NATURE 
°"° 
LORY 
THURSDAY, MAY 28, tor. 
GREEK PHYSICS AND DYNAMICS. 
Le Systéme du Monde: Histoire des Doctrines 
Cosmologiques de Platon & Copernic. By Prof. 
Pierres Duhem. Tome Premier: ~ Fp. 
(Parise A. Hermann ep pitiless; 1913.) 
18.50 francs. 
Bis 
HIS book contains a good deal more than 
one might expect from the title. It not only 
gives an account of the cosmical systems of the 
Greeks from Pythagoras to Ptolemy, but dis- 
cusses in considerable detail the views of the 
different schools of the same period as to the con- | 
matter, and their principles of 
As was to be expected from the 
author’s previous publications on the history of 
natural philosophy, he shows himself well ac- 
quainted with ancient literature, and also (with a 
few exceptions) with the very extensive modern 
literature of monographs on Greek science. The 
most recent editions of the classical writers are 
always quoted, but with one notable exception, 
Diels’s edition of the Doxographi Greci not 
having been made use of. 
The astronomical chapters, which fill less than 
half the book, do not call for any extended notice, 
as the subject treated in them has been dealt with 
in more than one book accessible to English 
readers, the last being Sir Thomas Heath’s book 
on Aristarchus, published only a year ago. 
the origin of the heliocentric idea, the author 
follows in the main the theory of Schiaparelli, that 
it was really due to Herakleides, fifty years before 
the time of Aristarchus, and he seems uncon- 
vinced by the weighty arguments brought forward 
against it by subsequent writers. 
The most valuable part of M. Duhem’s book 
is undoubtedly that dealing with the physics and 
dynamics of the Greeks, especially of Aristotle, 
and it gives a very clear and thorough account 
of this difficult subject. While Plato’s views on 
nature were characterised by doubts as to facts 
stitution of 
dynamics. 
learned by perception, as the immutability which | 
is regarded as the essence of things is not re- 
vealed thereby, Aristotle rehabilitated experience 
and observation, though often led astrxy by pre- 
conceived notions. In his dynamics the idea of 
mass does not enter; every moving body is neces- 
sarily subject to two forces, a power and a 
resistance ; without a power it would not move at 
all, without resistance the motion would be 
accomplished in an instant. The velocity with 
which the body moves depends both on the mag- 
nitude of the power and on that of the resistance ; 
NO.A2 226, VOL.’ 93) 
Price | 
As to | 
| if both are constant the resulting motion 1s 
supposed to be uniform; if the resistance 
| decreases the velocity will increase, if the same 
| power be employed to move resisting bodies, the 
velocities which it communicates to them 
inversely proportional to the resisting weights. 
Velocity is therefore proportional to the ratio 
_of power to resistance, and yet, how can motion 
| cease when they become equal? Aristotle secs 
| this difficulty and tries to get over it by remarking 
that because a certain power moves a_ body 
through a certain length it does not follow that 
any fraction of the power will move the body 
| through the same fraction of the length. A bedy 
falling through air or water represents © to 
Aristotle the simplest motion we can conceive ; 
the power is here the weight of the body, while 
| the resistance is caused by the medium i! 
| traverses, and the velocity of the fall is propor- 
tional to the weight. On the other hand, by the 
fundamental principle of Aristotelian dynamics, 
the velocity is inversely proportional to the resist- 
ance, and Aristotle seems to admit that this re- 
sistance is proportional to the density of. the 
medium. But he maintained that if a fall 
empty space were possible (which he denies), 
bodies of different weight would not fall with the 
same . velocity.. ‘‘This,” he says (‘ Physics.” 
iv. 8, p. 216a), “is impossible, for what should 
then cause one body to move faster? This 
is necessarily. the case in a medium because 
the body which has the greater power divides 
the medium more quickly, but in the void all 
bodies would have the same velocity, which is 
impossible.” 
The author also discusses very fully the theories 
prevalent after Aristotle so far as John Philoponus 
| in the sixth century. In opposition to Aristotle, 
Philoponus taught that weight is something which 
belongs to a body and represents the downward 
motion it would have in empty space; the resist- 
ing medium prolongs the time of the fall, but if 
the resistance is diminished to zero the fall docs 
/ not become instantaneous, the limit of 
| velocity being that with which it would fall 
| through empty space. This doctrine, so differei' 
| from that of Aristotle, was not accepted in the 
Middle Ages, though it was not without some 
influence on the views of Simplicius, who other- 
wise was a severe Critic of Philoponus. We shall 
look forward with interest to M. Duhem’s second 
volume, in which he will doubtless discuss the 
views of Thomas Aquinas and other philosophers 
of the Middle Ages, which did not always coincide 
completely with those of Aristotle. 
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