_Accumulation of Power_ arises from lifting a weight and then allowing
it to fall. A man, even with a heavy hammer, might strike repeated blows
upon the head of a pile without producing any effect. But if he raises a
much heavier hammer to a much greater height, its fall, though far less
frequently repeated, will produce the desired effect.
_Regulating Power._--A contrivance for regulating the effect of
machinery consists in a vane or a fly, of little weight, but presenting
a large surface. This revolves rapidly, and soon acquires an uniform
rate, which it cannot greatly exceed, because any addition to its
velocity produces a much greater addition to the resistance it meets
with from the air. The interval between the strokes on the bell of a
clock is regulated by this means; and the fly is so contrived, that this
interval may be altered by presenting the arms of it more or less
obliquely to the direction in which they move. This kind of fly or vane
is generally used in the smaller kinds of mechanism, and, unlike the
heavy fly, it is a destroyer instead of a preserver of force. It is the
regulator used in musical boxes, and in almost all mechanical toys.
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