250 
MOVING FORCE. 
Cases of diff- 
ciilty in the 
doctrines of 
moving force. 
the prosecution of this enquiry. It is stated in order to show^ 
that the same-effect is produced by the same force, whether 
it act by gradual pressure or by sudden percussion. If the 
piece of clay be placed so near to A as to touch the prism when 
it begins to fall, the whole impression will be produced by gra- 
dual pressure. In estimating the force in this case, a practical 
man thinks of nothing but the quantity of mechanical force, 
or the pressure into the space, necessary to raise the prism to 
the given height ; and as the same quantity of force will always 
raise it to the same height, he concludes, that the same eflect 
must always be produced by its fall, although the times in which 
these equal effects are produced may be very different. If, 
instead of a piece of clay, we place a much harder substance, 
a block of iron, for example, under the prism, we shall hav« 
an impression produced on the prism as well as on the block j 
and, unless the centre of motion be of a very permanent kind, 
we shall, when the block is placed near to A, have a change 
of figure in that centre also. But still, if all these changes of 
figure could be accurately measured, by the pressure and the 
space expended in producing each of tliem, their sum would 
be equal to the whole change of figure produced on the clay, 
or on any other comparatively soft substance, placed under P. 
There are many very complicated cases of this kind, such as 
the hammering and rolling of metals, which may, I appre- 
hend, be all distinctly explained upon the same principles. 
In the 14th case the same effects are produced by percussion, 
which, in the 5th case, are produced by gradual pressur® 
through sensible spaces j and we must either admit, that the 
moving force of D (fig. 14) is greater than that of C, or con- 
clude, that the rotatory motion is produced without force. It 
may be said that there is, in both cases, only the same quantity 
of motion in one direction. I must observe, however, that Sir 
Isaac Newton understood the sum of the viotkms of the two 
bodies to include the rotatory, as well as the progressive, mo- 
tion. “If two globes,” he says, “ joined by a slender rod, 
revolve about their common centre of gravity with an unifoim 
motion, while that centre moves on uniformly in a right lino 
drawn 
