MOVING FORCE. 241 
desjorces v’wes ii’a lieu que dans les cas, oil les mouvemens C4»e« of diffi- 
des corps chaiigeut |)ar des nuances insensiblcs. Si les mouve- joctri'ncs'^f 
mens eprouvent des changemens brusques, la force vive cst di- moving force, 
nilnuce d’une quantile que Ton determinera de cette mani- 
cre >” * and taking it for granted, in the usual way, that 
where the change of motion is sudden, ihe bodies must be non- 
elastic, he investigates the motions which are known to result 
from the collision of nonelastic soft bodies* But that conclu- 
sion is not justified by experience j for the characters of elas- 
ticity are often the most apparent where the changes of motion 
are, at tar as we can judge, the most sudden. 
The suppobitiot) of the fpssible existence of a perfectly hard 
body, appears to involve another inconsistency which J will 
endeavour to state in a few words. The resistance, or pressure, 
against c (fig. y) being increased, and the depth of its prenetra- 
tion being diminished, in pnoportion as tire hardness of A is 
increased, it follows, that if, by suppiosiug A to be perfectly 
hard, the depth of tlie penetration be reduced to noUiing, 
the pressure must be increased to infinity. That is, the 
pressure mu«t be infinitely great to communicate even tlie 
smallest finite quantity of motion. But I believe the " law 
of continuity” is not so much objected to now as it was for- 
merly, and few will be disposed to contend, that a body may, 
from a state of rest, arrive at any given velocity, without pras- 
siiig through the iutermediate degrees of velocity between that 
and rest ; and consequently few will now contend for the pws- 
sible existence of a prerfectly hard substance. 
If, instead of a nonelastic soft substance, we supp>ose A to 
be a hollow spliere filled with a dense elastic fluid, and c to 
pass through a hole in the side of the sphere, so as to move 
without friction and be uniformly pressed outw’ards by the 
fluid. A will tlien rep>resent a pierfectly elactic body. 
It may be proper to observe, that although we supipose c to 
make no penetration into B, we do not suppiose B to be pierfectly 
bard. We only suppose it to be so much harder than A, that 
the penetration shall be very small when compxnied with the 
f M^chaniqne cfleite, vol I. p. 5?. 
pene- 
