34 O^ PERCUSSION. 



and is nowhere ^^^ of this quantity of force Newton no where treats, 

 treated of by and has accordingly given no definition of it. If, after 

 defining what he meant by the quantitas acceleratrix, 

 and quantitas mofrix^ he had had occasion to convey an 

 eqnally distinct idea of the quantitas mechanica resulting 

 from the continued action of any force, he might, not 

 improbably, have proccededconformably to the definition 

 given by Smeaton, and have added 



quantitas mechanica est mensura proportionalis 



spatio per quod data vis motrix cxercetur ; 



or, if speaking with reference to the accumulated energy 



communicated to a body in motion, 



proportionalis quadrato velocitatis quam in dato 



corpore generat. 



But, if we attend to the words of his preface to the 

 first edition of his Principia^ he evidently had no need of 

 such a definition; 



'•'• Nos autem non artibus sed philosophia; consulentcs, 

 " deque potentiis non matmalibus sed naturalibus scri- 

 ^« bentes," &c. 



And again, nearly to the same effect in the SchoUiim^ 

 which follows the laws of motion, '' Caeterum mechani- 

 '' cam tractare non est hujus instituti." 

 Newton speaks Jn the third law of motion he has on the contrary been 

 not of percus- supposed to speak of this force from an ambiguity in the 

 sion. signification of the words actio and reactio. ^y these, 



however, Newton certainly meant a mere vis motrix or 

 pressure, as he himself explains them. '• Quicquid pre- 

 " mitvel trahit alterum, tantundem ab eo premitur vel 

 " trahitur. Si quis lapidem digito preniit, premitur et 

 " hujus digitus a lapide," &c. The same meaning is 

 equally evident from his demonstration of the third corol- 

 lary to the laws, in which he asserts that the quantitas 

 motits of two or more bodies estimated in any given di- 

 rection is not altered by their action upon each other. 

 The demonstration begins thus : 



'' Etenim actio eique contraria reactio a?quales sunt 

 •<> per legem tertiam, ideoque per legem secundam aequa- 

 '^ les in motibus efficient mutationes versus contrarias 

 ^^ partes." Now, if he had considered the third law as 

 iQjplying equality of more than mere moving forces, there 



could 



'f 



