NA TURE 



49 



THURSDAY, MAY 17, 1906. 



THE MECHANISM OF THE UNIVERSE. 

 History of the Planetary Systems from Thales to 

 Kepler. By Dr. J. L. E. Dreyer. Pp. xii + 432. 

 (C.TmbridsfO : University Press, i()(i6.) Price 10,';. 6d. 

 net. 



BY the publication of this masterly account of the 

 development of cosmogonic ideas and the history 

 ui planetary systems from the early dawn of Greek 

 philosophv to the final establishment of the Copernican 

 system, Dr. Dreyer has rendered a great service to 

 those who take an interest in the fascinating history 

 of astronomical science. Throughout the work we 

 have reason to admire his lucid e.\position, based on 

 profound historical studies, of the manifold views 

 formed by the thinking minds of antiquity and of the 

 middle ages on the mechanism of the universe, his 

 dilinite conclusions on many controversial points, and, 

 itbove all, his endeavour to trace out, as much as 

 possible, the influences of the philosophic and re- 

 ligious tendencies of the time on the cosmological 

 conceptions under view. 



The opening chapters describe the development of 

 astronomical ideas among the tjreelis. After a brief 

 review of the cosmogony of the early atomistic school 

 we are introduced to the more distinct teachings of 

 the Pythagoreans. To their conception of the revo- 

 lution of the earth round the central fire, notwith- 

 standing its crudeness, distinct merit must be attri- 

 buted, if judged by the favourable influence which 

 the teaching of Philolaus exerted on the propagation 

 of the Copernican system among those " who could 

 <inly admire philosophers of classical antiquity." 

 Compficated and erroneous as the Pythagorean idea 

 was, it nevertheless paved the way for the true con- 

 ception of the earth's rotation, but not, as was often 

 believed, for the heliocentric system. The chapter 

 on the primitive geocentric cosmology of Plato may, 

 at first sight, appear somewhat too prominent. Con- 

 sidering, however, the various controversies to which 

 Plato's astronomical system has given rise, a clear 

 si.itement of his natural philosophy on the basis of 

 an exhaustive analysis of the Dialogues is of import- 

 .ince, even apart from the author's apology that 

 " there is a charm in the poetical conception of the 

 ' soul of the world ' which makes the study of the 

 'I'tmaeus peculiarly attractive." 



With Eudoxus and Kalippus astronomy has started 

 on its career as a science. Eudoxus is the first to go 

 beyond mere speculative reasoning; he distinctly bases 

 his system of the homocentric spheres on the demand 

 of satisfying the observed motions. The possibilities 

 of this mathematically elegant system, recently 

 pointed out by Ideler and Schiaparelli, were, however, 

 not much appreciated by the ancients, although 

 .Aristotle had accepted it in the improved version of 

 Kalippus. 



Dr. Dreycr's criticism of Aristotelian melaphysics 

 is severe. 



NO. 1907, VOL 74] 



" His careful and critical examination of the 

 opinions of i)rcvious philosophers makes us regret 

 all the more that his search for the causes of pheno- 

 mena was often a mere search among words. This 

 tendency, which to us is his great defect, appealed 

 strongly to the medieval mind and helped to retard 

 the development of science in the days of Copernicus 

 and Galileo." 



The attempts of finding the physically true system 

 of the world terminated with Herakleides, who taught 

 the rotation of the earth, and .Vristarchus, who pro- 

 posed, as a way of " saving the phenomena," that 

 the earth performed an annual motion round the sun. 

 But the rapid rise of practical astronomy in the 

 Alexandrian school had so enormously increased the 

 knowledge of the complexities of the planetary 

 motions, which none of the proposed systems could 

 explain, that henceforth the idea of grasping the 

 physical truth was altogether abandoned, and atten- 

 tion became concentrated upon a purely geometrical 

 interpretation of the celestial phenomena. 



The subsequent exposition of the theory of the 

 epicycles and their utilisation in the Ptolemaic system 

 naturally forms an important part of Dr. Dreyer's 

 work. We cannot enter here upon his valuable ac- 

 count of the work of .^pollonius, Hipparchus, Eratos- 

 thenes, and Ptolemy ; nor can we dwell upon his 

 fascinating picture of the advances in astronomical 

 knowledge during the Alexandrian era, including, as 

 they do, the discovery of the precession by Hipfparchus, 

 the mensuration of the earth by Eratosthenes, and 

 general conclusions as to the dimensions of the 

 universe. The chapter closes with a graphic account 

 of the rapid decline of Hellenic culture after the 

 destruction of the .Mexandrian library, when " the 

 curtain went down for ever on the great stage where 

 Greek science had played its part so well and so 

 long." 



The return to archaic cosmology in Europe under 

 patristic influence and the supreme sway of Aristo- 

 telian metaphysics in scholasticism, mark an epoch 

 of scientific decadence in gloomy contrast to the 

 flourishing period of Hellenic culture. But the same 

 epoch is distinguished by the intense cultivation of 

 Greek astronomy among the .Arabs, whose labours, 

 devoted to the further development of the Ptolemaic 

 system, are e.xpounded in chapter xi. Although no 

 direct advance in cosmic ideas accrued from their 

 work, they influenced the subsequent advancement of 

 European astronomy in a most important manner bv 

 their invention of the trigonometric calculus. 



The description of the revival of astronomy in 

 Europe brings us face to face with some interesting 

 philosophers of the pre-Copernican period. The vague 

 and often mystic speculation of Cusa is contrasted 

 with the practical merits of Peurbach and Regio- 

 montanus, who successfully endeavoured to utilise the 

 Ptolemaic system for the purposes of astronomical 

 calculation, while we also find an interesting account 

 of the hopelessly complicated attempts made by 

 PVacastoro and .Amici to revive the theory of solid 

 spheres, and thereby to prove the physical truth of 

 the .Alexandrian svstem. 



