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fig. 17. Simple tumbler lock.

One of the most elementary forms of tumbler-lock is shewn in fig. 17. In this case the bolt, instead of having two notches in the bottom edge, like those in the back-spring lock, fig. 6, has two square notches or slots in the upper edge; and as the key acts upon the bolt, these notches must of course share in whatever movements the bolt is subjected to. Behind the bolt is a kind of latch or tumbler (the lower part of which is shewn by dotted lines), with a stump or projecting piece of metal at a; the tumbler moves freely on a pivot at the other end, and is made to rise through a small arc whenever the key acts upon the bolt. When the bolt is wholly shot, the stump falls into one notch and prevents the motion of the bolt; when wholly unshot or withdrawn, the stump falls into the other notch, and equally prevents the motion of the bolt. It is not, therefore, until the key, by elevating the tumbler, has raised the stump out of the notch, that the bolt has freedom of movement. If the shape of the key does not enable its web to effect this elevation to a sufficient degree, the bolt remains immovable; and to this extent a certain additional security is obtained by making the shape of the key significant as well as the wards.