The keys ninet, of course, be cut of a corresponding form, by which the difficulty of imitation is greatly increased.
By to the various forms cf key represented in Figs. 3, 4, 5, and 6, the peculiar advantages and defects of that principle of security which depends on the use of wards, may be reaclily comprehended. The greatest defect of the system arises from the circumstance that, in omi.nary awes, it is not absolutely necessary that a surreptitious instrument shoukl perfectly thread the mazes of the wards. But while the wards afford security against ordinary keys, they afford none whatever against a pick or skeleton key lie k, Fig. 3, which would open position, by inserting a blank key with the bit covered with wax or tallow, so as to receive an impression of the concealed obstructions in the lock. It is a very common practice to cut more notches in the key than there are wards in the lock ; so that the complex appearance of a key is no certain proof of the secure construction of the lock to which it belongs. Some contrivance is necessary to keep the bolt steadily in the position in which it is left by the key ; and in locks which depend upon wards for their security, this is usually effected by means of a spring, as illustrated by Fig. 7. which represents the interior of a small cupboard lock, with the bolt a d b half shot, or in a position inter mediate between locking and unlocking, and capable of being moved either backwards or forwards by the action of the bit of the key at c in a curved hollow formed in the lower edge of the bolt. The neces sary raising of the bolt and compression of the spring are properly any of the locks suited for se, A, and i, the only part essential to the moving of the bolt being the extremity of the bit, which is retained in the skeleton key with nothing but a slender piece to connect it with the pipe or shank. The security may be greatly increased by the use of other wards, attached to the opposite plate of the lock, and requiring notches in that part of the bit of the key which is represented by the slender connecting piece in the skeleton k. Such is the case in all the keys represented in Figs. 4, 5, and 6. In Fig. 4,1 represents a key for a solid-warded lock, which might, however, be easily picked by a skeleton key resembling r, Fig. 6. The greater complication of the wards in es, Fig. 4, increases the difficulty of picking; while by the adoption of the arrangement shown at a, Fig. 4, the difficulty of intro &wirer a false key is made perhaps as great as possible, since no instru ment that does not thread all the intricacies of the wards could answer the purpose. All the keys hitherto represented, as well as those in 6, are pipe-keys, adapted for such locks as have a fixed pin or axis, and can only be opened from one aide. It is, therefore, of no conse
quence that the wards attached to the back and front plates of the lock should resemble each other. In ordinary door-locks, however, in which the key may have to be inserted from either side, it is essential that the wards attached to the two plates, if such be used, should either be precisely similar, or should bear such a relation to each other that notches may be cut in both sides, or rather edges of the bit, to suit both sets of wards; it being a necessary condition that the two sides of the bit, marked a and b in the cut, Fig. 5, should be perfectly effected by the action of the key ; but as they may also be effected by pressure upon the end of the bolt, the security of locks in which such an arrangement is adopted, which are called back-spring locks, is much impaired.
Tamale? Locks.—We pass to the explanation of the second principle of security : that which consists in the use of moveable impediments to the motion of the bolt, Pig. S represents a lock provided with a common tumbler. In this figure the bolt a, 6, though shot backwards and forwards, has no spring or notches to catch on the back rim of the lock, to hold it in any required position ; but it is provided with two notches in its upper edge, at c and d. Behind the bolt is a piece of metal called the tumbler, pivoted to the plate of the lock at f, and con tinually forced downwards by a spring which presses upon its upper edge. Near the end e, the tumbler carries a projecting stud, which, when the bolt is fully shot, falls into the notch d, and holds it firmly until, by the application of the key, the tumbler is lifted up to the position shown in the cut ; by this action the bolt is released, so that the further turning of the key shoots it back, when the stud of the tumbler falls into the notch c, and again secures the bolt. So long as the tumbler remains in its proper notch, the bolt cannot be moved backwards or forwards by any pressure upon its ends ; and the lock cannot be opened by any false key unless its bit be so formed as to reach the tumbler as well as the bolt. To render this more difficult the tumbler is often made to fall a little lower than the bolt, eo as to be acted upon by a step formed on the bit of the key ; while further security may be obtained by the use of two or more tumblers, which may be acted upon by different steps on the key. The great exactness requisite in the length of the bit forms a strong recommendation of even the commonest tumbler locks; for if the bit be ever so little too short it will not lift the tumbler out of its notch, while if it be but a very little too long, it will not enter the curved portion of the bolt..