LOCK, a fastening in which a moveable bolt may be prQjected by the action of a key, introduced into the leek in such a manner that the bolt cannot be returned to its original position, so as to release the door or cover, excepting by another application of the key. There are, however, many locks in which the bolt is projected by the action of a spring, without the aid of the key, and in which it may be drawn back by means of a handle ; although it cannot be moved from the other side without the application of the key. In other looks, again, there are two or more bolts, one of which only is under the control of the key, the others being moved by handles ; while in another ()lass two or more bolts are shot or projected by the action of the key alone. In addition to such differences, the varieties in the form and size of locks, and in the arrangement of their parts, are almost infinite. In door and closet locks, the bolt is usually projected into a fixed socket. In various kinds of box and cabinet locks the bolt is not projected beyond the casing of the lock, but is caused to shoot into or through a staple which drops into the lock to receive it. In desk-locks and some others the bolts project permanently, and are of a hooked shape, adapted to catch, by a lateral. movement, into staples provided to receive them. Padlocks are a kind of detached lock in which a curved bar of iron, pivoted to the lock at one end, may be passed through a staple, and then so secured by shooting the bolt into a cavity in its free end, that it cannot be removed from the staples through which it has been passed.
By far the greater number of the contrivances for rendering locks inviolable may be classed under one of three systems of security.
Warded key of an ordinary lock consists of a cylindrical shank with a loop-shaped handle at one end, and a piece called the bit projecting from it at or near the other. The bit end of the shank is, in the keys of locks which are to be entered by the key from one side only, made hollow or tubular, to fit on to a pin fixed in the lock ; but in locks which may have to be opened from either side, such as ordi nary room-door locks, the shank is made solid, and is prolonged beyond the bit, so as to enter the upper part of the key-hole of that plate of the lock which happens to be farthest from the person applying the key. The projecting bit, after being introduced through the key-hole,
is turned round within the lock by a rotatory motion imparted, until it comes in contact with a part of the bolt so shaped that the bit cannot pass it, to complete its revolution, without shooting the bolt either backwards or forwards, as the case may be. When thus moved the bolt is retained in its position by a spring, or some other means, until it is again moved by the reverse action of the key.
The first and simplest means by which the entrance of a false key may be rendered difficult, is by giving a peculiar form to the substance of the bit, and either adapting the form of the keyhole exactly to it, or inserting pieces of metal in the lock in such a way as to prevent the admission of a bit of different shape. Fig. 1 represents several forms of bit commonly used. Of these a, b, c, and d are adapted for key holes of various corresponding forms ; while e and f, though suitable for key-holes of the same general form, admit of further security by forming projections upon the sides of the key-holes.
The next and principal means of security is the use of pieces of metal of various forms, fixed within the lock in such a way that no key can be turned round within it unless corresponding notches are cut in its projecting bit. Fig. 2, which represents a portion of the interior of a look, with the bit end of the key in its place, will illus trate this. Attached to the back-plate are two concentric prominent rings, of different degrees of elevation. These prominent rings are the wards, which impede the of a false key. No key could be put into a lock provided with them, unless a notch were made in its bit to correspond with the larger of the two rings ; and it is evident that no key could be turned round without having also a notch to correspond with the smaller circle. In the commoner kind of locks the wards seldom form a complete circle, but their effect is the same if they occupy only a small segment. Many wards are of a more complicated character, such as what are termed L, T, or Z wards, from the resemblance of their sectional form to those letters respectively.