MOVINfi FORCE. 
95 
If the amount of the force conld be increased or diminished Case* of diffi- 
by ^ny variation of the length of the lever, we might expect 
to find its measure to be of that indefinite kind which might moving force, 
be estimated by the product of the mass into any function of 
its velocity. Such a conclusion, however, is rjuite inconsistent 
with experience; for under every variation of the proportions 
of the lever, the effect, if measured by the mass into the square 
of its velocity, is uniformly found to be in proportion to the 
moving force by which it is produced ; if that force be mea- 
sured by the pressure multiplied into the space through which 
it acts. But if we multiply the mass into any other function 
than, the square of its velocity, no such general correspondence 
between the force and its effects is to be found. 
Mr. Smeaton has well illustrated this principle by many va- 
luable experiments on the more complicated cases of the action 
of water on mill-wheels, and on force generating rotatory 
motion in masses of matter about fixed axes*. • 
The Edinburgh reviewers of Dr. Wollaston's lecture on the 
force of percussion have urged some strong objections against 
Mr. Smeaton’s conclusions. I would willingly excuse myself 
from venturing to controvert any thing in a criticism written 
with so much candour and ability ; but some of the argu- 
ments it contains are pressed so powerfully against the appli- 
cation of the square of the velocity of a body in motion as the 
measure of its’ force, that they must, I apprehend, be answered 
before that measure can be consistently defended. 
In the first case it is argued, that the principle which Mr. 
Smeaton understood to be confirmed by the result of all his 
experiments, '• is, in fact, abandoned by him at the very outset 
of his investigation, in conse<]uence of having included the 
time in the measure of the etfect*." Now, I do not see how 
this supposed contradiction in Mr. Smeaton's reasoning can 
possibly be maintained. The measure of mechanical power 
adopted by him consists of the pressure multiplied into the space 
through which it acts. In cases where the pressure moves 
• See Phil. Trans, for 17.»9 and 1776. 
t Ediuburgh Review, vol. Iv’, p. « 
through 
