302 
MOVING fORCK. 
Cases of diifi Steam, the motion of the machine resembles the uniform 
doctrines ^of of ^ pendulum, and the moving force of the steam is 
inoviiig force, applied to the greatest advantage. 
By proceeding on the principle that when W is raised to B 
in the least time, the maximum of effect is produced, many 
erroneous conclusions have been drawn respecting the proper 
construction of machines. It is laid down for example, on 
this principle, that “ In an overshot water-wheel, the machine 
will be in its greatest perfection, when the diameter of the wheel 
is two-thirds of the height of the water above the lowest point 
of the wheel*.” But it is very well knowm that there would 
be lost, by that construction, nearly one-third of the moving 
force of the water, which is saved by making the wheel one- 
half larger in diameter, and by making its velocity much less 
than what is required by the above rule. 
It should be borne in mind, that the mechanical effects pro- 
,duced by means of machines, consist, almost invariably, of 
changes of figure. Even when a given mass is raised with an 
uniform velocity to a given height, a change of figure only is 
produced. For, if the mass were pressed to the earth by the 
elastic force of a spring instead of a force of gravity, we should 
not hesitate to say, that a mechanical change of figure is pro- 
duced when it is raised. Changes of figure of this kind being 
easily estimated, the raising of a given weight to a given height, 
has long been adopted as a convenient 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 measure, its fallacy would have been apparent. 
Dr. Wollaston has described a case of collision 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 dura- 
tion of its action, or by the space through which it acts, accord- 
ing to the particular views which may be taken of the pheno- 
mena. C (fig. 21) is supposed to be a ball Of clay, or any 
other soft and wholly inelastic substance, suspended at rest, but 
fr«« 
• Gregory’s IMcchanics, vol. 1, p. 447, 
