60 HISTORY OF MECHANICS. 



Newton deduced from his principles the conclusion, 

 that by the mutual action of bodies, the motion of 

 their center of gravity cannot be affected. Mar- 

 riotte, in his Traite de la Percussion (1684), had 

 asserted this proposition for the case of direct im- 

 pact. But by the reasoners of Newton's time, the 

 dynamical proposition, that the motion of the center 

 of gravity is not altered by the actual free motion 

 and impact of bodies, was associated with the sta- 

 tical proposition, that when bodies are in equili- 

 brium, the center of gravity cannot be made to 

 ascend or descend by the virtual motions of the 

 bodies. This latter is a proposition which was as- 

 sumed as self-evident by Torricelli ; but which may 

 more philosophically be proved from elementary 

 statical principles. 



This disposition to identify the elementary laws 

 of equilibrium and of motion, led men to think too 

 slightingly of the ancient solid and sufficient foun- 

 dation of Statics, the doctrine of the lever. When 

 the progress of thought had opened men's minds to 

 a more general view of the subject, it was consi- 

 dered as a blemish in the science to found it on the 

 properties of one particular machine. Descartes 

 says in his Letters, that "it is ridiculous to prove 

 the pulley by means of the lever." And Varignon 

 was led by similar reflections to the project of his 

 Nouvette Mecanique, in which the whole of statics 

 should be founded on the composition of forces. 

 This project was published in 1687; but the work 



