tients' pulse-beats. His son, however, is supposed to have applied the
pendulum to clocks. There is at the Victoria and Albert Museum a copy of
an early clock, said to be Galileo's, in which the pins on a rotating
wheel kick a pendulum outwards, remaining locked after having done so
till the pendulum returns and unlocks the next pin, which then
administers another kick to the pendulum (fig. 2). The interest of the
specimen is that it contains the germ of the chronometer escapement and
free pendulum, which is possibly destined to be the escapement of the
future.

The essential component parts of a clock are:--

1. The pendulum or time-governing device;

2. The escapement, whereby the pendulum controls the speed of going;

3. The train of wheels, urged round by the weight or main-spring,
together with the recording parts, i.e. the dial, hands and hour motion
wheels;

4. The striking mechanism.

[Illustration: FIG. 3.--Section of House Clock.]

The general construction of the going part of all clocks, except large
or turret clocks, is substantially the same, and fig. 3 is a section of
any ordinary house clock. B is the barrel with the cord coiled round it,
generally 16 times for the 8 days; the barrel is fixed to its arbor K,
which is prolonged into the winding square coming up to the face or dial
of the clock; the dial is here shown as fixed either by small screws x,
or by a socket and pin z, to the prolonged pillars p, p, which (4 or 5
in number) connect the plates or frame of the clock together, though the
dial is commonly set on to the front plate by another set of pillars of
its own. The great wheel G rides on the arbor, and is connected with the
barrel by the ratchet R, the action of which is shown more fully in fig.
25. The intermediate wheel r in this drawing is for a purpose which will
be described hereafter, and for the present it may be considered as
omitted, and the click of the ratchet R as fixed to the great wheel. The
great wheel drives the pinion c which is called the centre pinion, on
the arbor of the _centre wheel_ C, which goes through to the dial, and
carries the long, or minute-hand; this wheel always turns in an hour,
and the great wheel generally in 12 hours, by having 12 times as many
teeth as the centre pinion. The centre wheel drives the "second wheel" D
by its pinion d, and that again drives the scape-wheel E by its pinion
e. If the pinions d and e have each 8 teeth or _leaves_ (as the tee