Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/768

744 LOCK

LOCK – not being a canal lock – means the fastening of a door, or box, or drawer, which requires a key, or else some secret contrivance or manipulation, to open it. It is generally fixed to the door, but it may also be loose, and then it is called a padlock, which is internally like other locks, but externally has a half link or bow turning on a hinge at one end, while the other, after being put through a chain or staple on the door, enters the lock and is fastened by the bolt therein. The bolt may be moved by the key, or may close by a spring, but require a key to open it, as in the case of handcuffs, which are a pair of padlocks of this kind united by a short chain. A common door lock also comprises a spring latch which opens by a handle, and sometimes a small bolt held by friction either shut or open, which is moved by a smaller handle inside the room only; but neither of these is the lock proper, any more than a hook or a button, or a common lifting latch. Therefore, omitting them, a lock is as defined above.

Fig. 1.

The earliest lock of which the construction is known is the Egyptian, which was used four thousand years ago. In fig. 1, aa is the body of the lock, bb the bolt, and cc the key. The three pins p, p, p drop into three holes in the bolt when it is pushed in, and so hold it fast; and they are raised again by putting in the key through the large hole in the bolt and raising it a little, so that the pins in the key push the locking pins up out of the way of the bolt. The security of this is very small, as it is easy enough to find the places of the pins by pushing in a bit of wood covered with clay or tallow, on which the holes will mark themselves; and the depth can easily be got by trial.

Fig. 2. Fig. 3. Fig. 4.

Mr Chubb, the well-known lock-maker, used to show a wooden Chinese lock very superior to the Egyptian, and, in fact, founded on exactly the same principle as the Bramah lock, which long enjoyed the reputation of being the most secure lock ever invented; for it has sliders or tumblers of different lengths, and cannot be opened unless they are all raised to the proper heights, and no higher. Until about a century ago no lock so good as this was known in England. The locks then in use (fig. 2) were nothing better than a mere bolt, held in its place, either shut or open, by a spring b, which pressed it down, and so held it at either one end or the other of the convex notch aa; and the only impediment to opening it was the wards which the key had to pass before it could turn in the keyhole. But it was always possible to find the shape of the wards by merely putting in a blank key covered with wax, and pressing it against them; and when this had been done, it was by no means necessary to cut out the key into the complicated form of the wards (such as fig. 3), because no part of that key does any work except the edge bc farthest from the pipe a; and so a key of the form fig. 4 will do just as well; and a small collection of skeleton keys, as they are called, of a few different patterns, were all the stock in trade that a lock-picker required.

Fig. 5.

The common single-tumbler lock (fig. 5) was rather better than this, as it requires two operations instead of one to open it. The tumbler at turns on a pivot at t, and has a square pin at a, which drops into a notch in the bolt bb, when it is either quite open or quite shut, and the tumbler must be lifted by the key before the bolt can be moved again. But this also is very easy, unless the lock is so made that the tumbler will go into another notch in the bolt if it is lifted too high, as in the lock we shall now describe, which was the foundation of all the modern improvements in lock-making.

Fig. 6. Fig. 7. Fig. 8.

Barron's Lock. – This was the first lock with several tumblers. It was patented in 1778. Fig. 6 is a front view, and fig. 7 a horizontal section. First consider it with reference to one tumbler at only. Unless the square pin a is lifted by the key to the proper height, and no higher, the bolt cannot move, and that alone adds very considerably to the difficulty of picking, except by a method not discovered for many years after. But Barron added another tumbler, and unless both were raised at once to the proper height, and no higher, the lock could not be opened. The face, or working edge, of the key of a many-tumblered lock assumes this form (fig. 8), the steps corresponding to the different heights to which the tumblers have to be raised, and one of them acting on the bolt, and they may have a much wider range of difference than in this figure. The key here drawn is also one with the wards of such a shape that no skeleton except itself can pass them. The form, however, can be got in the usual way by a wax impression; and as it weakens the key very much, and is expensive to cut, it is not often used.

Fig. 9.

Bramah's Lock. – The next lock of any importance was the celebrated one patented ten years after Barron's, by Joseph Bramah (see BRAMAH). In figs. 9 and 10 aaaa is the outer barrel of the lock, which is screwed to, or cast with, the plate; cccc is a cylinder, or inner barrel, turning within the other. It is shown sepa-