x OK A 
Moving Force. 249 | 
regulated pressure of the steam, the motion 
of the machine resembles the uniform vibra- 
tions of a pendulum, and the moving force 
of the steam is applied to the greatest ad- 
vantage. 
By proceeding on the principle that when 
W is raised to B in the least time, the maxi- 
mum 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 known 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 me- 
chanical effects produced by means of ma- 
chines, 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 
* Gregory’s Mechanics, vol. J, p. 447. 
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