eir cords at an equal rate; and
on investigation he discovered the principle that the shorter a pendulum
is the more quickly will it swing to and fro. As has already been
observed, Huygens first applied the principle to the governing of
clocks. In Fig. 202 we have a simple representation of the "dead-beat"
escapement commonly used in clocks. The escape-wheel is mounted on the
shaft of the last cog of the driving train, the pallet on a spindle
from which depends a split arm embracing the rod and the pendulum. We
must be careful to note that the pendulum _controls_ motion only; it
does not cause movement.

The escape-wheel revolves in a clockwise direction. The two pallets _a_
and _b_ are so designed that only one can rest on the teeth at one time.
In the sketch the sloping end of _b_ has just been forced upwards by the
pressure of a tooth. This swings the pallet and the pendulum. The
momentum of the latter causes _a_ to descend, and at the instant when
_b_ clears its tooth _a_ catches and holds another. The left-hand side
of _a_, called the _locking-face_, is part of a circle, so that the
escape-wheel is held motionless as long as it touches _a_: hence the
term, "dead beat"--that is, brought to a dead stop. As the pendulum
swings back, to the left, under the influence of gravity, _a_ is raised
and frees the tooth. The wheel jerks round, and another tooth is caught
by the locking-face of _b_. Again the pendulum swings to the right, and
the sloping end of _b_ is pushed up once more, giving the pendulum fresh
impetus. This process repeats itself as long as the driving power
lasts--for weeks, months, or years, as the case may be, and the
mechanism continues to be in good working order.

COMPENSATING PENDULUMS.

Metal expands when heated; therefore a steel pendulum which is of the
exact length to govern a clock correctly at a temperature of 60 deg.
would become too long at 80 deg., and slow the clock, and too short at
40 deg., and cause it to gain. In common clocks the pendulum rod is often
made of wood, which maintains an almost constant length at all ordinary
temperatures. But for very accurate clocks something more efficient is
required. Graham, the partner of Thomas Tompion, took advantage of the
fact that different kinds of metal have different ratios of expansion to
produce a _self-compensating_ pendulum on the principle illustrated by
Fig. 203. He used steel for the rod, and formed the _bob_, or weighted
end, of a