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 PHYSICS

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PHYSICS

rors", in which the principles of catoptrics were cor- rectly set forth. Dioptrics, being more difficult, was developed less rapidly. Ptolemy already knew that the angle of refraction is not proportional to the angle of incidence, and in order to determine the ratio be- tween the two he undertook experiments the results of which were remarkably exact.

Statics reached a fuUer development than optics. The "Mechanical Questions" ascribed to Aristotle were a first attempt to organize that science, and they contained a kind of outline of the principle of virtual velocities, destined to justify the law of the equi- librium of the lever; besides, they embodied the happy idea of referring to the lever theory the theory of all simple machines. An elaboration, in which Euclid seems to have had some part, brought statics to the stage of development in which it was found by Ar- chimedes (about 287-212 b. c), who was to raise it to a still higher degree of perfection. It will here suffice to mention the works of genius in which the great Syracusan treated the equilibrium of the weights suspended from the two arms of a lever, the search for the centre of gravity, and the equilibrium of liquids and floating bodies. The treatises of Ar- chimedes were too scholarly to be widely read by the mechanicians who succeeded this geometrician; these men preferred easier and more practical writings as, for instance, those on the lines of Aristotle's "Mechan- ical Questions". Various treatises bj' Heron of Alex- andria have preserved for us the type of these de- cadent works.

II. Science .a.nd E.\rly Christian Schol.vhs. — Shortly after the death of Ptolemy, Christian science took root at Alexandria with Origen (about 180-253), and a fragment of his "Commentaries on Genesis", preserved by Euscbius, shows us that the author was familiar with the latest astronomical discoveries, especially the precession of the equinoxes. However, the writings in which the Fathers of the Church com- ment upon the work of the six days of Creation, notably the commentaries of St. Basil and St. Ambrose, bor- row but little from Hellenic physics; in fact, their tone would seem to indicate distrust in the teachings of Greek science, this distrust being engendered by two prejudices: in the first place, astronomy was becoming more and more the slave of astrology, the superstitions of which the Church diligently combatted; in the second place, between the essential propositions of peripatetic physics and what we believe to be the teaching of Holy Writ, contradictions appeared; thus Genesis was thought to teach the presence of water above the heaven of the fixed stars (the firma- ment) and this was incompatible with the Aristotelean theory concerning the natural place of the elements. The debates raised by this question gave St. Augustine an opportunity to lay down wise exegetical rules, and he recommended Christians not to put forth lightly, as articles of faith, propositions contradicted by physical science based upon careful experiments. St. Isidore of Seville (d. 636), a bishop, considered it legitimate for Christians to desire to know the teach- ings of profane science, and he laboured to satisfy this curiosity. His "Etymologies" and "De natura rerum" are merely compilations of fragments bor- rowed from all the pagan and Christian authors with whom he was acquainted. In the height of the Latin Middle Ages these works served as models for numer- ous encyclopa;dias, of which the "De natura rerum" by Bede (about 672-735) and the "De universo" by Rabanus Maurus (776-856) were the best known.

However, the sources from which the Christians of the West imbibed a knowledge of ancient physics became daily more numerous, and to Pliny the Elder's "Natural History", read by Bede, were added Chalcidius's commentary on Pl.ato's "Timseus" and Martianus Capella's "De Nuptiis Philologite et Mer- curii", these different works inspiring the physics of

John Scotus Eriugena. Prior to a. d. 1000 a new Platonic work by Macrobius, a commentary on the "Somnium Scipionis", was in great favour in the schools. Influenced by the various treatises already mentioned, Guillaume of Conches (1080-1150 or 1154) and the unknown author of "De mundi con- stitutione liber", which, by the way, has been falsely attributed to Bede, set forth a planetary theory making Venus and Mercury satelhtes of the sun, but Eriugena went still further and made the sun also the centre of the orbits of Mars and Jupiter. Had he but extended this hypothesis to Saturn, he would have merited the title of precursor of Tycho Brahe.

III. A Glance at Ar.abian Physics. — The authors of whom we have heretofore spoken had only been acquainted with Greek science through the medium of Latin tradition, but the time came when it was to be much more completely revealed to the Christians of the West through the medium of Mussulman tradition.

There is no Arabian science. The wise men of Mohammedanism were always the more or less faith- ful disciples of the Greeks, but were themselves desti- tute of all originality. For instance, they compiled many abridgments of Ptolemy's "Almagest", made numerous observations, and constructed a great many astronomical tables, but added nothing essential to the theories of astronomical motion; their only inno- vation in this respect, and, b}' the way, quite an un- fortunate one, was the doctrine of the oscillatory motion of the equinoctial points, which the Middle Ages ascribed to Thabit ibn Ktlrrah (836-901), but which was probablj^ the idea of Al-Zarkali, who lived much later and made observations between 1060 and 1080. This motion was merely the adaptation of a mechanism conceived by Ptolemy for a totally differ- ent purpose.

In physics, Arabian scholars confined themselves to commentaries on the statements of Aristotle, their attitude being at times one of absolute ser\-ility. This intellectual ser\'ility to Peripatetic teaching reached its climax in Abul ibn Roshd, whom Latin scholastics called Averroes (about 1120-98) and who said: Aris- totle "founded and completed logic, physics, and metaphysics . . . because none of those who have followed him up to our time, that is to say, for four hundred years, have been able to add anything to his writings or to detect therein an error of any impor- tance". This unbounded respect for Aristotle's work impelled a great many Arabian philoso]jhers to attack Ptolemy's "Astronomy" in the name of Peripatetic physics. The conflict between the hypotheses of eccentrics and epicycles was inaugurated by Ibn Bddja, known to the scholastics as Avempace (d. 1138), and Abu Bekr ibn el-Tofeil, called Abubacer by the scholastics (d. 1185), and was vigorously con- ducted by Averroes, the protcg<j of Abubacer. Abu Ishak ibn al-Bitrogi, known by the scholastics as Alpetragius, another disciple of Abubacer and a con- temporary of Averroes, advanced a theorj' on plan- etary motion wherein he wished to account for the phenomena peculiar to the wandering stars, by com- pounding rotations of homoccntric spheres; his trea- tise, which was more neo-Platonic than Peripatetic, seemed to be a Greek book altered, or else a simple plagiarism. Less inflexible in his Peripateticism than Averroes and Alpetragius, Moses ben Maimun, called Maimonides (1139-1204), accepted Ptolemy's astron- omy despite its incompatibility with Aristotelean physics, although he regarded .-iristotle's sublunary physics as absolutely true.

iV. Arabian Tr.adition and Latin Scholasti- cism. — It cannot be said exactly when the first trans- lations of Arabic writings began to be received by the Christians of the West, but it was certainly previously to the time of Gerbert (Sylvester II; about 930-1003). Gerbert used treatises translated from the Arabic,