MECHANICAL PHILOSOPHY. 421 



given of the soundness of Bacon's views. " We wish," says 

 Des Cartes, "to deduce effects from their causes, and not, con- 

 versely, causes from their effects. We appeal to experience only, 

 that out of innumerable effects which may he produced from the 

 same cause, we may direct our attention to one rather than to 

 another." The axioms, to which he endeavoured to reduce all his 

 theories, were grounded on metaphysical notions of the attributes 

 of the Deity ; and we have a remarkable instance of the presump- 

 tion and error, which characterized most of his reasonings, in the 

 dogma, that the quantity of motion in the universe must remain 

 always the same, because the Divine nature is immutable. 



To Des Cartes, however, we are indebted for the first distinct 

 enunciation of the laics of motion; although his own conception 

 of these laws was far from accurate, and he seems, in particular, to 

 have regarded the inertia of matter as a kind of active force. 



The first actual contributions of importance which dynamical 

 science received subsequently to the discoveries of Galileo are due 

 to Huygens. By the aid of the most refined geometry, of which 

 he was so perfect a master, this mathematician discovered the 

 relation which subsists between the length of a pendulum and the 

 time of its vibration ; and having developed the theory of the 

 instrument, he realized the conception of Galileo, and applied it to 

 the regulation of the clock. On pursuing his inquiries, however, 

 Huygens ascertained that the property of tautochronism belonged 

 to the circular pendulum only when the arcs of vibration are 

 indefinitely small; and he was thus led to inquire what curve 

 possessed the property universally. Finding this curve to be the 

 cycloid, he conceived the idea of making a pendulous body move 

 in it'; and his mathematical skill soon after enabled him to point 

 out the means of accomplishing this object. His account of this 

 construction, and of the principles on which it depends, was pub- 

 lished in the year 1670, in a work entitled Horologimn Oscillatorinm, 

 many years after the date of the discovery. But interesting as the 

 cycloidal pendulum is in theory, it has been long since abandoned, 

 as useless, in practice ; and the clocks to which the circular 

 pendulum was adapted were found far to excel the cycloidal 

 clocks of Huygens. 



To Huygens we are indebted also for the next great step 

 which was made in dynamical science the theory of enrufur 

 )uoti<i ; and the laws of centrifugal force were developed by him 



