750 LOCK is on, so that they will stand indifferently in any position. In the figure they are drawn all pressed down, so as to prevent the stump S from entering the gating, and this has been done by the long tail Y of the handle, which, it is easy to see, will raise the left end of the tumblers, and depress the right, after the fan-tailed piece X of the handle has shut the bolt. After the tumblers have been raised to the proper height by turning the key half round (where it may be stopped by the plates P, P), the stump can enter the gatings, and the bolt can be drawn back by the handle, the tail Y then doing nothing. So far as we have yet gone, the lock would possess no greater security than any other many-tumblered lock ; but there is a steel curtain CC, which does not revolve as usual, but slides on two pins set in the back of the lock, and is pressed up against the front plate by two spiral springs, so as to close the keyhole completely, except when it is pressed in. From the back of the curtain there goes a kind of square plug (shown in section at fig. 21), which can be pushed through a hole in the back plate, and has a notch in it just in the plane of the bolt, and the bolt itself has a corner there ; in this way, when the curtain is up, the bolt can be drawn back through the notch in the curtain plug; but when the "" plug is pushed in ever so little the bolt cannot be drawn back, because its corner cannot pass the curtain plug, and in " " this position the stump cannot be made to touch the tumblers, except one of Fig- 21. them, which is made a little longer than the rest (as shown at T in fig. 20), in order to keep the bolt steady. It is evident then that as soon as the curtain is pushed in, to admit any instrument whatever, the bolt is held fast, and it becomes impossible to put any pressure of the stump upon the tumblers ; in other words, the tentative mode of picking is impossible. In small locks the curtain has no plug, but merely works against the edge of a second stump of the bolt, which can only pass when the curtain is up, and it slides on the drill-pin and another pin below it. The security of the lock is farther increased by the addition (DEG) of what may be called a detector, as it does detect if the bolt has not been shot far enough by the person who locked it, and, what is of more consequence, prevents it from being opened in that state. It turns on a hinge or pin at G, and is held up or down by a jumper- spring at E, as in Chubb s first detector. In fig. 20 it is shown as held down, or out of the way of the bolt ; but, as the handle turns back again and draws back the bolt, the pin below X raises the detector a little, and then the spring is ready to throw its tooth into the notch in the bolt as soon as it is shot only about half-way. In that state the bolt cannot be drawn back without turning the handle far enough for the fan-tail X to send the detector down again below the corner of the spring, and by doing that you will also have locked all the tumblers, and so made the lock fast until the key comes to open it. And it is to be observed that the curtain cannot be pushed in until the bolt is fully shot, so that no exploration of the lock can take place while it is open, or even partially open. By adding a spring catch under the curtain, to be freed by one of the tumblers when it is fully locked, it may be arranged, if required, that the curtain could not be pushed in, not only until the bolt is shot, but until the tumblers are locked also. The following, therefore, are the advantages of this lock. 1. A very large lock, with all its parts strong, only requires a very small key, not weighing above a quarter of an ounce. 2. No key is re quired to lock it, and you cannot leave the key in the lock (a fruit ful source of mischief), and yet it is free from the inconvenience of spring-locks, which sometimes shut themselves when not intended, and moreover, when large, require large and strong keys to open them. 3. It cannot get out of order from the usual causes of the tumblers sticking together, or tumbler-springs breaking, because there are none, and the tumblers do not touch each other, but the friction-plates between them. 4. The keyhole being always quite closed by the curtain, except while the key is in, the lock is pro tected from dirt and from the effects of a damp or smoky atmo sphere, which injures other locks. 5. The smallness of the keyhole prevents the insertion of any instrument strong enough to force the lock, and also prevents inspection. 6. It is pronounced by the highest authority to be secure against any known mode of picking. 7. It requires no delicacy of construction or high finish in any of the parts, and the moving parts are few in fact, the whole of them together are fewer than the number of springs alone in the great American lock of Day & Newell. 8. It is free from the incum- brance of a patent, the inventor being one of those who agree with the opinion of the jury on philosophical instruments in the exhibition of 1851, and with many of the first engineers and most scientific men, that " patents are a great obstruction to the progress of science," and waste, on the whole, more money than they gain for real inventors. Changeable Key Locks. Any lock with many tumblers may be changed by taking it off and transposing any two or more of the tumblers; but it will then want a different key, and the process is too troublesome to be resorted to except when there is reason to apprehend that the original key has fallen into bad hands, or had a copy taken of it. A lock which can be locked by a great number of keys, but can only be opened by the one which locked it last, is evi- Fig. 22. dently an immense addition to security. Those keys may either be all distinct, or there may be a great number of different webs, or stepped parts, fitting to a common handle ; or each bit or step may be separate, and all screwed together into the key frame as shown in fig 22. In a lock with n tumblers, each raisable to m distinct heights, if you have only n loose bits of the m different heights you may compose 1 x 2 x 3 . . . . n keys, which for nine tumblers = 362, 880. But if you also have m bits for each place in the web, the possible number of different keys is very much greater, viz., m n ; for every possible length of bit in each place may be used with every possible length in every other place. If m is 7 and n 9, as they are in the locks we are going to describe next, each lock may have very nearly sixty million different keys, or key-webs capable of fitting into one shank or handle ; and the chances are so many against a lock-picker hitting on the right one. The first changeable key lock known in England was Day & Newell s " Parautoptic " (inspection-defying) lock, which was brought from America by Mr Hobbs at the time of the 1851 Exhibition, and has been largely used since for banks and other safes. It was fully described in the last edition of this work, and in the books above referred to ; but in this place we will describe instead a much simpler form of it, since introduced by Mr Hart of the firm of Fig. 23. Hobbs, Hart, & Co, The principle of all changeable key locks is that instead of a single stump fixed to the bolt, there are as many stumps as tumblers, and each one is the projecting part S of a slider PPS which can ride up and down on two pins 1 P in the bolt, and will stay at any height where it is left when the bolt B is shot. Fig. 23 shows the lock open and the slider stumps lying in the jaws or gating of the tumblers ready to be raised to
