Manchester Memoirs, Vol. xlvi. (1902), No. 10- 23 



to prevent the modern student of rational mechanics from 

 obtaining a comprehensive view of the principles of motion, 

 is the change that has recently been made in the signifi- 

 cation of the name vis viva. The term nionienijun is 

 admitted on all hands to be synonymous with the quantity 

 of motion of Descartes and Newton. It will also be 

 allowed that the term energy (kinetic energy), first applied 

 by Young to the product of the weight of a body into the 

 square of its velocity, is synonymous with the vis viva of 

 Leibnitz, as it was so afifirmed to be by Young.* 

 Nevertheless, recent writers have arbitrarily degraded the 

 vis viva or energy of a body to one half its original value, 

 while still retaining the same name, f The liberty so 

 taken with this fundamental principle of motion may be 

 fairly comparable with the action of an over-enterprising 

 trader who should, in like manner, reduce the imperial 

 standard of weights and measures to his own advantage. 

 Now, whatever assumed convenience (apart from actuality) 

 this change in the value of the vis viva might have in 

 relation to the duty of motors and machines through which 

 mechanical force is transformed into heat or other mode 

 of force, and where the work done is admittedly as the 

 velocity, the final result, if not indeed the actual object, 

 of reducing the vis viva of Leibnitz to one half its value 

 was to bring it into agreement with the Cartesian and 

 Newtonian quantity of motion or momentum with which it 

 is absolutely irreconcilable. 



Much of the opposition to the Leibnitzian measure of 

 moving force has doubtless arisen from the apprehension 

 that the Newtonian law of gravitation would be invalidated 



* Lectures on Natural Philosophy, Lecture VIII. 

 f Moseley's Engineering and Architecture , p. viii, 1843. 

 Matter and Motion, J. Clerk Maxwell, 1876, p. 79. 

 Newton'' s Laws of Motion, P. G. Tait, 1899, p. 27. 



