the minute-hand
shaft, which also carries one of the cogs of the escapement train. The
shaft is permitted by the escapement to revolve once an hour. Fig. 211
shows diagrammatically how this is managed. The hour-hand shaft A (solid
black) can be moved round inside the cog B, driven by the mainspring
drum. It carries a cog, C. This gears with a cog, D, having three times
as many teeth. The cog E, united to D, drives cog F, having four times
as many teeth as E. To F is attached the collar G of the hour-hand. F
and G revolve outside the minute-hand shaft. On turning A, C turns D and
E, E turns F and the hour-hand, which revolves 1/3 of 1/4 = 1/12 as fast
as A.[41]

* * * * *

LOCKS.

On these unfortunately necessary mechanisms a great deal of ingenuity
has been expended. With the advance of luxury and the increased worship
of wealth, it becomes more and more necessary to guard one's belongings
against the less scrupulous members of society.

[Illustration: FIG. 212.]

The simplest form of lock, such as is found in desks and very cheap
articles, works on the principle shown in Fig. 212. The bolt is split at
the rear, and the upper part bent upwards to form a spring. The under
edge has two notches cut in it, separated by a curved excrescence. The
key merely presses the bolt upwards against the spring, until the notch,
engaging with the frame, moves it backwards or forwards until the spring
drives the tail down into the other notch. This primitive device
affords, of course, very little security. An advance is seen in the

TUMBLER LOCK.

[Illustration: FIG. 213.]

The bolt now can move only in a horizontal direction. It has an opening
cut in it with two notches (Figs. 213, 214). Behind the bolt lies the
_tumbler_ T (indicated by the dotted line), pivoted at the angle on a
pin. From the face of the tumbler a stud, S, projects through the hole
in the bolt. This stud is forced into one or other of the notches by the
spring, S^1, which presses on the tail of the tumbler.

[Illustration: FIG. 214.]

In Fig. 213 the key is about to actuate the locking mechanism. The next
diagram (Fig. 214) shows how the key, as it enters the notch on the
lower side of the bolt to move it along, also raises the tumbler stud
clear of the projection between the two notches. By the time that the
bolt has been fully "shot," the key leaves the under notch and allows
the tumbler stud to fall into the rear locking-