232 On the Force of Percussion. 
wards by Smeaton, very aptly denominated mechanic force s 
and when by force of percussion is meant the quantity of 
mechanic force possessed by a body in motion, to be esti- 
mated by its quantity of mechanic effect, I apprehend it can- 
not be controverted that it is in proportion to the magnitude 
of the body and to the square of its velocity jointly. 
But of this quantity of force Newton no where treats, and 
has accordingly given no definition of it. If, after defining 
what he meant by the quantitas acceleratrix, and quantitas 
motrix, he had bad occasion to convey an equally distinct 
idea of the quantitas mechanica resulting from the continued 
action of any force, he might, not improbably, have pro- 
ceeded conformably to the definition given by Smeaton, and 
have added, 
quantitas mechanica est mensura proportionalis io 
per quod data vis motrix exercetur 5 
or, if speaking with reference to the accumulated energy 
communicated toa body in motion, 
proportionalis quadrato velocitatis quam in dato cor- 
pore 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 philosophie consulentes, 
deque potentiis non manualibus sed naturalibus scribentes,”” 
&e. 
And again, nearly to the same effect in the Scholiwm, 
which follows the laws of motion: ‘* Czterum mechani- 
cam 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, how- 
ever, 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 demon- 
stration of the third corollary to the laws, in which he as- 
serts 
s 
ee ee, ee ee ee 
——. 
ed 
