20 HISTORY OF MECHANICS. 



were destroyed, the body would instantly stop; if 

 Newton's were annihilated, the body would go on 

 uniformly in a straight line. Kepler compares the 

 action of his Forces to the way in which a body 

 might be driven round, by being placed among the 

 sails of a windmill ; Newton's Forces would be 

 represented by a rope pulling the body to the 

 center. Newton's Force is merely mutual attrac- 

 tion; Kepler's is something quite different from 

 this ; for though he perpetually illustrates his views 

 by the example of a magnet, he warns us that the 

 sun differs from the magnet in this respect, that its 

 force is not attractive, but directive 4 . Kepler's 

 essays may with considerable reason be asserted to 

 be an anticipation of the Vortices of Descartes ; but 

 they can with no propriety whatever be said to 

 anticipate Newton's Dynamical Theory. 



The confusion of thought which prevented ma- 

 thematicians from seeing the difference between 

 producing and preserving motion, was, indeed, fatal 

 to all attempts at progress on this subject. We 

 have already noticed the perplexity in which Aris- 

 totle involved himself, by his endeavours to find a 

 reason for the continued motion of a stone after the 

 moving power had ceased to act ; and that he had 

 ascribed it to the effect of the air or other medium 

 in which the stone moves. Tartalea, whose Nuova 

 Scienza is dated 1550, though a good pure mathe- 

 matician, is still quite in the dark on mechanical 

 4 Epitome Astron. Copern. p. 170. 



