Although no doubt can be entertained that Barron really invented the lock we have been noticing, it appears from the statements of Mr. Ainger, that the Egyptian locks now in use are constructed upon the same principle as Barron's; and as these modern Egyptian locks are the same as those observed upon the great temple at Karnac, the invention which we have been regarding as our countryman's, and of modern date, is upwards of 4000 years old. The bolt and a fixed part of the Egyptian lock are, as described by Mr. Ainger, each pierced with any number of holes, arranged in any chosen form; those in the bolt and in the fixed part coinciding when the bolt is locked. These bolts are occupied by pins, which are contained in the fixed part, and descend into the bolt, so as to prevent its motion till they are removed wholly into the fixed part. This is effected by a key having the same number and arrangement o pins, and of such a length, that they elevate the ends of the pins in the lock to the plane of motion between the bolt and the fixed part. This key is introduced laterally through a long tube, at the end of which it acts vertically upon the pins, whose position therefore it is difficult to ascertain.
The same principle was afterwards adopted by Mr. Bramah, (who took out a patent for it in 1784,) but without the assistance of wards; his mode of application was, however, very different from the Egyptian. In the latter the security arose from a concealment of the number and position of the impediments; in Mr. Bramah's these were discoverable on inspection, and the security depended on the various degrees of motion which the several impediments required before the bolt could be moved. The office which in ordinary locks is performed by the extreme point of the key, is, in Bramah's, assigned to a lever, which cannot approach the bolt till every part of the lock has undergone a change of position. The lock may be described as consisting of a common axis, on which six levers, crossing the face of the lock, are united as in a joint. Each of these rests upon a separate spring, sufficiently strong to bear its weight, or if depressed by a superior force, to restore it to its proper position when that force is removed. The levers pass through a frame by separate grooves, exactly fitted to their width, but of sufficient depth to allow them a free motion in a perpendicular direction.
The joint or carriage of the levers, and the springs on which they rest, are fixed on a circular platform, turning on a centre, and the motion of this platform impels the bolt in either direction by means of a lever. The inviolable restraint upon this lock, by which means it is subjected only to the action of the key, is lodged in a thin plate, bearing at each extremity on a block, and having of course a vacant space beneath, equal in height to the thickness of the block on which it rests. By this plate the motion of the machine is checked or guided in the following manner: - on the edge of the plate which faces the movement there are six notches, which receive the ends of the levers projecting beyond the frame; and while they are confined in this manner, the motion of the machine is so totally suspended as to defy every power of art to overcome. To understand in what manner the proper key of this lock over-comes these obstacles, it must be observed that each lever has a notch on its extremity, and that those notches are disposed as irregularly as possible.
To give the machine a capacity of motion, these notches must be brought parallel to each other, and, by a distinct but unequal pressure upon the levers, be formed into a groove in a direct line with the edge of the plate, which the notches are exactly fitted to receive. The least motion of the machine whilst the levers are in this position, will introduce the edge of the plate into the groove, which controlling the power of the springs, will give liberty to the levers to move in a horizontal direction, as far as the space between the blocks which support the plate will admit, and which is sufficient to give the machine the power of acting on the bolt. The key exhibits six different surfaces, against which the levers are progressively admitted in the operation of opening the lock: the irregularity of these surfaces shows the unequal and distinct degree of pressure which each lever requires to bring them to their proper bearings, in order to put the machine in motion. Hence it appears that unless the various heights of the surfaces expressed on the bit of the key are exactly proportioned to the several distances necessary to bring the notches into a straight line with each other, they must remain immovable.
On this principle it would be a matter of great difficulty for any workman, however skilful, to construct a key for the lock when open to his inspection; for the levers, being raised by the subjacent springs to an equal height in the frame, present a plane surface, and, consequently, convey no direction that can be of any use in forming a tally to the irregular surface which they present when acting in subjection to the key. Unless therefore we can contrive a method to bring the notches in the points of the levers in a direct line with each other, and to retain them in that position till an exact impression of the irregular surface, which the levers will then exhibit, can be taken, the workman will be unable to fit a key to the lock, or to move the bolt. If such difficulties occur even when the lock is open to the inspection of a skilful workman, much more must we suppose it out of the power of one who has not access to the internal parts to make a false key. These difficulties render it necessary in making locks of this kind not to fit the key to the lock, but to fit the lock to the key. The key must therefore be made first, and the inequalities upon the surface of the bit worked as chance or fancy may direct, without any reference to the lock.