Moving Force. 193 
suze eeleritatis, eadem semper est ante et post 
eccursum eorum.’’ * 
M. Laplace considers that “ Ce principe”’ 
de la conservation des forces vives “ n’a lien 
que dans les cas, ou les mouvemens des corps 
changent par des nuances insensibles. Si les 
mouvemens éprouvent des changemens brus- 
-ques, la force vive est diminuée d’une quantite 
que l’on determinera de cette maniére ;”———f 
and taking it for granted, in the usual way, that 
where the change of motion. is sudden, the 
bodies must be non-elastic, he investigates the 
motions which are known to result from the 
collision of non-elastic soft bodies. But that 
conclusion is not justified by experience; for 
the characters of elasticity are often the most 
apparent where the changes of motion are, as 
far as we can judge, the most sudden. 
The supposition of the possible existence of 
a perfectly hard body, appears to involve 
another inconsistency which I will endeavour 
to state in a few words.—The resistance, or 
pressure, against c (fig. 8) being encreased, 
and the depth of its penetration being dimi- 
nished, in proportion as the hardness of A is 
increased, it follows, that if, by supposing A 
to be perfectly hard, the depth of the pene- 
* Phil, Trans. 1669. p- 928. . : 
+ Méchanique céleste, vol. 4. p. 52. 
Bb 
