becomes a retarded one from the same cause. The body is now going
upwards, away from the earth, and the earth's attraction therefore drags
upon it and keeps it back instead of hastening it. As it travels up in
its curved path, more and more of its weight is taken off the string,
and thrown, so to speak, upon the moving impulse. In the descending
portion of the vibration the weight of the body increases its movement;
in the ascending portion it diminishes its movement. At last the upward
movement becomes so slow, that the impulse of momentum is lost, and the
earth's attraction is again unopposed. The body then begins to
retrograde, acquires progressively increasing velocity as it descends,
overshoots the place of its original repose, and once more commences the
ascent on the opposite side.

Whenever, then, a heavy body suspended by a flexible string is drawn to
one side, and dropped from the hand, a vibrating pendulum is made,
because weight and acquired impulse influence it alternately with a sort
of see-saw action, the power of the one diminishing as the power of the
other augments. Weight pulls down--confers velocity and impulse during
the pulling--and then velocity carries up. As velocity carries up,
weight diminishes its impulse, and at last arrests it, and then begins
to pull down again. In the middle of the vibration velocity is at its
greatest, and weight at its least, as regards their influence on the
motion. At the extremes of the vibration velocity is at its least, and
weight at its greatest. Now here it is the earth's attraction clearly
that confers the impulse of the downward movement, just as much as it is
the earth's attraction that causes the downward movement of running
water. Therefore the power which makes the pendulum swing is the same
with the power which grinds the corn in the water-mill--the attraction
of the earth's vast mass for the mass of a smaller body placed near to
its surface under certain peculiar conditions of position.