MOVING KORCK. 
257 
produced, may be estimated with tolerable precision. The Cases of Jiili- 
force expended in driving piles into tlie earth, has been exa* <Tf 
mined by many mathematicians. In this case, the whole force 
of a body in mo'ioa is supposed to be expended in driving the 
pile, and this quantity of force is understood to be as the height 
from which the body falls, or as the square of its velocity. Rut 
there appears to be a material inconsistency in this application 
of tiiM prevailing theory. For there is, in fact, no ditference 
in kind between tliis case and the 8th case which we have be- 
foreexamined ; although, inthatci.se, there is, according to 
the theory, no force expended in driving the cylinder into the 
ball of clay. I do not .see how this inconsistency can possibly 
be removed, but by adopting Mr. Sineaton’s explanation of the 
collision of non-elastic bodies. 
I am aware, tliat many object to the comparison of changes 
of figure with changes of motion, as etfects of force. Our 
knowledge of both, however, appears to be acquired by the 
same means. They are both protlnced by pressure acting 
through some portion of space ; and there appears to be no 
difficulty in estimating the forces by which they are produced 
by the same measure. 
Of all tlie various terms that have been adopted in explaining 
the phenomena which we have been examining, none has been 
so uniformly used with the same meaning as the word pressure. 
All our notions of force appear to be derived from pressure, as 
it is perceived by the sense of touch. By balancing and com- 
paring all other pressures with that of gravity, we obtain a 
common measure of pressure. Although pressures are ba- 
lanced by pressures relatively at rest, under an almost infinite 
variety of circumstances, their most intricate combinations are 
distinctly explained and estimated by the application of a small 
number of general principles, and upon that subject no diffe- 
rence of opinion exists. 
If pressure be applied to a mass of matter at rest, but free 
to move in any direction, the ma.ss is put in motion. But that 
motion of the mass implies motion of the pressure ; for, un- 
less the pressure follow and act upon the mass through some 
portion 
