The key being thus completed and applied to the surface of the levers, will, by a gentle pressure, force them to unequal distances from their common station in the frame, and sink their points to unequal depths into the space beneath the plate. While the levers are in this position, the edge of the plate will mark the precise point at which the notch on each lever must be expressed. The notches being cut by this direction, the irregularity which appears when the levers resume their station in the frame, and the inequality of the recesses on the bit of the key, will appear as a seal and its corresponding impression. The moving of the bolt, or other parts of the lock whereby it may be opened, entirely depends on the positive motion of the levers, etc, as any of them would, by being pushed the least degree too much or too little, entirely prevent the bolt from being moved or set at liberty: and as the whole of the levers are restored to their situation when the bolt is withdrawn, the tally, or impression, is totally destroyed, and, consequently, the opening of the lock is left wholly dependent on chance whilst the said key is absent, as there is no rule whatever to assist in discovering the required position of each or any of the levers, or other movables, whereby the form of the key necessary to the opening of the lock might be ascertained.
Mr. Bramah calculated the number of changes of position that the levers of such a lock are capable of before the right one might be discovered, in the following manner: - "Let us suppose the number of levers, sliders, or other movables, by which the lock is kept shut, to consist of twelve, all of which must receive a different and distinct change in their position or situation by the application of the key, and each of them likewise capable of receiving more or less than its due, either of which would be sufficient to prevent the intended effect; it remains, therefore, to estimate the number producible, which may be thus attempted: - Let the denominations of these levers, etc be represented by twelve arithmetical pro-gressionals, we find that the ultimate number of changes that may be made in their place or situation, is 479,001,500; and by adding one more to that number of levers, etc, they would then be capable of receiving a number of changes equal to 6,227,019,500, and so on progressively, by the addition of others in like manner, to infinity.
From this it appears that one lock, consisting of thirteen of the above-mentioned levers, sliders, or other movable parts, may (by changing their places only, without any difference in motion or size) be made to require the said immense number of keys, by which the lock could only be opened under ail its variations."
Statements like the foregoing, apparently founded upon just reasoning, obtained for Bramah's patent an extraordinary degree of reputation, and, for the patentee, dining many years, a very lucrative trade; but this and other improvements induced a corresponding study in the art of picking, which finally obtained a triumph over Bramah's invention; and had it not been for the discovery of new means of baffling the picker's art, by the introduction of false notches, the reputation of these admirable locks would have been destroyed; but, from the apparent impossibility of discovering the false from the true notches, or of ascertaining those which assist from those which do not assist in the effect the lock is now deemed inviolable; it is manufactured very extensively, and sold at very moderate prices.
In 1805, Mr. Stansbury, an American, came over to this country with a new lock, which he patented, and was very assiduous in endeavouring to get it introduced; in which attempt, however, he met with so little encouragement, that it might be deemed a failure. Nevertheless, there was sufficient originality in his contrivance to merit a notice in this place: the key was of the ordinary shape of those with a pipe, but longer and narrower in the bit, on the lower side of which were a number of pins projecting from its surface; the key had no wards, and the lock, consequently, none; the bolt was not moved by the key immediately, hut through the instrumentality of a revolving circular plate, attached to, and underneath which, was a fixed pin, that took into a notch in the bolt; it was therefore the office of the key to remove the impediments to the motion of the revolving plate, which impediments consisted in a number of pins passing through it and another fixed circular plate or bridge underneath, the said pins being pressed through both, and made flush with the surface of the upper by the action of springs rivetted to the bridge.
The two plates thus locked together were separated by the projecting pins upon the key, which, entering the holes in the upper plate, pressed the spring pins out of them and turned the plate round. The pin-holes in the circular plates were not opposite to the key-hole, but on one side leading towards the bolt, so that to find them out it was necessary to push the key slightly against the plate whilst turning it round.
Mr. Lawson subsequently took out a patent for a lock, the additional security in which consisted in the employment of a sliding curtain, which is drawn before the key-hole in the act of unlocking, thus rendering it impossible to move the bolt whilst a pick remained in the aperture.
In 1816 a lock was invented by Mr. Kemp, of Cork, the security of which consisted in the adaptation of tumblers or sliders, operated upon by two, three, of more small concentric tubes, of different lengths, placed inside the barrel of the key. These tubes were made of such a length as to push back the pins or sliders that detain the bolt, to the required positions, until each one corresponds with the notch that is cut in it for the projecting part of the bolt. Mr. Kemp calls his invention the union loch, from the circumstance that it unites the qualities of Barron's and Bramah's locks; and from the manner in which the combination is effected, it affords, according to the inventor, a greater degree of security than either of the former, or than both of them together, supposing a lock of each kind was placed on the same door; and that a dishonest servant, who does not possess any particular ingenuity, may be instructed by a locksmith how to take the requisite impressions of either Barron's or Bramah's keys, even if he could be intrusted with them only for a few minutes: but this cannot be done with the key of the union lock, as it would require the locksmith to examine it himself, and to make several tools to ascertain its different dimensions, which he could not do without having it in his possession for some considerable time, with leisure to make repeated trials.