ght itself it will drive the wheel to the left for a short distance,
when its end s is held fast, with the same force as if that end was
pulled forward by the weight; and as the great wheel has to move very
little during the short time the clock is winding, the spring will keep
the clock going long enough.

In the commoner kind of turret clocks a more simple apparatus is used,
which goes by the name of the _bolt and shutter_, because it consists of
a weighted lever with a broad end, which shuts up the winding-hole. When
it is lifted a spring-bolt attached to the lever, or its arbor, runs
into the teeth of one of the wheels, and the weight of the lever keeps
the train going until the bolt has run itself out of gear. Clocks are
not always driven by weights. When accuracy is not necessary, but
portability is desirable, springs are used. The old form of spring
became weaker as it was unwound and necessitated the use of a device
called a fusee or spiral drum. This apparatus will be found described in
the article WATCH.

_Striking Mechanism._--There are two kinds of striking work used in
clocks. The older of them, the _locking-plate_ system, which is still
used in most foreign clocks, and in turret clocks in England also, will
not allow the striking of any hour to be either omitted or repeated,
without making the next hour strike wrong; whereas in the _rack_ system,
which is used in all English house clocks, the number of blows to be
struck depends merely on the position of a wheel attached to the going
part, and therefore the striking of any hour may be omitted or repeated
without deranging the following ones. We shall only describe the second
of these, which is the more usual in modern timepieces.

Fig. 26 is a front view of a common English house clock with the face
taken off, showing the repeating or rack striking movement. Here, as in
fig. 3, M is the hour-wheel, on the pipe of which the minute-hand is
set, N the reversed hour-wheel, and n its pinion, driving the 12-hour
wheel H, on whose socket is fixed what is called the snail Y, which
belongs to the striking work exclusively. The hammer is raised by the
eight pins in the rim of the second wheel in the striking train, in the
manner which is obvious.

[Illustration: FIG. 26.--Front view of common English House Clock.]

The hammer does not quite touch the bell, as it would jar in striking if
it did, and prevent the full sound. The form of the hammer-shank at the
arb