90 HISTORY OF MECHANICS. 



attacked the vis viva in 1728; "with strong and 

 victorious reasons," as the Marquise du Chatelet de- 

 clared, in the first edition of her Treatise on Fire 15 . 

 But shortly after this praise was published, the 

 Chateau de Cirey, where the Marquise usually lived, 

 became a school of Leibnitzian opinions, and the 

 resort of the principal partisans of the vis viva. 

 "Soon," observes Mairan, "their language was 

 changed ; the vis viva was enthroned by the side of 

 the monads." The Marquise tried to retract or ex- 

 plain away her praises; she urged arguments on the 

 other side. Still the question was not decided; 

 even her friend Voltaire was not converted. In 

 1741 he read a memoir On the Measure and Na- 

 ture of Moving Forces, in which he maintained 

 the old opinion. Finally, D'Alembert in 1743 de- 

 clared it to be, as it truly was, a mere question 

 of words; and by the turn which Dynamics then 

 took, it ceased to be of any possible interest or im- 

 portance to mathematicians. 



The representation of the laws of motion and of 

 the reasonings depending on them, in the most 

 general form, by means of analytical language, can- 

 not be said to have been fully achieved till the time 

 of D'Alembert ; but as we have already seen, the 

 discovery of these laws had taken place somewhat 

 earlier; and that law which is more particularly 

 expressed in D'Alembert's Principle (the equality 

 of the action gained and lost) was, it has been 

 15 Mont. iii. 640. 



