Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/769

LOCK 745 rately at fig. 11; and fig. 12 is a cross section of it, the black ring being the keyhole, and the light spot in the Fi". 10. middle the drill-pin, which goes into the key. The short pin b in figs. 9, 10, 11 is set in the end of the cylinder, b TI Fig. 12. near its edge ; and, when the cylinder turns round, that pin shoots or draws the bolt, by acting in a slit of the form shown in fig. 13. The security of the lock depends upon a number of sliders, s, s, of , which the shape is shown in t/J fig. 14, and the cross section in fig. 12. They are made of plates of steel doubled, and sprung open a little, so as to make them move with a little friction in the slits of the cy linder or revolving barrel in which they lie, and are pressed up against the cap of the lock by a spiral spring. They are shown so pressed up in fig. 9, and pressed down by the key in fig. 10. There is a deep groove cut round the barrel, and in each of the sliders there is a deep notch which can be pushed down to that place in the barrel by a key slit to the proper depth ; and it is evident that when all the sliders are pushed down to that position the barrel will present the appearance of having no sliders on it. A steel plate (fig. 15), made in tvo pieces in order to get it on, embraces the barrel at the place where the groove is, having notches in it corresponding to the sliders, and is fixed to the body of the lock by two screws marked d, d in figs. 9, 10, and 15. When the sliders are pushed up by the spring they fill the notches in the plate, and prevent the barrel from turning ; but when they are pushed down by the key the notches in the sliders all lie in the plane of the plate, and so the barrel can turn with the key, and the pin 6 in the end of it drives the bolt as before described. The key has a bit, k, sticking out from the pipe, the use of which is to fix the depth to which it is to be pushed in, and then, as the bit slips under the cap of the lock, it keeps the key at the same depth while it is being turned. This was the construction of the lock for a good many years, and Bramah pronounced it in that state "not to be within the range of art to produce a key, or other instrument, by which a lock on this principle can be opened." It was found, however, long before the defeat of the improved challenge Bramah lock by Mr Hobbs in 1851, that the inventor had made the common mistake of pronouncing that to be impossible which he only did not see how to do himself. As it has been generally supposed that what is called the tentative method of lock-picking was unknown in England before it came over from America in the year of the Exhibition of 1851, we must remind our readers that it was described in the 7th edition of this work fifty years ago, though the lock-picking fraternity were not of sufficiently literary habits to make themselves acquainted with it. Mr Hobbs, it is true, carried the process further than had been supposed possible before ; but all the Barren and Chubb and other many-tumblered locks, which were supposed impregnable, might long ago have been opened by anybody who had paid attention to the method by which the Bramah locks were known to have been picked some seventy years ago, before the intro duction of the false notches designed in 1817 by Mr Russell, then one of Mr Bramah s workmen. If you apply back ward pressure to the bolt of a tumbler lock vhen locked, or twisting pressure to the barrel of a Bramah lock, first pressing down the spiral spring, there will be a greater pressure felt against some of the tumblers or sliders than against others, in consequence of inevitable inequalities of workmanship ; and if you keep the pressure up, and gently move any of the tumblers or sliders on which the pressure is felt, you will at last get it to some point where it feels loose. That may or may not be the exact place to which the key ought to lift it ; but as soon as you feel it loose leave it alone, it will not fall again, as the friction is sufficient to prevent it ; and, if necessary, you may fix it there by a proper instrument, or measure the depth and keep the measure till you begin again. Then try another tumbler which feels tight, and raise it till it also feels loose. And if you go on in that way, always leaving the loose tumblers alone, and raising the one which feels tight, they will at last all be got into the position of complete freedom, i.e., to the place where the stump of the bolt can pass them. The operation is just the same in principle in the Bramah lock and in tumbler locks ; only, as all the sliders are acted on by one spring in the Bramah as now made, you need only just push down that spring, and hold it there, and then the sliders maybe moved freely either way by means of a hook or a small pair of self-acting forceps to pull them up if they accidentally get pushed too far. At first each slider had a separate spring. But if the sliders have some false notches in them not so deep as the true ones (see fig. 14), and the corners of the notches in the plate dd are cut out a little (as in fig. 15), then you might by trial get all the sliders into such a position that the barrel could turn a very little, but no more ; and when it is turned that little, you cannot push the sliders in any further, and so (as was long supposed) the tentative process is defeated ; and undoubtedly it is made much more troublesome, but it only requires more time and patience. You can still feel that the pressure is greater against some one or more of the tumblers or sliders than against others, and, wherever that is the case, XIV. - 94