Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/773

LOCK 749

generally used for a lock which shuts of itself on a box or drawer, or articles of that kind; and the latter for street or room-door locks which shut of themselves, and open with a handle on the inside, but only with a key on the outside. In the simplest and cheapest form of these locks there is no pretence of any security except a few fixed wards, which the key has to pass; and, as before explained, that is no security at all against anybody with the smallest dexterity, and with a serious intention of opening the lock. Next to them, or rather below them, pretending to be what they are not, come the locks which lock a certain distance themselves by means of a spring, but can be locked further by the key, and have tumblers, but no fixed wards (which a good tumbler-lock does not require). But though this kind of lock cannot be opened when it is thus double locked, except by the key, or some efficient mode of picking, yet when they are only self-locked the tumblers are of no more use than if they did not exist, and the lock can be opened by any bit of bent wire that will go into the keyhole. It should be remarked, however, that the Bramah lock is just as secure as usual when used for a spring or latch lock, because the key cannot turn at all without pushing in the sliders properly. But in this, as in all latch-locks, it is very unsafe to have a handle which palls back, as it can easily be reached by a wire put through a hole in the door; the handle should always be made to turn, like a common room-door handle.

There are two ways in which spring-locks with tumblers are made as safe as the same lock with an ordinary bolt. One is to make a click or catch fall into the bolt when it is drawn back, and not to make the tumblers to fall when the bolt is drawn back; in the shutting of the door this catch is pushed back by some knob projecting for the purpose, and then the tumblers fall and hold it fast. Prison locks are made in that way. But this will not do for a latch-lock which is intended to open by a handle on one side of the door. For that purpose the proper plan is that which is now adopted in all good latch and spring locks, not to let the key act directly on the bolt, which has no stump, but on the false bolt which lies on the top of the real one, and has the stump fixed in it. When the real bolt is shut by the spring it carries the false one with it, and that is then locked by the tumblers. But the real bolt can be pushed back by the door shutting, or pulled back by the handle, without moving the false bolt, though it cannot be reached through the keyhole. In buying a lock, the test of this is to see whether the stump moves as you push or pull the bolt back. If it does the lock is good for nothing, unless it is on some other peculiar construction.

Latches and Latch Locks. – The latter of these, so called by Mr Chubb, is substantially on the principle just now described, and so is Hobbs's and Hart's latch lock, which also has the protector stump, and therefore is as unpickable as their other locks, provided neither the handle nor the bolt can be got at. Chubb's latch (not his latch-lock) consists only of four tumblers, which come out and form the bolt, and fit between a sort of mouth in the striking-plate on the door post, and have all to be lifted to the same height by the key; but that can easily be picked by the tentative method, though it might delay a common street thief for an inconvenient time.

Spring Curtain. – All latch locks for street doors are liable to stick fast through dirt getting between the tumblers, especially in a smoky town. They will keep clean much longer if the keyhole is protected by Sir E. Beckett's self-acting spring curtain, which can be added to any lock which is worth it. It consists only of a small thin steel plate, sliding on the key-pin, and another pin just below the keyhole to guide it, with a slight spring behind, for which there is room in any latch lock with a sliding locking plate over the bolt. They are not patented, and the cost is quite insignificant, and it is odd they are not more commonly insisted on by purchasers.

Safe Locks with Small Keys. – In all the locks we have yet mentioned the bolt is acted on by the key, even though the key may not touch it; the key must therefore be strong enough to move the bolt besides lifting the tumblers, or whatever is substituted for them, and this makes the key for a large lock too large and heavy to be conveniently carried in the pocket, and a bunch of such keys impossible. To get over this difficulty, most of the makers of iron safes have adopted the plan (we do not know by whom invented) of shooting a large bolt, or a number of bolts, by means of a handle, and then a small lock with a small key locks into one of them, and thus fastens them all. The security then depends upon the impregnability of the small lock against fraudulent picking or forcible evisceration. There are certain thieves instruments by which a force sufficient to tear open the inside of a lock can be inserted through a keyhole of the common size. This, however, is now defeated by cutting out a piece of the back plate, and then screwing it on again with only a few small screws; and so that alone gives way under any bursting pressure, whether from the instrument called the jack-in-the-box, or from gunpowder, which is another of the thieves methods for cutting the knot which they cannot untie. If the small lock, therefore, cannot be picked, or forced, this mode of locking the bolts of a large door is quite safe, and you have the advantage of a very powerful lock with nothing to carry in your pocket larger than a small desk key.

Sir E. Beckett's Lock. – A lock was invented by Sir Edmund Beckett (formerly Denison) in 1852, but not patented, which combines the advantages of large and strong works with a keyhole so narrow that no instrument strong enough to injure the lock could be got in, nor a reflector to observe the bellies of the tumblers; and the bolt is not only shot by turning the handle, but locked besides, without using any key at all. This lock enjoys the distinction of being the only one of English invention which was pronounced secure against any known method of picking, by Messrs Hobbs and Tomlinson, in the treatise before referred to.

Fig. 20.

In fig. 20 are shown the tumblers T, turning on a pin at or near the middle of their length, so as to be nearly balanced, though in small locks this is unnecessary. Between every two adjacent tumblers, and between the bolt and the tumbler next to it, there is a thin steel plate, which occupies the position shown by the dotted lines PP. These plates have one edge lying against the upper side of the lock, so that they cannot turn at all on the tumbler-pin, which goes through them quite loosely. One or two of the plates should be bent a little to make them act as friction springs on the tumblers when the cap of the lock