or also shows that the
lock has been tampered with, since even the right key cannot move the
bolt until the overlocking has been performed.

[Illustration: FIG. 218.--A Chubb key raising all the tumblers to the
correct height.]

Each tumbler step of a large Chubb key can be given one of thirty
different heights; the bolt step one of twenty. By merely transposing
the order of the steps in a six-step key it is possible to get 720
different combinations. By diminishing or increasing the heights the
possible combinations may be raised to the enormous total of 7,776,000!

[Illustration: FIG. 219.--Section of a Yale lock.]

THE YALE LOCK,

which comes from America, works on a quite different system. Its most
noticeable feature is that it permits the use of a very small key,
though the number of combinations possible is still enormous (several
millions). In our illustrations (Figs. 219, 220, 221) we show the
mechanism controlling the turning of the key. The keyhole is a narrow
twisted slot in the face of a cylinder, G (Fig. 219), which revolves
inside a larger fixed cylinder, F. As the key is pushed in, the notches
in its upper edge raise up the pins A^1, B^1, C^1, D^1, E^1,
until their tops exactly reach the surface of G, which can now be
revolved by the key in Fig. 220, and work the bolt through the medium of
the arm H. (The bolt itself is not shown.) If a wrong key is inserted,
either some of the lower pins will project upwards into the fixed
cylinder F (see Fig. 221), or some of the pins in F will sink into G. It
is then impossible to turn the key.

[Illustration: FIG. 220.--Yale key turning.]

There are other well-known locks, such as those invented by Bramah and
Hobbs. But as these do not lend themselves readily to illustration no
detailed account can be given. We might, however, notice the _time_
lock, which is set to a certain hour, and can be opened by the right key
or a number of keys in combination only when that hour is reached.
Another very interesting device is the _automatic combination_ lock.
This may have twenty or more keys, any one of which can lock it; but the
same one must be used to _un_lock it, as the key automatically sets the
mechanism in favour of itself. With such a lock it would be possible to
have a different key for every day in the month; and if any one key got
into wrong hands it would be useless unless it happened to be the one
which last locked the lock.

[Illustration: FIG. 221.--