notch.

A lock of this type also can be picked very easily, as the picker has
merely to lift the tumbler and move the bolt along. Barron's lock,
patented in 1778, had two tumblers and two studs; and the opening in the
bolt had notches at the top as well as at the bottom (Fig. 215). This
made it necessary for both tumblers to be raised simultaneously to
exactly the right height. If either was not lifted sufficiently, a stud
could not clear its bottom notch; if either rose too far, it engaged an
upper notch. The chances therefore were greatly against a wrong key
turning the lock.

[Illustration: FIG. 215.--The bolt of a Barron lock.]

THE CHUBB LOCK

is an amplification of this principle. It usually has several tumblers
of the shape shown in Fig. 216. The lock stud in these locks projects
from the bolt itself, and the openings, or "gates," through which the
stud must pass as the lock moves, are cut in the tumblers. It will be
noticed that the forward notch of the tumbler has square serrations in
the edges. These engage with similar serrations in the bolt stud and
make it impossible to raise the tumbler if the bolt begins to move too
soon when a wrong key is inserted.

[Illustration: FIG. 216.--Tumbler of Chubb lock.]

Fig. 217 is a Chubb key with eight steps. That nearest the head (8)
operates a circular revolving curtain, which prevents the introduction
of picking tools when a key is inserted and partly turned, as the key
slot in the curtain is no longer opposite that in the lock. Step 1 moves
the bolt.

[Illustration: FIG. 217.--A Chubb key.]

In order to shoot the bolt the height of the key steps must be so
proportioned to the depth of their tumblers that all the gates in the
tumblers are simultaneously raised to the right level for the stud to
pass through them, as in Fig. 218. Here you will observe that the
tumbler D on the extreme right (lifted by step 2 of the key) has a stud,
D S, projecting from it over the other tumblers. This is called the
_detector tumbler_. If a false key or picking tool is inserted it is
certain to raise one of the tumblers too far. The detector is then
over-lifted by the stud D S, and a spring catch falls into a notch at
the rear. It is now impossible to pick the lock, as the detector can be
released only by the right key shooting the bolt a little further in the
locking direction, when a projection on the rear of the bolt lifts the
catch and allows the tumbler to fall. The detect