs on his Ninetieth Birthday, Leiden 1996 165-181. al-Kwârizmi (Abü Abda11āh Muhammad ibn Ahmad ibn Yüsuf a1-kātib). Liber Mafàtih al-ulūm, ed. G. van Vloten, Leiden 1895, repr. 1968. Lee, H.D.P, see Aristotle Lejeune, A. (ed.), see Ptolemaeus, Opt. Lettinck, P, Aristotle's Physics and its Reception in the Arabic World, Leiden, New York, Köln 1994 (Aristoteles Semitico-Latinus 7). Lindberg, D.C, "The Cause of Refraction in Medieval Optics", in: The British Journal for the History of Science 4 (1968) 23-38, repr. in: Studies in the History of Medieval Optics, London 1983 (Variorum Collected Studies Series 186). , Theories of Vision from al-Kindi to Kepler, Chicago 1976. al-Maqdisî (Mutahhar ibn Tâhir), Kitàb Bad' al-kalq wa-t-ta'rik, ed. and transi. C. Huart, 2 vols, Paris 1899, 1901.' Masselink, J.F, De Grieks-Romeinse Windroos, Utrecht, Nijmegen 1956. al-Mas'üdi (Muhammad ibn Mas'üd), Kitàb at-Tanbih wa-l-isràf, ed. M.J. de Goeje, Leiden 1894. Najm ad-Din (Ahmad ibn Hamdān ibn Šabīb al-Harräni al-Hanbali), Jàmi' al-funün wa-salwat al-mahzūn} an-Nāši' (Abda11āh ibn Muhammad), al-Kitàb al-Awsat, see van Ess. Neugebauer, Ο, A History of Ancient Mathematical Astronomy, 3 vols, Berlin, Heidelberg, New York 1975. an-Nuwayri (Ahmad ibn Abda1wahhāb), Nihàyat al-'arab fi funūn al-adab, 30 vols, Cairo 1923—1991. Olympiodorus, in Meteor. = Commentaria in Aristotelis Meteora, ed. G. Stüve, Berlin 1900 (Commentaria in Aristotelem Graeca XII 2).

1

See Brockelmann, GAL Suppl. II 161-162.

(Pseudo) Olympiodorus, "Tafsir A1imfīdùrūs 1i-kitāb Aristātā1īs fī 1-ātār al-'ulwiyya tarjamat Hunayn ibn Ishāq", in: Α. Badawī (ed.), Commentaires sur Aristote perdus en grec et autres èpîtres, Beyrouth 1971. Pauly, Α., Wissowa, G. (ed.), Real-Encyclopädie der klassischen Altertumswissenschaft, Stuttgart 1894 ff. Peters, F.E., Aristoteles Arabus, Leiden 1968 (New York University. Department of Classics. Monographs on Mediterranean Antiquity I). , Aristotle and the Arabs, New York, London 1968 (New York University Studies in Near Eastern Civilization 1). Petraitis, C. (ed.), The Arabic Version of Aristotle's Meteorology (Kitäb al-ät_är al-'ulwiyya li-Aristütälis), Beyrouth 1967 (Recherches publiées sous la direction de l'Institut de Lettres Orientales de Beyrouth, série. I, t. 39), see Aristotle. Philoponos, in Meteor. = In Aristotelis Meteorologicorum librum primum commentaria, ed. M. Hayduck, Berlin 1901 (Commentaria in Aristotelem Graeca XIV 1). Pseudo-Olympiodorus, Tafsir = see Olympiodorus. Ptolemaeus, Opt. = A. Lejeune (ed.), L'Optique de Claude Prolémée dans la version latine d'après l'arabe de l'émir Eugène de Sicile, Louvain 1956. , Almagest = G.J. Toomer (transi.), Ptolemy's Almagest, London 1984. Puig Montada, J., see Ibn Bājja. al-Qazwïnï, Kitäb 'Ajä'ib al-maUüqät, ed. F. Wüstenfeld, 2 vols., Göttingen 1848, 1849. al-Qifti, Ta'rik al-hukamä', ed. J. Lippert, Leipzig 1903. Rashed, R., "Le modèle de la sphère transparante et l'explication de l'arc-en-ciel: Ibn al-Haytham, al-Fârisi", in: Revue d'Histoire des Sciences 23 (1970) 109-140, repr. in: R. Rashed, Optique et Mathématiques, Aldershot 1992 (Variorum, Collected studies series 388). , "A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses", in: Isis 81 (1990) 464-491, repr. in: R. Rashed, Optique et Mathématiques, Aldershot 1992 (Variorum, Collected studies series 388). , Géométrie et dioptrique au Xe siècle. Ibn Sahl, al-Qûhi et Ibn al-Haytham, Paris 1993. ar-Rāzī (Abü Bakr Muhammad Zakarīyā), Kitäb al-Asrär, transi, by J. Ruska as Al-Räzi's Buch Geheimnis der Geheimnisse2 Ruska, J., Al-Räzi's Buch Geheimnis der Geheimnisse, Berlin 1937. Sabra, A.I., "The physical and the mathematical in Ibn al-Haytham's theory of light and vision", in: The Commemoration Volume of Birūni International Congress in Tehran, Tehran 1976 1-20, repr. in: Optics, Astronomy and Logic. Studies in Arabic Science and Philosophy, Aldershot 1994 (Variorum Collected Studies Series 444). , (ed. and transi.), The Optics of Ibn al-Haytham. Books I-III: On Direct Vision, 2 vols., London 1989. Samso, J. Islamic Astronomy and Medieval Spain, London 1993. Sayih, Α., "The Aristotelian Explanation of the Rainbow", in: Isis 30 (1939) 65-83. , "A1-Qarāfī and his Explanation of the Rainbow", in: Isis 32 (1940) 16-26. Schramm, M., Ibn al-Haytham's Weg zur Physik, Wiesbaden 1963 (Boethius 1). Sersen, W.J., Arab Meteorology from Pre-islamic Times to the Thirteenth Century, thesis London 1976. Sezgin, GAS = Sezgin, F., Geschichte des arabischen Schrifttums 9 vols. Leiden 1967-84. Solmsen, F., Aristotle's System of the Physical World, Ithaca, Mew York 1960 (Cornell Studies in Classical Philology 33). Steinmetz, P., Die Physik des Theophrastos von Eresos, Bad Homburg, Berlin, Zürich 1964 (Palingenesia I).

2

See Sezgin, GAS IV 280. This is not a translation of a1-Rāzī's Kitäb Sirr

al-asrär.

Steinmetz, P., "Ansatzpunkte der Elementenlehre Theophrasts im Werk des Aristoteles", in: Naturphilosophie bei Aristoteles und Theophrast, ed. I. Düring, Heidelberg 1969 224-249. Strohm, H., "Untersuchungen zur Entwicklungsgeschichte der aristotelischen Meteorologie", in: Philologus, Suppl. 28/1, Leipzig 1935. 1984, see Aristotle. at-Tabari ('Ali ibn Rabban), Firdaws al-hikma, ed. M.Z as-Siddiqi, Berlin 1928. Theophrastus, Der syrische Auszug der Meteorologie des Theophrast, see Wagner, Steinmetz. , Meteorology, Syriac and Arabic translations, see Daiber 1992. , Theophrastus of Eresus. Sources for his Life, Writings, Thought and Influence, ed. W. Fortenbaugh, P.M. Huby, R.W. Sharpies, D. Gutas, 2 vols, Leiden 1992, 19932 (Philosophie antiqua 54). at-Tīfāšī, Surūr an-nafs bi-madärik al-hawäss al-kams, excerpt by Ibn Manzür (+ 1311) from Fasl al-kitäb fi madàrik al-hawäss al-kams li-ūlī l-albäb, ed. I. 'Abbās, Beirut 1980. Toomer, G.J. (transi.), see Ptolemaeus. Wagner, E„ Steinmetz, P., Der syrische Auszug der Meteorologie des Theophrast, Wiesbaden 1964 (Akademie der Wissenschaften und der Literatur (Mainz). Abhandlungen der geistes- und sozialwissenschaflichen Klasse Jg. 1964/1). Walzer, R„ Greek into Arabic, Oxford 1962. al-Watwät (Jamal ad-Dīn), Manāhij al-fikar wä-mabähij al-'ibar. Encyclopaedia of Four Natural Sciences, ed. F. Sezgin, Frankfurt am Main 1990 (Publications of the Institute for the History of Arabic-Islamic Science, Series C, Facsimile Editions v o l 49). Webster, E.W., see Aristotle. Wiedemann, E, "Auszüge aus arabischen Enzyklopädien und Anderes" (Beiträge zur Geschichte der Naturwissenschaften V), in: Sitzungsberichten der PhysikalischMedizinischen Sozietät zu Erlangen 37 (1905) 392-455, repr. in: Aufsätze I, 109-172. , "Über die Lage der Milchstrasse nach Ibn al-Haytam", in: Sirius 39 (1906) 113-5, repr. in: Gesammelte Schriften I, 174-6. , "Über die Brechung des Lichtes in Kugeln nach Ibn al-Haytam und Kamäl ad-Din a1-Fārisī" (Beiträge zur Geschichte der Naturwissenschaften XIX), in: Sitzungsberichten der Physikalisch-Medizinischen Sozietät zu Erlangen 42 (1910) 15-58, repr. in: Aufsätze I, 597-640. , "Zur Chemie bei den Arabern" (Beiträge zur Geschichte der Naturwissenschaften XXIV), in: Sitzungsberichten der Physikalisch-Medizinischen Sozietät zu Erlangen 43 (1911) 72-113, repr. in: Aufsätze I, 689-730. , "Zu Ibn al-Haytams Optik", in: Archiv für die Geschichte der Naturwissenschaften und der Technik 3 (1912) 1-53, repr. in: Gesammelte Schriften I, 541-593. , "Zur Optik von Kamäl al-Din", in: Archiv für die Geschichte der Naturwissenschaften und der Technik 3 (1912) 161-177, repr. in: Gesammelte Schriften I, 596-612. , "Arabische Studien über den Regenbogen", in: Archiv für die Geschichte der Naturwissenschaften und der Technik 4 (1913) 453-60, repr. in: Gesammelte Schriften II, 745-52. , "Theorie des Regenbogens von Ibn al-Haytam" (Beiträge zur Geschichte der Naturwissenschaften XXXVIII), in: Sitzungsberichten der Physikalisch-Medizinischen Sozietät zu Erlangen 46 (1914) 39-56, repr. in: Aufsätze II, 69-86. , "Anschauungen von muslimischen Gelehrten über die blaue Farbe des Himmels", in: Festschrift Elster und Geitel (1915) 118-26, repr. in: Gesammelte Schriften II, 812-20. , "Über die Milchstrasse bei den Arabern" (Beiträge zur Geschichte der Naturwissenschaften LXXIV), in: Sitzungsberichten der Physikalisch-Medizinischen Sozietät zu Erlangen 58/59 (1926/27) 348-62, repr. in: Aufsätze II, 662-76. , Aufsätze zur arabischen Wissenschaftsgeschichte, 2 vols., Hildesheim, New York 1970.

Wiedemann, E„ Gesammelte Schriften zur arabisch-islamischen Wissenschaftsgeschichte, 3 vols., Frankfurt am Main 1984. Würschmidt, J„ "Die Theorie des Regenbogen und des Halo bei Ibn al-Haytam und Dietrich von Freiberg", in: Meteorologische Zeitschrift 31 (1914) 484-7. Zimmermann, F.W, Brown, H.V.B., "Neue Uebersetzungstexte aus dem Bereich der spätantiken griechischen Philosophie", in: Der Islam 50 (1973) 313-24.

INDEX LOCORUM Abü 1-Barakāt, al-Mu'tabar II 141,1-10: 85 202.7-21: 199 202^2-203,14: 200 203,14-204,1: 200 204,1-205,10: 200 205.10-21: 201 206,1-207,6: 202 207.6-24: 202 208.1-13: 202 20848-209,19: 146 209,19-211,13: 147 211.13-212,8: 148 212.8-21: 148 213.7-11: 57 213,7-215,1: 115 215.2-24: 115 215,24-216,13: 116 216.14-21: 116 217.11-219,4: 184 219.12-220,66: 184 220,7-15: 185 220.15-22: 185 2213-12: 221 221.13-20: 237 221.21-222,4: 238 222,11-223,12: 83 223,13-21: 83 223.23-224,23: 84 224.24-225,21: 84 225.22-226,6: 85 226,7-23: 85 227,7-228,19: 305 228,19-229,16: 305 229,17-231,18: 306 230,13-231,18: 306 Alexander, in Meteor. 1344-23: 40 14,19-28: 64 1430-15,7: 40 15,8-22: 40 15,30-16,8: 63 16,12-15: 40 18,8-19,13: 42 1943-19: 42 1932-20,15: 40 21,27-33: 67 2247-18: 67 23,21-29: 72

Alexander (continued) 23.29-2440: 72 25,8-13: 72 2549-22: 72 38,28-32: 71 40,3-4: 71 44,25-6: 97 4430-45,6: 99 45.30-46,7: 100 53,5-16: 100 86,1-87,10: 133 9031-32: 34 9332-94,2: 162 97,30-9843: 164 98,21-10142: 165 99,15-16: 164 100,28-101,9: 190n105 105,33-107,12: 165 110.6-10: 187n98 110,7: 159n2 111.2-25: 159n3 126,33: 34 12835-129,9: 227 13432: 226 141.3-30: 245n4 142,28-1433 252 143.7-14: 252 1493-15042 253 15132-152,16: 253 152,18: 254 159,9-21: 249n9 160,21-33: 254 161,8: 269n72 178,13-15: 301 1793-11: 310 Bahmanyār, at-Tahsīl 700,6-70141: 57' 710.6-7113: 57 711.4-7123: 114 712.7-8: 283 712.13-713,5: 283 713.14-714,3: 237 714,4-8: 283 714,9-10: 183 714,11-715,6: 83 715,7-14: 237 716,1-3: 183 716,4-5 : 146 716,6-11: 146

Bahmanyār (continued) 716,12-717,11: 146 717.12-718,2: 221 718,5-8: 146 718,9-720,6: 304 Fakr ad-DIn, al-Mabähit II 99,13-17: 86 140.7-141,19: 58 142.1-143,9: 149 172.2-9: 58 172.16-174,8: 116 174.9-175,14: 117 177.4-9: 283 1783-6: 284 182.17-183,1: 285 184,16-186,15: 285 187.5-188,1: 238 188.3-13: 238 188.15-21: 238 189,2-1903: 86 190.6-12: 239 190.19-191,15: 185 191.16-192,19: 186 192.20-193,2: 186 193.5-194,9: 186 194.10-17: 186 194.18-196,13: 186 196,14-197,19: 186 198.4-199,14: 203 199,14-200,19: 203 200.20-201,16: 203 201,18-203,8: 204 203,9-17: 204 203.18-204,14: 204 204.19-205,12: 148 205.13-15: 148 205,16-206,7: 221 206.8-10: 222

206.11-18: 222 206,19-207,1: 222 210.21-211,6: 306 211,8-212,4: 307 212.6-11: 307 212,16-214,2: 307 214.4-16: 307 214,18-215,2: 308 2153-216,4: 308 216.5-218,1; 309 Hunayn, Jawämi' 44-70: 105 51-2: 47 68-70: 47 74: 105 94-119: 135

Hunayn (continued) 122-38: 136 139-40: 47 144-6: 169 162-8: 168 168-78: 168 189-98: 169 201-6: 212 206-8: 213 213-20: 213 222-4: 213 224-9: 213 229-31: 214 239: 47 239-46: 229 258-63: 230 263-6: 229 277-80: 262n40 281-93: 266 298-302: 47 302-5: 77 312-5: 77 324-7: 78 Ibn al-Bitriq, Meteor. 12,5: 311 17,11-18,8: 45 19.5-20,4: 45 20,11: 53, 108 20,12: 52 23.3-4: 76 24.1-3: 76 25.11-26,3: 76, 87 29.2-8: 75 30.4-6: 75 30.12-31,9: 46 31,9-32,4: 75 32.4-7: 75 33,9-34,5: 75 34,9-11: 75 36.2-9: 46 36.6-37,5: 105 37,16-38,3: 105 39.5-9: 105 39.13-40,9: 105 40,15-41,3: 105 42,11-46,9: 134 47.8-9: 135 51,5-53,10: 135 55.3-10: 135 57.9-58,1: 154 59.3-60,10: 136 63,5: 46 66.5-67,7: 167 70.4-71,5: 168 71.6-12: 207n41

al-Bitriq (continued) 72^-6: 168 72,7-11: 169 75,7-10: 212 75,10-773: 213 77,8-78,1: 213 78,4-7: 213 79,1-4: 213 79,11-80,2: 214 81,6: 46 81,10-82,5: 229 85,5-9: 229 86,1-5: 229 86,7-12: 230 87,2-10: 230 87,12-88,2: 230 88,8-11: 262 89,1-5: 262 89,7-8: 263 89,12-14: 263 90,5-91,4: 263 91,4-6: 264 9U2-16: 264 92,10-93,12: 265 953-11: 263, 265 96,2-14: 266 97,2-10: 266 98,8: 302n5 117,11: 309n28 118,1: 309n28 119,10: 302n7 ι Rušd, Middle Commentary 25,1-4: 63 25,15-16: 63 27,10-29,7: 64 29,8-30,18: 64 31,1-32,4: 65 32,15-34,20: 65 35,10-36,5: 63 36,6-40,10: 92 40,14-42,17: 93 44,16-46,10: 93 47,12-48,2: 93 54,7-56,9: 94 54,9: 76 57,7-58,15: 95 58,16-61,17: 95 63,10-17: 63 66,6-67,2: 118 68,7-69,13: 119 69,14-71,5: 119 73,10-74,4: 152 743-19: 152 75,8-15: 153 76,13-77,6: 153

Ibn Rušd (continued) 843-9: 153 84,12-85,12: 153 893-18: 154 91,2-16: 154 91,17-9242: 154 973-5: 63 100,2-12,19: 191 104,13-105,10: 192 107,7-10: 167n20 108,6-109,11: 193 111,1-112,4: 207 112,9-116,17: 208 124,6-125,4: 224 132,6: 63 139,7-12: 241 140,16-141,2: 292 141,10-143^8: 293 144,1-146,12: 294 147,2-9: 262n40 147,11-13: 263 147,20: 263 148,6-8: 264 148,14: 264 148,15-149,7: 294n144 149,9-12: 264 151,7-11: 264 151,16-152,7: 264 152,8-20: 294 153,1-154,2: 295 154,4-156,20: 296 1543: 264n51 157,2-158,8: 296 158,10-13: 296 158,15-159,14: 297 159,14-160,7: 297 160,15-163,9: 298 165,7-166,2: 309 165,12: 302n4 165,13-15: 302n5 165J.6: 302n7 167,4-6: 310, 312 201,10: 309n28 201,11: 309n28 203,4: 302n7 Ibn Rušd, Short 2,2-5,7: 61 3,18-4,5: 312 5,8-6,11: 62 6,19-7,1: 62 7,1-8: 63 8,10-12: 62 9,8-15: 62 10,2-16: 88 11,3-18: 89

Commentary

Ibn Rušd (continued) 11,18-12,1: 89 12.2-3: 89 12,4-133: 89 133-15,2: 89 1542-1849: 91 20,1-3: 62 20,1-21,10: 117 21,12-19: 117 22.3-14: 118 24.6-25,4: 118 25,16-2742: 150 28.1-17: 150 28,18-303 151 303-31,14: 152 3145-3347: 150 342-17: 187 35.2-36,1: 187 35.3-4: 62 36,2-37,6: 188 37.7-19: 188 38.1-14: 188 38,15-41,6: 190 41.7-44,8: 191 44.9-45,2: 205 453-14: 205 45.15-47,2: 205 473-49,4: 206 50.2-51.8: 207 52.1-53,18: 223 52.2-3: 62 53,18-54,14: 223 54.16-56,5: 240 56.6-57,15: 240 57.18-58,7: 240 58.8-59,1: 241 59.3-61,11: 288 61.14-64,6: 289 64.15-653: 289 65.7-66,2: 289 66,2-68,9: 290 68.10-7247: 291 73.19-74,18: 291 74,19-76,18: 292 76,18-77,9: 292 97,4: 302n7 98,8: 309n28 98,9: 309n28 Ibn Sīnā, aš-Šifā', Tab. 4 2023-204,2: 55 ' 2043-13: 56 205,4-206,3: 144 206,4-17: 144 207.1-208,1: 144 208.2-10:144

Ibn Sīnā (continued) 208,11-210,6: 145 210,7-18: 145 Ibn Sīnā, aš-Sifā', Tab. 5 5,15-20: 239 10,4-11,12: 142 13.4-14,10: 143 15.5-16,4: 218 16.5-15: 218 17.1-7: 219 17,7-16: 219 17,17-18,12: 220 18,13-192: 220 192-9: 220 19.9-14: 220 20,4-23,15: 302 24,7-25,12: 196 25.13-20: 196 25,20-26,6: 196, 204n29 26.6-14: 196 26.14-27,15: 197 27,16-28,13: 197 28,13-30,15: 198 30.16-31,14: 199 35,4-6: 55 35,4-36,5: 112 36.7-37,17: 113 38,6-9: 113 394-18: 55 403-433: 278 43,6-44,2: 279 443-45,6: 279 45,6-46,12: 280 47,4-48,19: 280 48,19-50,7: 281 50.8-53,6: 281 53,6-8: 283 53.9-54,4: 282 54,4-56,2: 283 58,4-19: 177 59,4-11: 177 59,12-16: 178 59.17-60,9: 178 60.10-12: 178 60,12-61,5: 179 61,6-8: 179 61,12-62,2: 179 62.2-16: 180 62,17-633: 180 63,4-8: 180 63.10-16: 180 63,17-64,1: 180 64,1-7: 181 64,9-15: 181 64,17-65,15: 181

Ibn Sīnā (continued) 65,16-66,1: 182 66,2-9: 182 67,4-68,13: 235 68,13-18: 235 68,19-69,8: 235 69,9-14: 235 69,15-18: 236 69,19-21: 236 70,1-6: 236 70,6-713: 237 71,4-14: 81 71,15-72,17: 81 72,18-743: 81 743-15: 82 Ibn Sīnā, an-Najāt 152,12-154,5: 56 157,12-13: 303 Ibn Tibbon, Otot ha-Shamayim 1,17-23: 7 1,181-213: 46 11,273-289: 167, 192 II312: 167n20 II322-6: 168 11,436-9: 213 11,470-81: 213 111,2-6: 229 111,18-20: 230 11137-62: 230 111,113-38: 263 111,174: 264n51 111,189-229: 265 111,288: 302n4 111,289: 302n5 al-Kindï, Rasà'il II 70-75: 107 76-78: 108 80-85: 109, 233 90-100: 50 103-108: 274 Olympiodorus, in Meteor. 1,18-233: 39 5,24-630: 311 10,25-26: 39 1645-22: 41 32,7-33,16: 44 35,15-21: 44 38,10-16: 71 38,28-31: 67 40,19-22: 41 43,25-6: 72 44,8-15: 72

Olympiodorus (continued) 44,33-45,10: 72 44,37-45,1: 72 533-9: 70n7 5931-60,14: 73, 79 6831-33: 71 73,2-5: 71 74,17-763: 80 75,24-76,5: 74 79,32-80,29: 102 80,30-81,10: 100, 103 81,13-82,3: 103 82,17-25: 39 84,19-26: 101 86,14-873: 104 88,37-89,10: 104 9330-5: 104 94,4-17: 100 973-34: 164 99,12-100,6: 127, 137i 103,24-30: 127 106,23-31: 127 111,28-112,22: 128 112,22-113,17: 128 1163-33: 128 126,3-127,14: 129 127,14-128,8: 130 133,8-13432: 130 13432-135,15: 131 135,29-137,11: 124n3 150,29-153,11: 132 156,26-159,23: 133 1623-8: 133 168,21-171,23: 163 172,6-11: 34 172,17-29: 163 174,30-175,30: 164 176,30-177,13: 164 177,13-27: 165 181,11-23: 165n15 1823-1833: 165 184,16-18: 195n2 185,8-189,10. 166 186,11-187,1: 159n2 187,3-14: 166 192,28-34: 165 194,14-25: 159n3 19533-9: 166 200,6-201,10: 228 201,10-20: 228 202,14-27: 233n19 204,24-205,3: 228 206,7-10: 228 207,16-23: 228 208,18-26: 233n19 210,15-38: 254

Olympiodorus (continued)

211,1-214,28: 255 217,27-218,35: 256 218,35-219,7: 256 219,8-23: 256 222.29-223,17: 257 229.30-230,9: 257 232.21-23340: 253 233.22-239,28: 259 245,3-6: 260 248,8-249,14: 260 255.19-258,5: 260 261,18-28: 261

263.20-264,26: 261 26633-6: 301 2723-274,2: 311 Philoponus, in Meteor.

1,24-2,7: 39 3,14-16: 311 6,19-20: 39 2632-27,9: 40 28,36-29,1: 40 31,28-38: 40 32,31-33,18: 40 35,30-36,7: 40 36.11-22: 40 41,24-43,25: 43 45,24-35: 43 47.27-53,27: 44 55.12-27: 40 58.4-32: 66 61.8-39: 67 62,17-24: 40 65.7-13: 67 67.28-37: 72 68.5-9: 72 693-9: 72 69,21-36: 72 72,36-73,23: 72 105,25-106,13: 71 113,34-11635: 74 122.21-4: 97 12231-123,17: 101 125,10-126,15: 101 126,16-127,1: 102 127,24-6: 102 128,24-129,19: 102 12949-21: 104 Pseudo-Olympiodorus,

83,3-88,2: 47, 311 953-96,7: 78 96.9-18: 79 96,20-97,6: 79 97.8-98,4: 79

Tafsir

Pseudo-Olympiodorus (continued)

98,6-99,19: 80 99,16-100,3: 106 100.5-101,8: 106 101.10-102,9: 107 101.11-17: 107 102,11-103,8: 138 103.10-104,21: 138 105,4-116,12: 141 116,14-117,6: 170 117,9-17: 170 117.20-118,6: 170 118,9-20: 171 118,23-119,23: 171 119,25-120,2: 172 120,4-10: 172 120.21-121,18: 172 122,3-16: 173 122.18-124,23: 195 125.2-8: 195 125.11-127,9: 173 127.11-128,1: 174 128.3-24: 174 1293-12: 174 129.14-130,15: 175 130,17-1313: 175 131,7-11: 175 131.12-20: 175 131.22-1333: 175 133.7-135,21: 215 135.23-138,17: 216 138.19-139,2: 216 139.4-17: 216 139,19-140,6: 217 140.8-17: 217 140.19-141,2: 217 141,4-13: 217 141.15-142,12: 231 142,17-143,14: 232 143.16-144,15: 232 144.17-145,12: 233 145.14-146,10: 267 146.12-21: 267 147.1-15: 268 147,17-148,2: 268 148,4-149,11: 268 149.13-18: 268 149.20-150,7: 268 150.9-24: 269 151.2-10: 269 152.6-21: 270 152,33-153,2ft 270 153,22-154,26: 270 155,2-25: 271 156,2-13: 271 156.15-157,2: 271

Pseudo-Olympiodorus (continued) 157,4-158,14: 272 158,16-159,7: 272 159,9-160,22: 273 161,2-10: 273

Pseudo-Olympiodorus (continued) 161,12-162,9: 274 162,11-21: 274 181,2: 309n28 182,14: 302n7

GREEK-ENGLISH-ARABIC GLOSSARY Not all occurrences are mentioned, only the first occurrence and/or those in key passages. B: Ibn al-Bitriq's translation of the Greek, such as it occurs in the passage(s) corresponding to the mentioned place(s) in Aristotle. The same word is used in all other Arabic treatises, except when another translation is mentioned. H: Hunayn ibn Ishâq's Compendium of the Meteorology. O: The Arabic translation of Pseudo-Olympiodorus' Commentary on the Meteorology by Hunayn ibn Ishāq, revised by Ishāq ibn Hunayn. S: Ibn Sînâ's Kitāb aš-Šifā'. The Arabic verbs may be given here in a form different from that used in the texts, in order to correspond better to the Greek entry. α ί μ ο α ώ δ ε ς χ ρ ώ μ α — blood-red colour 342a36 — humra Β 33,9 — lawn damawi Ο 96,9 o u ξ _ 'goat' 341b3; 341b31 — 'am Ο 96,4 ά λ μ υ ρ ό ι η ς , ά λ μ υ ρ ό ς — saltness, salty 353a33 — mulūha, mālih Β 51,5 ά λ ω ς — halo 371b22, 372b12 — hāla Β 88,3; istidāra Β 88,4 — da ira H 271; O 147,2 άναθυμίασίς — dry, hot exhalation 340b26 — dukàn B 19,7; wahaj B 19,11 exhalation 341b7 — bukār Β 30,13; 344a10; 358a22 — bukār Β 60,9; 359b28 — bukār Β 63,5; 369a12 — bukār Β 81,6;^ 378a18 άνακλαω, ά ν ά κ λ α σ ί ς — reflect; ά ν α κ λ ά ο μ ο α — reflect (intr.) 340a28 — in'akasa Β 17,1; 342b6 — raja'α Β 34,3 άνεμος — wind 360a19 — rīh Β 64,5 ά ν τ ί π ε ρ ά σ τ η μ ί , ά ν ΐ ί π ε ρ ί σ τ α σ ί ς — concentrate by surrounding, recoil 347b6; 348b2 — dādda Β 40,6; nāfara Β 41,1; nàfara Η 74; ta'âqaba S 18,8; 37,1 αστραπή — lightning 369a10 — barq Β 80,13 άτμίς, ά ί μ ι δ ώ δ η ς — vapour (moist exhalation) 340a34 — bukār Β 17,5; 340b3; 340b27 — bukār Β 19,7; 341b8; 341b10; 359b30; 360a23; 378a19 β ό θ υ ν ο ς — trench 342a36 — wādin Β 34,9; tajwïf Β 97,2 β ο ρ έ α ς — north wind 361a6 — šimàl Β 66,5 β ρ ο ν τ ή — thunder 369a10 — ra'd Β 80,13 γ ά λ α — Milky Way 338b22 — majarra Β 12,1 δαλός — torch 341b3; 341b32 — misbāh Ο 95,21; N.B. sirāj Β 32,2 does not correspond to 341b32 δ ι α δ ρ ο μ ή — shooting-star 341a33, 342a7 δ ι α θ έ ω ν α σ τ ή ρ , see also διάττων α σ τ ή ρ — shooting-star 341b2 δ ι α κ ρ ί ν ω , διάκρίσίς — dissolve, evaporate, rarefy 340a10; 340a29; 340b3; 340b13; 341a17; 345a8; 346b22 — atāra Η 46; hallala Ο 99,16; 354b30 δ ι α λ ύ ω — dissolve 347a35 — 'hallala Β 38,2 διάττων ά σ τ ή ρ — shooting-star 341b35 — sihāb Β 32,4; Η 305; Ο 85,11 — kawkab munqadd Η 314; Ο 95,3 διαφαίνω — shine through (tr.>, διαφαίνομαι — shine through (intr.) 342b6

διηθέω, see ήθέω δ ρ ό σ ο ς — dew 347a16 — nadan Β 37,10 — tall Ο 100,20

έκκαίω, έ'κκαυσίς — ignite, inflame; έ κ κ α ί ο μ α ΐ — be ignited 341b20 — iltahaba Β 31,8; Ο 95,5 — ista'ala Ο 95,5 έ κ ν ε φ ί α ς — hurricane 365a1; 370b8 — rīh sahābiyya Ο 142,18 έκπυρούμενον ΚΟίί Κίνούμενον φ ά σ μ α — phenomenon of inflammation and motion 338b23 — šihāb wa-nayzak pl.: šuhub wa-nayāzik Β 12,2 ε κ π υ ρ ό ω , έ κ π ύ ρ ω σ ί ς - ignite, inflame 340b13 — alhaba Β 18,10 ετησίαΐ — Etesian winds 361b35 — riyāh hawliyya Ο 120,22 ήθεω, διηθέω — filter, seep (through) 353b15 — saffā Β 52,8 θ ά λ α π α — sea 353a32 — bahr Β 51,5 ίδρώς — sweat 353b12 — 'araq Β 52,6 TptÇ - rainbow 371b26, 373a32 — qaws quzaha Β 89,12 κ α ί ο μ έ ν η φ λ ό ξ — burning flame 341b2 — 'amid an-nār Β 30,11; 341b26 — 'amūd an-nār Β 31,10 — lahīb Ο 95,17 καπνός, κ α π ν ώ δ η ς — smoke (hot, dry exhalation) 341b10; 359b32 — wahaj dukāni Β 63,6; 360a25; 378a19 — dukān Β 98,4 κ α τ α κ λ υ σ μ ό ς — flood 352a33 — garaq Β 48,10 — lù fan Ο 104,12 κ ε ρ α υ ν ό ς — thunderbolt 339a3; 371a19 — sà'iqa Β 87,9 κομήτης — comet 338b23 — kawkab du d-du'âba, pl.: kawākib dawàt ad-dawà'ib Β 12,1; 344a21 — kawkab du d-du'àba B 30,4; O 97,11; kawkab dû d-danab O 97,8 κ ρ η ν η — spring 350a5 — yanbū' Β 44,4 κ ρ ύ σ τ α λ λ ο ς - ice 347b36 κ ΰ μ α — tidal wave 368a34 — tufàn Β 80,2 λ ί μ ν η — lake 353b24 — buhayra Ο 107,16 λ ο ξ ο ς — oblique, horizontal 361a23 — mu'wajj Β 67,8 λ ύ χ ν ο ς — lamp 342a3 — sirāj Β 32,8 μεταλλευόμενος — metal, lit: what is mined 384b32 — mā karaja min al-ma'ādin Β 116,4-5 μ ε τ α λ λ ε υ τ ό ς — metal, lit.: what can be mined 378a21 ν έ φ ο ς — cloud 340a25 — sahāb Β 16,9; 346b33 — gaym Β 35,3 sahàb Β 36,10 ν ό τ ο ς — south wind 361a6 — janūb Β 66,5 ό μ ι χ λ η - mist 346b33 — dabāb Β 374 π α ρ ή λ ι ο ς — mock sun 372a10, 377b15 — Sams, pl. "sumūs Ο 161,12; šumaysa S 56,3 π ά χ ν η — hoarfrost 347a16 — jalid Β 37,9 — saqī' S 36,9 περίττωμα — residue 355b8; 356b2; 357a33 — jadla Β 59,3 π η γ ή — source 350b28 — 'ayη Β 45,15; 353a35 — yanbu Β 51,7; 353b20 — 'ayn Β 533; Ο 108,2 — yanbū' Β 533; Ο 108,2 π ή γ ν υ μ ί — freeze 347a17 — jamada Β 37,9 π ν ε ύ μ α , π ν ε υ μ α τ ώ δ η ς — wind 338b26 — rīh Β 12,4; 341b9; 360a13 π ο τ α μ ό ς — river 349b2 — nahr Β 42,11 πρηστήρ — fire-wind 339a4; 371a16 — not in Β π ρ ο σ χ ο ω , π ρ ό σ χ ω σ ι ς - silt 35lb7 — tamara ο 103,22 π υ κ ν ό ω , π ύ κ ν ω σ ί ς — condense 342a12; 344a16; 348b11 ο ρ υ κ τ ό ς — mineral, lit.: what is quarried 378a20 ρ ά β δ ο ς — rod 372a11, 377a31 — 'amūd, pl. a'mida Β 972; 'asan, pl. 'isīy Ο 16241; nayzak S 56,5 ρίπτέω, ρίψις — throw, project 342a2 — qadafa Β 32,7 σ ε ι σ μ ό ς — earthquake 338b26 — zalzala Β 12,4; 365a14; 365b23 σ υ γ κ ρ ί ν ω , σ υ γ κ ρ ί σ ι ς — gather, cluster, make denser, condense 341a4; 342a29; 344b9; 346a23; 346b22; 346b34 σ ύ μ φ α σ ί ς — conjunction 342b28 — ijtimà' Β 26,9 σ ύ ν ε ΐ μ ΐ (εΤμΐ) — condense 342a19; 342b17 συνίστημι, σ ύ σ τ α σ ι ς — gather, cluster, make denser, condense; σ υ ν ί σ τ α μ α ι — gather (intr.), become denser 340a34 — ijtama'a Β 173 takātafa Β 17,6; 342a1; 342b1 — katufa Β 34,1; ijtama'a Β 34,2; takātafa Η 318; gaÍuza Η 318; 344b11; 345b34; 346b29 — galuza; takàtafa Β 36,7; takātafa; inqabada Η 55; 360a1 τ έ λ μ α — pool 353b24 — a jama Β 53,6; Ο 107,20

GLOSSARY

ΐ υ φ ώ ν — whirlwind 339a3; 371a2 — zawba'a Β 86,1 ύετός — rain 347a12 — malar Β 37,5 ύ π έ κ κ α υ μ α — fuel, inflammable material 341b19 ύ π ό σ χ α σ ί ς — sediment 355b8 φ λ ό ζ , see κ α ι ο μ έ ν η φ λ ό ζ φ ρ έ α ρ — well 347b9; 353b26 — bir Ο 107,16; S 13,5 χ ά λ α ζ α — hail 347b28 — barad Β 39,10 χ ά σ μ α — chasm 342a35; 342b17 — jawba Β 34,9; hāwiya Ο 96,19 χ ι ώ ν — snow 347b16 — talj Β 39,4 ψ α κ ά ς — drizzle 347a11 — nadan Β 37,4

INDEX OF NAMES AND SUBJECTS Abü 1-Barakāt al-Bagdädi 2 survey of his works on meteorology 11 Aeschylus (pupil of Hippocrates of Chios) his theory of comets 70 al-'Alawi 191n106, 293 alchemy Ibn Sīnā 29, 304 Abū 1-Barakât 29, 306 Fakr ad-Din 29, 307-9 Alexander of Aphrodisias 3 survey of his commentary on the Meteorology 6 - 7 Alfred of Sareshel 4 antapodosis a cause for wind; Theophrastus 160-1 ant i peris tasis 20, 97-98 Philoponus 101-2 Ibn al-Bitriq 105 Hunayn 105 a cause for shooting-stars; Olympiodorus 71; Fakr ad-Din 86 a cause for rain in Ethiopia 20, Alexander 20, 100; Olympiodorus 103 a cause for the rising of the Nile; Olympiodorus 100 a cause for hail 20, 99; Pseudo-Olympiodorus 20, 21, 106-7; al-Kindi lia, Ibn Sīnā 20, 21, 112; Bahmanyār 114; Fakr ad-din 116; Ibn Rušd 118 Ammonius 74, 132, 166, 259, 261 Anaxagoras his theory of comets 69 his theory if the Milky Way 70-71 his theory of earthquakes 209, 214, 218 his theory of thunder and lightning 225 Anaximenes his theory of earthquakes 209, 214, 218 Aristotle Meteorology Book IV 3-4; Alexander 29, 301, 310; Olympiodorus 29, 301, 310; Philoponus 311; PseudoOlympiodorus 29, 311; Ibn al-Bitriq 311; Ibn Sînā 29, 311; Ibn Bäjja 29, 60, 311; Ibn Rusd 29, 61, 312; Meteorology Latin versions 4 Meteorology its subject matter 14 ff.

atmosphere structure of the — 15-16, 33-5, 37-9; Olympiodorus 39; Ibn al-Bitriq 16, 45-6; al-Kindi 51-3; Ikwān asSafā' and al-Qazwini 53; Ibn Sīnā 17, 55-6, 56-7; Ibn Bājja 59, 61; Ibn Rusd 62; Gilbert 38-9 Bahmanyār ibn al-Marzubân 2 survey of his works on meteorology 11 Baffioni 4, 310n31, 312 blood-red colours see red colours blue colour (of the sky) al-Kindi 274-5 Bahmanyār 283 burning flames 18, 67 Ibn al-Bitriq 74 Pseudo-Olympiodorus 78 Ibn Rusd 89, 92 celestial world the material of the — 35-6 canal 143 chasm 18, 67 Greek commentators 71-2 Ibn al-Bitriq 75 Pseudo-Olympiodorus 79 Ibn Sīnā 82 Ibn Rusd 89 circular motion of the upper atmosphere 37, 38; Alexander 40; Philoponus 40; al-Kindi 53; Ibn Bäjja 58, 60; Ibn Rusd 63 clouds 20, 97 Philoponus 101 Olympiodorus 101 Pseudo-Olympiodorus 106 Ibn Sīnā 111-2 Bahmanyār 113 Abü 1-Barakāt 115 where are — formed? 15-6, 36-7, 38; Alexander 40; Philoponus 40; al-Kindi 51-2 Cleidemus his theory of lightning 226 clustering 66n2 colour see light, rainbow

comet 18, 68 a weather-sign 68 Aristotle's refutation of other opinions 69-70, Ibn al-Bitriq 75; Ibn Rusd 93 Olympiodorus 72-3 Pseudo-Olympiodorus 79 Ibn Sīnā 81 Abù 1-Barakāt 83 Fakr ad-Din 85 Ibn Rusd 89, 93-4 compression a cause for precipitation; Theophrastus 20, 100, 102; Alexander 20, 100; Olympiodorus 20, 100; PseudoOlympiodorus 20, 106; Ibn Sīnā 20, 112 a cause for the rising of the Nile; Olympiodorus 100 concentrate by surrounding see antiperistasis condensation 20-21 conjunction a comet as a — of planets 69 Daiber 2, 5, 9 Democritus his theory of comets 69 his theory of the Milky Way 70-1 his theory of the sea 125, 138 his theory of earthquakes 209, 215, 218 dew 20, 97 see also precipitation al-Kindi 1081 Ibn Sīnā 112 Dietrich von Freiberg 28, 299-300 ad-Dimašqī 2 dissolution 66n2 drizzle 97 Abū 1-Barakāt 114 Ibn Rusd 117 downpour Ibn Sina 112 Ibn Rušd 117 earthquake 25, 209-12 Ibn al-Bitriq and Hunayn 25, 212-4 Pseudo-Olympiodorus 25, 214-7 Ibn Sina 25, 218-21 Abū 1-Barakāt 221 Fakr ad-Din 221-2 Ibn Rusd 25, 222-4 Aristotle's refutation of other opinions 209

earthquake (continued) various kinds of — 212; PseudoOlympiodorus 217; Ibn Sīnā 220; Ibn Rusd 223 circumstances under which an — may occur 209-10; Ibn al-Bitriq and Hunayn 212-3; Pseudo-Olympiodorus 215; Ibn Sina 219-20 Ibn Rušd 222, 223-4 phenomena which may accompany an — 210-1; Ibn al-Bitriq and Hunayn 213; Pseudo-Olympiodorus 215-7; Ibn Sina 219, 220; Ibn Rusd 222, 223-4 Empedokles his theory of the saltness of the sea 125, 144 his theory of thunder and lightning 225 Etesian winds see wind error (in vision) see vision ether 36 evaporation see also dissolution — from the sea 124; Olympiodorus 131, 133; Pseudo-Olympiodorus 138, 139; Ibn Sīnà 144; Ibn Rusd 151 exhalation theory of the double — 15, 33-5, 37, 37-8; Alexander 40; Olympiodorus 16, 41; Philoponus 16, 40; Ibn al-Bitriq 45-7; Hunayn 47; Pseudo-Olympiodorus 16-7, 48-9; al-Kindi 17, 50-2; Ibn Suwār 17, 54, 275-6; Ibn Sīnā 17, 54, 56-7; Ibn Bājja 17, 59, 59-60; Ibn Rusd 17, 62, 63; Gilbert 38 three exhalations distinguished: Ibn al-Bitriq 16, 46; Ibn Rusd 17, 62, 63 phenomena due to —; PseudoOlympiodorus 49-50; Ibn Sina 57 moist —; Philoponus 40 moist —, hot or cold? 15, 34n4; Philoponus 40; Olympiodorus 16; al-Kindi 17, 51 phenomena due to moist —; see precipitation phenomena due to dry — in the upper atmosphere 18, 66, 68, 69; Ibn al-Bitriq 74-5; Hunayn 77-8; Pseudo-Olympiodorus 78-80; Ibn Sīnā 80-2; Bahmanyār 82-3; Abū 1-Barakāt 83-5; Fakr ad-Din 85-6; Ibn Bäjja 59-60; Ibn Rušd 88

exhalation (continued) greasy exhalation; Ibn Sīnā 235; Fakr ad-Din 238-9, 284 expansion a cause for wind; al-Kindi 23, 107 Fakr ad-Din ar-Rāzī 2 survey of his works on meteorology 11 a1-Fārisi 28, 299 filtration 126, 133, 135, 144 fire Ibn Sīnā 19, 56, 81; Fakr ad-Din 86; Ibn Rusd 63-5 firewind (prèstèr) 227 Pseudo-Olympiodorus 232 flame see burning flame flood 122, 138 Fontaine 5 Gerard of Cremona 4 Gilbert, O. 34n4, 38, 38-9 'goat' 18, 67 Pseudo-Olympiodorus 78 Ibn Rušd 89 hail 20, 98-9 see also precipitation, antiperistasis Philoponus 102 Olympiodorus 104 Ibn al-Bitriq 21, 105 al-Kindi 21, 109-10 Pseudo-Olympiodorus 21, 106-7 Ibn Sīnā 21, 112-3 Abu 1-Barakāt 21, 114-5 Ibn Rušd 118, 118-9 more — in autumn than in spring; Olympiodorus 104; Pseudo-Olympiodorus 107; Ibn Sīnā 112; Fakr ad-Din 116 halo 26, 243, 247 see also reflection Theophrastus 26, 251-2 Alexander 27, 252 Olympiodorus 27, 254-7 Ibn al-Bitriq 262-3 Pseudo-Olympiodorus 27, 267-9 Ibn Suwār 27, 276 Ibn Sīnā 280-1 Abû 1-Barakāt 28, 85, 283 Ibn al-Haytam 28, 285-6 Ibn Rusd 28, 288-9, 292-4 description 243; Pseudo-Olympiodorus 267-8 its diameter 257, 268, 281

halo (continued) is an optical phenomenon; Olympiodorus 254, 257; Ibn Sīnà 278-9, 280; Fakr ad-Din 283; Ibn Rusd 287 why is the — a ring? Theophrastus 251-2; Alexander 252; Olympiodorus 256; Pseudo-Olympiodorus 269; Ibn Suwār 276; Ibn Sīnā 280 geometrical explanation of its form 26, 247; Ibn Sīnā 280; Ibn al-Haytam 28, 286; Ibn Rusd 288-9, 293 a weather-sign 247; Ibn al-Bitriq 262; Ibn Sīnā 280; Ibn Rusd 294 more than one — visible; PseudoOlympiodorus 268-9; Ibn Suwâr 276; Ibn Sīnā 280-1; Ibn Rusd 289. comparison with eclipses; PseudoOlympiodorus 273 heaven see celestial world Henricus Aristippus 4 Hippocrates of Chios his theory of comets 70 hoarfrost 20, 97 see also precipitation Ibn al-Bitriq 105 Ibn Sīnā 112 Holmyard 4 horizontal motion (of wind) see wind Hunayn ibn Ishäq 3 survey of his Compendium of the Meteorology 8-9 hupekkauma 15, 34, 66 hurricane 25, 160, 226 Pseudo-Olympiodorus 175, 231, 232 Ibn Sīnā 178 Ibn al-'Amid 1, 2 Ibn Bäjja 2, 6 survey of his Commentary on the Meteorology 12 Ibn al-Bitriq 3 survey of his version of the Meteorology 7-8 influence on other works 8 Ibn al-Haytam 3 halo 11, 285-6 rainbow 11, 286-7 Ibn Rusd 2 survey of his commentaries on the Meteorology 12-14 Ibn Sinä survey of his work on meteorology 10-11

Ibn Sīnā (continued) De Mineralibus 4 Ibn Suwār ibn al-Kammâr 2, 3, 5-6 survey of his Treatise on Meteorological Phenomena 10 Ibn Tibbon 4 - 5 survey of his version of the Meteorology 14 ice 63, 64, 98 Olympiodorus 102 ignition of the hupekkauma 18, 66, 68, 69; Ibn Sina 19 of greasy exhalation; Ibn Sina 235; Fakr ad-Din 238-9, 284 Ikwän as-Safä' 1, 2 precipitation 9 wind 9 inflammable material see hupekkauma inflammation see ignition inhabitable regions 24, 157-8, 194 Ptolemaeus 195, 205 Pseudo-Olympiodorus 194-5 Ibn Sina 24, 195-9 Abû 1-Barakāt 24, 199-202 Fakr ad-Din 24, 203-4 Ibn Rusd 24-5, 204-8 how are the — determined by the sun? Ibn Sina 196-9; Abü 1-Barakät 199-202; Fakr ad-Din 203-4; Ibn Rušd 205-6, 207-8 does the excentricity of the sun play a role? Ibn Sina 196; Fakr ad-Din 204 Jābir ibn Hayyān 2 juxtaposition (of elements) 58, 62, 131, 132, 140, 171 al-Kindi 2 survey of his works on meteorology 10 leakage water 143 lake Olympiodorus 127, 129; PseudoOlympiodorus 139 lamp (a phenomenon in the upper atmosphere) Ibn al-Bitriq 74 Ibn Rušd 92 light Aristotle's theory of — and colour 243-6

light (continued) survey of various theories of — by Ibn Sīnā 277-8 lightning 25, 225 Alexander 227 Ibn al-Bitriq 25, 228-9 Pseudo-Olympiodorus 231, 232 al-Kindi 25, 233-4 Ibn Sina 26, 234-6 Bahmanyār 237 Abū 1-Barakāt 26, 237 Fakr ad-Dīn 238 Ibn Rušd 26, 239, 241 Aristotle's refutation of other opinions 225-6 why does — move downward? 225; Ibn al-Bitriq 229-30 al-Maqdisi 1, 2 Marinus 53 a1-Mas'ūdī 1, 2 metals 28-9, 301 see also minerals Ibn al-Bitriq 302 Ibn Sina 145, 302-4 Abü 1-Barakāt 29, 304-6 Fakr ad-Din 306-7 meteor see shooting star Milky Way 18, 69 Aristotle's refutation of other opinions 70-1 Philoponus 18, 73-4 Olympiodorus 18, 74 Ibn al-Bitriq 18-9, 76-7 Pseudo-Olympiodorus 18, 79-80 Abü 1-Barakāt 19, 85 Fakr ad-Din 19, 86 Ibn al-Haytam 82n53 Ibn Bäjja 19, 86-8 Ibn Rusd 19-20, 90-1, 94-6; influence of Ibn Bājja 91-2 minerals 28-9, 301 Ibn al-Bitriq 302 Ibn Sina 28, 302-4 Bahmanyār 304 Abü 1-Barakāt, 304-6 Fakr ad-Din 306-7 mist 97 Alexander 99 Pseudo-Olympiodorus 106 al-Kindi 108-9 Ibn Sīnā 113 Fakr ad-Din 117 mixing (of elements) 58, 62, 131, 140, 171

mock sun 243, 251 Olympiodorus 256 Pseudo-Olympiodorus 273-4 Ibn Suwār 277 Abū 1-Barakāt 85 Fakr ad-DIn 284-5 moisture local variation of — 121-2; Olympiodorus 128; Ibn al-Bitriq 135; Pseudo-Olympiodorus 137-8; Ibn Sīnā 144-5; Ibn Rusd 150, 153 mountains formation of —; Ibn Sīnā 142; Abü 1-Barakāt 146 advantages of —; Ibn Sīnā 142 a mountain is like an alembic; Ibn Sīnā 142; Ibn Rusd 150 Najm ad-Din 2 an-Nāši' 1, 2 Nile the rising of the —; Olympiodorus 100 an-Nuwayri 2 Olympiodorus 3 survey of his commentary Meteorology 7

on

the

Philippus (Plato's associate) 253 Philoponus 3 survey of his commentary on the Meteorology 7 Plato his theory of rivers and the sea 124-5 pool Olympiodorus 129; Ibn al-Bitriq 135; Pseudo-Olympiodorus 139 Posidonius real and imagined phenomena 39 halo 252 precipitation 20, 97-9 Theophrastus 20, 100, 102-3 Alexander 20, 100 Olympiodorus 20, 100, 102-4; relative temperatures of — 103-4 Ibn al-Bitriq 105 Pseudo-Olympiodorus 20, 106-7 al-Kindi 21, 107-11 Ikwān as-Safā' and al-Qazwini 111 Ibn Sinā 20, 112-3 Bahmanyār 113-4 Abü 1-Barakāt 114-6 Fakr ad-Din 116-7 Ibn Rušd 117-9

Pseudo-Olympiodorus 3 survey of his commentary on Meteorology 9 Ptolemaeus 53, 72n14, 74, 82, 197n11 on the inhabitable world 195, 205 Pythagoreans their theory of comets 69 their theory of the Milky Way 70

the

al-Qazwini 1, 2 precipitation 9 wind 9 rain 20, 97 see also precipitation Pseudo-Olympiodorus 106 al-Kindi 107-8 Abü 1-Barakāt 114-6 in Ethiopia see compression, antiperistasis bitter or salty — (esp. in Thebes) 126; Olympiodorus 103, 132, 133 Pseudo-Olympiodorus 139-40; Ibn Rusd 151 size of raindrops in a thunderstorm see thunder rainbow 26, 243, 247-50 see also reflection Olympiodorus 257-61 Ibn al-Bitriq 27, 263-6 Hunayn 266 Pseudo-Olympiodorus 269-73 Ibn Suwār 276-7 Ibn Sīnā 27-8, 281-3 Abü 1-Barakāt 28, 85, 283 Ibn al-Haytam 28, 286-7 Ibn Rusd 289-92, 294-8 a1-Fārisi 298-9 Dietrich von Freiberg 299-300 description 243; Pseudo-Olympiodorus 269 is an optical phenomenon; Alexander 253; Olympiodorus 254; Fakr ad-Din 283; Ibn Rusd 287 difference between the — and the halo 247-8; Olympiodorus 255-6, 257 ; Pseudo-Olympiodorus 270, Ibn Rusd 295-6 geometrical explanation of its form 26, 249-50; Olympiodorus 260, Pseudo-Olympiodorus 270; Ibn Suwār 277; Ibn al-Haytam 28, 286-7; Ibn Rusd 28, 290-1, 294, 297-8

rainbow (continued) its colours 26, 243, 248-9; Olympiodorus 27, 258-60, Ibn al-Bitriq 263, 264-5; Pseudo-Olympiodorus 27, 270-2; Ibn Suwār 27, 276; Ibn Sīnā 27-8, 282-3; Ibn Rusd 28, 291-2, 295-7 its radius 243, 250; Olympiodorus 261; Ibn al-Bitriq 263; Pseudo-Olympiodorus 270; Ibn Sīnā 282 the area between both rainbows is dark; Alexander 253-4 comparison with eclipses; PseudoOlympiodorus 273 comparison with lunar phases; Ibn Suwār 277 rarefaction see dissolution recoil see antiperistasis red colours (in the sky) 18, 67 Greek commentators 71-2 Ibn al-Bitriq 75 Hunayn 77 Pseudo-Olympiodorus 78-9 Ibn Sīnā 82 Abü 1-Barakāt 84-5 Ibn Rušd 89, 92-3 reflection see also halo, rainbow Olympiodorus 254-5, 256 Pseudo-Olympiodorus 267 laws of — 246; Olympiodorus 255; Fakr ad-Din 283 — in the case of the halo and rainbow 246-7; Olympiodorus 256; Pseudo-Olympiodorus 267; Ibn Sīnā 279-80; Fakr ad-Din 283-4; Ibn Rusd 295 of light from the upper atmosphere against clouds 68; Ibn al-Bitriq 75; Pseudo-Olympiodorus 78-9; Ibn Sīnā 82 refraction Olympiodorus 254-5 Pseudo-Olympiodorus 267 what is seen by — seems larger 246-7; Alexander 252; Olympiodorus 255; Pseudo-Olympiodorus 267 of light from the upper atmosphere through clouds; Pseudo-Olympiodorus 78 of light from the stars through the upper atmosphere; Ibn Bājja 87

residue is the sea is like a —? 124; Ibn alBitriq 136; Ibn Rusd 151 rivers 21, 120-1 Plato's theory 124-5 Olympiodorus 127, 129 Ibn al-Bitriq 134 Hunayn 21, 134-5 Pseudo-Olympiodorus 137, 139 Ibn Sīnā 142 Abü 1-Barakāt 21, 146-7 Ibn Rušd 149-50, 152-3 rods 243, 250-1 Olympiodorus 256, 261 Ibn al-Bitriq 266 Abū 1-Barakāt 85 Pseudo-Olympiodorus 274 Ibn Suwār 277 saltness (of the sea) 21, 125-7 Empedokles' view 125 Theophrastus' view 132 Olympiodorus 22, 130-3 Ibn al-Bitriq 22, 135-6 Hunayn 136 Pseudo-Olympiodorus 22, 139-41 Ibn Sina 22, 143-5 Abū 1-Barakāt 148 Ibn Rušd 22, 151-2, 154-5 sandbank 122 sea 22, 122-7 see also evaporation, residue, saltness, sediment, sweat Democritus' theory 125, 138 Plato's theory 124-5, 138 Olympiodorus 128-133 Ibn al-Bitriq 135-6 Pseudo-Olympiodorus 138-41 Ibn Sina 143-5 Abü 1-Barakāt 148 Fakr ad-Din 149 Ibn Rusd 150-2, 153-5 the — has no sources 123; Olympiodorus 129-30; Ibn al-Bitriq 135; Pseudo-Olympiodorus 138-9; Ibn Rusd 150 the — is flowing 123; PseudoOlympiodorus 139; Ibn Sina 145 is the — the main body of the element water? 123-4; Olympiodorus 130-1; Pseudo-Olympiodorus 138; Abü 1-Barakāt 148; Ibn Rušd 150, 153 seawater is heavier than fresh water 126; Olympiodorus 103; PseudoOlympiodorus 140; Ibn Sīnā 144

sediment is the sea is like a — ? 124 Sersen 1, 53 Sezgin 2 shells found in Egypt; Olympiodorus 128; Pseudo-Olympiodorus 138; Ibn Sīnā 145; Ibn Rusd 150 shining through shining of light from the upper atmosphere through clouds 68; Ibn Rusd 89 shooting star 18, 67 Alexander 67 Olympiodorus 67 Philoponus 67 Ibn al-Bitriq 75 Hunayn 77 Pseudo-Olympiodorus 78 Ibn Sīnâ 80-1 Abü 1-Barakāt 83 Fakr ad-Din 85 Ibn Rusd 88-9, 92 sight see vision silt 121, 122 Olympiodorus 128; Pseudo-Olympiodorus 137; Ibn Rušd 150 smoke see exhalation, dry — snow 20, 98 see alo precipitation Ibn al-Bitriq 105 al-Kindi 110 Ikwān as-Safā' 111 Ibn Sīnā 112 Abü 1-Barakāt 114 zamharīr 53, 115; al-Kindi 110 Sosigenes 252 source the sea has no sources; see sea of a river 121; Ibn al-Bitriq 134; Ibn Sīnā 142, 143; Fakr ad-Din 148 spring 121 Olympiodorus 127, 129 Stapleton 4 Steinmetz 5, 34n4 stone formation of —; Ibn Sīnā 142; Abü 1-Barakât 146

sun how is the earth heated by the — ? 16, 37; Alexander 16, 41-2; Philoponus 16, 42-4; Olympiodorus 16, 44; Ibn Sīnā 17, 56, 197-9; Abü 1-Barakāt 199; Ibn Bājja 17, 58-9; Ibn Rušd 17, 62, 65 how is the climate determined by the —? see inhabitable regions vertically inciding rays of the sun are stronger than obliquely inciding rays; Ibn Sīnā 197; Abü 1-Barakāt

200

is the — fed by vapour? 124n3; Ibn al-Bitriq 135 sweat is the sea the — of the earth? 123, 125; Olympiodorus 129, 131-2; Ibn Rusd 154 at-Tabari 1, 2 Theophrastus 5, 301 thunder see lightning size of raindrops in a thunderstorm; Ibn al-Bitriq and Hunayn 229; Ibn Rusd 241 ' colours of a thundercloud; Ibn al-Bitriq 230; Ibn Rusd 242 thunderbolt 25, 227 Ibn al-Bitriq 230 Pseudo-Olympiodorus 232-3 al-Kindi 234 Ibn Sīnā 236-7 Abü 1-Barakāt 237-8 Fakr ad-Din 238 Ibn Rusd 239-40, 241-2 why does a — move downward? Ibn Sīnā 236; Ibn Rusd 240 tidal wave 211 Ibn al-Bitriq and Hunayn 214; Ibn Rusd 223 tides Ibn Rusd 153, 154 at-Tīfāšī 1 torch 18, 67 Pseudo-Olympiodorus 78 Ibn Rusd 89 trench 18, 67 Greek commentators 71-2 Ibn al-Bitriq 75 Pseudo-Olympiodorus 79 Ibn Rusd 89

vapour see also exhalation, moist — Philoponus 101 Olympiodorus 101 Pseudo-Olympiodorus 106 vision see also light, reflection, refraction, visual rays weakening of — 245-6; Ibn Rusd 288 error in —; Olympiodorus 258-9; Pseudo-Olympiodorus 268, 270-3; Ibn Suwār 276; Ibn Sīnā 279 visual rays theory of visual rays 245 cone of —; Olympiodorus 256, 259; Pseudo-Olympiodorus 272 a1-Watwāt 2 Webster 34n4, 68, 67n1 well 121 Olympiodorus 127, 129; PseudoOlympiodorus 138; Ibn Sīnā 143; Abü 1-Barakāt 147-8 whirlwind 25, 226-7 Olympiodorus 228 Ibn al-Bitrlq 229 Pseudo-Olympiodorus 231-2 Ibn Sīnā 178-9 Ibn Rusd 240 William van Moerbeke 4 wind 22, 120, 156-60 Theophrastus 22, 160-1 al-Kindi 23, 107-8, 176 Ikwān as-Safā' and al-Qazwini 176-7 Ibn Sīnā 23, 177-8 Bahmanyār 183 Abü 1-Barakāt 23, 183-5 Fakr ad-Din 185-6 dry exhalation is the matter of — 156; Olympiodorus 162; PseudoOlympiodorus 169-7fr, Ibn Rusd 187 the north and south winds 156, 157, 158; Olympiodorus 162-3, 165-6; Ibn al-Bitriq 167, 169; PseudoOlympiodorus 170-1; Ibn Sīnā 181; Ibn Rusd 188-90, 191

wind (continued) Etesian winds and the times at which they start to blow 157; Alexander 164-5; Olympiodorus 164-5; Ibn al-Bitriq 168; Pseudo-Olympiodorus 172, 173; Ibn Sīnā 182; Ibn Rusd 188-90, 192-3 white south wind 158; Greek commentators 165; PseudoOlympiodorus 172; Ibn Sina 181 bird (or chicken, or egg) wind 158; Greek commentators 165; PseudoOlympiodorus 172; Ibn Sīnā 181 east — is warmer than west — 159; Pseudo-Olympiodorus 174; Ibn Rusd 190-1 names and directions of winds 158-9; Olympiodorus 166; Ibn al-Bitriq 168; Hunayn 168, PseudoOlympiodorus 173—4; Ibn Sini 179; Fakr ad-Din 186; Ibn Rusd 187 characteristics of various winds 159-60; Pseudo-Olympiodorus 174-5; Ibn Sínā 180-1; Ibn Rusd 190-1 horizontal (oblique) motion of — 22, 157; Theophrastus 22, 161-2; Alexander 22, 161-2; Olympiodorus 22-3, 163—4; Pseudo-Olympiodorus 23, 171; Ibn Sīnā 23, 177; Abü 1-Barakāt 183-4; Fakr ad-Din 185; Ibn Rusd 23, 187-8, 192 formation of — supported by moisture 22, 156; Olympiodorus 163; Pseudo-Olympiodurus 49, 170; Ibn Sina 178; Ibn Rusd 187 zamharir see snow