Moving Force. 247 
represent the whole mioving force of a given 
quantity of water descending from A to B. This 
effect is considerably greater than that which 
the same quantity of water would produce if 
applied to an undershot water-wheel, but less 
than that which it would produce if properly 
applied to an overshot water-wheel. 
Respecting the maximum of effect produced 
by machines, I wish to observe, that in the 
actual construction of machines it is necessary 
to aim at a maximum quite different from 
that which is usually proposed in books on the 
theory of mechanics. 'This will perhaps be 
best explained by examining the simple case 
where a given weight P, (fig. 20) connected 
with another W, by a string passing over the 
pulley F, descends vertically and raises W, 
without, friction, from the horizontal line AC 
along the inclined plane AB. If we make 
AB: BC::2W:P, W will be raised to B in 
the least time ;* and upon this principle, the 
maximum of effect in machines is usually de- 
monstrated in theory. In practice, however, 
the object is not merely to raise W to B 
in the least time, but to raise it with the 
least expenditure of moving force. When 
* If the ascent be made in the least possible time, W 
must ascend not along the plane AB, but along a concave 
surface AGB. 
