t until they have long
accumulated. The weight does not stand still at once, but its arc of
vibration grows imperceptibly less and less, until at last there comes
a time when the eye cannot tell whether the body is still moving or in
absolute repose.

Now, suppose that a careful and patient observer, aware of the exact
length of the suspending-rod of a vibrating pendulum, were to set
himself down to count how many beats it would make in a given period,
he would thenceforward be able to assign a fixed value to each beat,
and would consequently have acquired an invariable standard whereby he
might estimate short intervals. If he found that his instrument had
made exactly 86,400 beats at the end of a mean solar day, and knew
that the length of its rod was a trifle more than 39 inches, he would
be aware that each beat of such a pendulum might always be taken as
the measure of a second. The length of the rod of a pendulum which
beats exact seconds in London is 39.13 inches.

But there are few persons who would be willing to go through the
tedious operation of counting 86,400 successive vibrations. The
invention of a mechanical contrivance that was able to break the
monotony of such a task, would be hailed by any one who had to perform
it as an invaluable boon. Even a piece of brass with sixty notches
upon it, which he might slip through his fingers while noting the
swinging body, would enable him to keep his reckoning by sixties
instead of units, and so far would afford him considerable relief. But
if the notched brass could be turned into a ring, and the pendulum be
made to count the notches off for itself, round and round again
continuously, registering each revolution as it was completed for
future reference, the observer would attain the same result without
expending any personal trouble about it. It is this magical conversion
of brass and iron into almost intelligent counters of the pendulum's
vibrations, that the clock-maker effects by his beautiful mechanism.

In the pendulum clock, the top of the swinging-rod is connected with a
curved piece of steel, which dips its teeth-like ends on either hand
into notches deeply cut in the edges of a brass wheel. The notched
wheel is connected with a train of wheel-work kept moving by the
descent of a heavy weight; but it can only move onwards in its
revolution under the influence of the weight, as the two ends of the
piece of steel are alternately lifted out of the notches by