on the Force of .Percussion. 17 
“ Nos autem non artibus sed philosophise consulentes, deque 
“ potentiis non manualibus sed naturalibus scribentes,” &c. 
And again, nearly to the same effect in the Scholium , which 
follows the laws of motion, “ Cseterum mechanicam tractare 
“ non est hujus instituti.” 
In the third law of motion he has on the contrary been 
supposed to speak of this force from an ambiguity in the sig- 
nification of the words actio and reactio. By these, however, 
Newton certainly meant a mere vis motrix or pressure, as he 
himself explains them. “ Quicquid premit vel trahit alterum, 
“ tantundem ab eo premitur vel trahitur. Si quis lapidem 
“ digito premit, premitur et hujus digitus a lapide,” &c. The 
same meaning is equally evident from his demonstration of 
the third corollary to the laws, in which lie asserts that the 
quantitas motus of two or more bodies estimated in any given 
direction is not altered by their action upon each other. The 
demonstration begins thus : 
“ Etenim actio eiqtie contraria reactio asquales sunt per legem 
“ tertiam, ideoque per legem secundam sequales in motibus 
“ efficient mutationes versus contrarias partes.” Now, if he 
had considered the third law as implying equality of more 
than mere moving forces, there could have been no occasion 
to refer to the second law, with a view thence to deduce the 
equality of momenta produced. 
Some authors however have interpreted the third law 
differently, and accordingly have expressed a difficulty in 
comprehending the simple illustration given by Newton. 
"When they say that action is equal to reaction, they mean 
not only that the instantaneous intensity of the moving forces, 
or pressures opposed to each other, are necessarily equal, but 
mdcccvi. D 
