240 
MOVING FORCE, 
C.^cs of diffi- yery reniarkabiej that while Mr. Soicaton's other dissertations 
doc/rines ©f principles of moving force, have met with considerable 
moyicg force, attemion abroad as well as at home, this last treatise on the 
collision of bodies, (which he himself considered a most im- 
portant one, as containing the best confirmation of his former 
conclusions) has been almost totally neglected by all succeeding 
writers. It is impossible for me to do justice to it by giving 
an abstract of it 3 but I would earnestly recommend the entire 
treatise to the attention of all those who take an interest in 
investigations of this kind. 
With regard to the collision of bodies, which are supposed to 
be perfectly hard, as well as none^istic, Mr. Smeaion under- 
stood a contradiction to be involved in the very supposition of 
the existence of such bodies. It has never been contended, 
that any such are to be found in nature. But it is very generally 
argued, with Mr. Maclaurin, that “ there is the same objection 
(of non-existence) against admitting and treating of bodies of 
a perfect elasticity*,” In reply to this I would observe, that 
the objection does not appear to be of the same weight against 
perfectly elastic, as .against perfectly nonelastic bard bodies. 
For we have substances which approach very nearly to perfect 
elasticity ; but we can find no substance of which the qualities 
approach to hardness and nonelasticity united. In general, 
the elasticity increases as the hardness increases, and no sub- 
stance has ever been produced that can be called hard, without 
possessing, at the same time, great elasticity. 
It does not appear that the possible existence of a perfectly 
hard nonelastic body was qbyious to the first discoverers of the 
laws of percussion. Hu)gens appears to have understood n 
hard body to be one that is perfectly elastic. His O'lh law of 
peicusslon is as follow.s : “ Summa productorum factorum a 
/ mole cujuslibet corporis duridMcla. in quadratum suae celeritatis, 
eadem semper est ante et post occursumeorumf 
M. Laplace cousiders that " Ce principe” de la conservation 
* Account of Sir Isaac Newton’s Discoveries, p. 9 ?, 
t I’hiJ. Trans. 1669, p. 928. 
des 
