the
swaying of the pendulum. The other wheels and pinions of the movement
are so arranged that they indicate the number of turns the wheel at
the top of the pendulum completes, by means of hands traversing round
a dial-plate inscribed with figures and dots.

It is found convenient in practice to make the direct descent of a
weight the moving power of the wheel-work, instead of the swinging of
the pendulum, for the simple reason, that the excess of its power
beyond what is required to overcome the friction of the wheel-work, is
then employed in giving a slight push to the pendulum; this push just
neutralises the retarding effects before named as inseparable from the
presence of air and imperfect means of suspension. The train of
wheel-work in a clock, therefore, serves two purposes--it records the
number of beats which the pendulum makes, and it keeps that body
moving when once started. As far as the activity of the pendulum is
concerned, the wheel-work is a recording power, and a preserving
power, but _not_ an originating power. If there were no air, and no
friction in the apparatus of suspension, the pendulum would continue
to go as well without the wheel-work as with it. With the wheel-work
it beats as permanently and steadily upon material supports and
plunged in a dense atmosphere, as it would if it were hung upon
nothing, and were swinging in nothing; and also performs its backward
and forward business in solitude and darkness, to the same practical
purpose that it would if the eyes of watchful and observant guardians
were turned incessantly towards it.

Galileo published his discovery of the isochronous property of the
pendulum in 1639. Richard Harris of London took the hint, and
connected the pendulum with clock-work movement in 1641. Huyghens
subsequently improved the connection, and succeeded in constructing
very trustworthy time-keepers, certainly before 1658.

But notwithstanding all that the knowledge and skill of Huyghens could
do, his most perfect instruments were still at the mercy of
atmospheric changes. It has been said, that the time of a pendulum's
vibration depends upon the length of its suspending-rod. This length
is measured, not down to the bottom of the weight, but to the centre
of its mass. For the weight itself is necessarily a body of
considerable dimensions, and in this body some particles must be
nearer to, and others further from the point of suspension. Those
which are nearest will, o