250 On the Measure of 
were pressed to the earth by the elastic force 
of a spring instead of the force of gravity, we 
should not hesitate to, say, that, a mechanical 
changé of figure is preduced when it is raised. 
Changes of figure of this kind being easily 
estimated, the raising of a given. weight toa 
given height, has Jong been adopted | asa cons 
venient common measure for almost-every kind 
of, moving force. If the ‘rule, - quoted above, 
for. the construction, of an_ overshot water- 
wheel, had, been tried by this IDSA Grn Hs 
fallacy would have. been apparent. . 
Dr. Wollaston has described a case. lt adi 
lision. and change of. figure, which has been 
understood to prove, | that the force of a body 
in motion may. be properly estimated either 
by the duration of its action, or by, the space 
through which it acts, according to the par- 
ticular views which may be taken of the 
phenomena, Cc (fig. 21) is supposed to be a 
ball of clay, or any other soft and wholly 
inelastic substance, suspended at rest, but free 
to move in any direction with the slightest 
impulse ; the two pegs, O and P, to be similar. 
and equal in every respect, and to meet with 
uniform and equal resistance in penetrating 
C; the weight of A to be double that of B, 
ane velocity of A moving in the direction AC, 
to be half that of B, moving in the opposite 
