194 On the Measure of 
tration be reduced to nothing, the pressure — 
must be increased to infinity. That is, the 
pressure must be infinitely great to communi- 
cate even the smallest finite quantity of motion. 
But I believe the “ law of continuity ” is not. 
so much objected to now. as it was formerly, 
and few will be disposed to contend that a 
body may, from a state of rest, arrive at. any) 
given velocity, without passing through the. 
intermediate degrees of velocity, between that 
and rest; and consequently, few will now 
contend for the possible existence of a per- 
fectly hard substance. 
- If instead of a non-elastic soft substance,. 
we suppose A to be a hollow sphere filled with 
a dense elastic fluid, and ¢ to pass through a 
hole im the side of the sphere so as to move 
without friction, and be uniformly pressed 
outwards by the fluid; A will then ee 
a perfectly elactic body. 
It may be proper to observe, that although 
we suppose c'to make no penetration into B, 
we do not suppose B to be perfectly hard. 
We only suppose it to be so much harder than 
A, that the penetration , shall be very. smal 
when compared with the penetration into A. 
If we were to suppose A and B to yield equally 
to c, the same explanation of the phenomena, 
as when A only is supposed to be penetrated, 
