a well-known instrument used for fastening doors, chests, &c. generally opened by a key.

The lock is reckoned the masterpiece in smithery; a great deal of art and delicacy being required in contriving and varying the wards, springs, bolts, &c. and adjusting them to the places where they are to be used, and to the various occasions of using them.

From the various structure of locks, accommodated to their different intentions, they acquire various names. Those placed on outer doors are called block-locks; those on chamber doors, spring-locks; those on trunks, trunk-locks, pad-locks, &c.

Of these the spring-lock is the most considerable, both for its frequency and the curiosity of its structure. Its principal parts are, the main-plate, the cover-plate, and the pin-hole: to the main-plate belong the key-hole, top-hook, cross-wards, bolt-toe or bolt-knob, drawback-frink tumbler, pin of the tumbler, and the flaps; to the cover-plate belong the pin, main-ward, cross-ward, step-ward or dap-ward; to the pin-hole belong the hook-ward, main cross-ward, flank, the pot or bread, bow-ward, and bit.

As on the proper construction of locks the security of the most valuable kinds of property almost entirely depends, and as numberless devices are continually fallen upon to elude the utmost efforts of mechanical invention in this respect, it thence becomes an object of no small importance to invent a lock which it should be impossible to open except by its proper key. A treatise upon this subject has been published by Mr Joseph Lock.

soph Bramah; who is confident that he has brought the matter to the requisite perfection, and that every one may rest assured of the security of his property when under the protection of a lock of his invention. He begins with observing, that the principle on which all locks depend, is the application of a lever to an interior bolt, by means of a communication from without; so that, by means of the latter, the lever acts upon the bolt, and moves it in such a manner as to secure the lid or door from being opened by any pull or push from without. The security of locks in general therefore depends on the number of impediments we can interpose betwixt the lever (the key) and the bolt which secures the door; and these impediments are well known by the name of wards, the number and intricacy of which alone are supposed to distinguish a good lock from a bad one. If these wards, however, do not in an effectual manner preclude the access of all other instruments besides the proper key, it is still possible for a mechanic of equal skill with the lockmaker to open it without the key, and thus to elude the labour of the other.

"Locks (lays our author) have been constructed, and are at present much used and held in great esteem, from which the picklock is excluded: but the admission of false keys is an imperfection for which no locksmith has ever found a corrective; nor can this imperfection be remedied whilst the protection of the bolt is wholly confided to fixed wards." This position is proved by a remark, that the wards, let them be as intricate as we please, must all be expressed on what is called the bit or web of the key: and therefore when all the varieties that can be expressed on this bit or web have been run through, every succeeding lock must be the counterpart of some other; and consequently the same key which opens one will open the other also. This is evident from the locks usually put upon drawers; and which, though they should be made to resist the picklock, are still liable to be opened by ten thousand other keys besides that appropriated to each of them. But though the variety of wards could be augmented even to infinity, still there could be no security against false keys; for as every one of the wards must be expressed on the web of the key, if another key with a web quite plain be made to fit the key-hole exactly, we have only to cover it over with some colouring substance upon which the wards may make an impression; after which, it is easy to cut out the web in a proper manner for admitting them, when the lock will be as easily opened by the false as by the true key.

The first person, according to our author, who had any claim to merit in the branch of lock-making, is Mr Baron; whose lock he acknowledges to be by far more perfect and secure than any that ever appeared before: though he still considers it as unfit for giving that absolute security which is to be wished for. His improvement consisted in the proper application of what are called tumblers. "These (lays Mr Bramah) are a kind of grapple: by which the bolt is confined, as well in its active as in its passive situation, and rendered immovable till set at liberty by the key. One of these instruments is commonly introduced into all locks that are of any use or value; it is lodged behind the bolt, and is governed by a spring which acts upon the tumbler as the tumbler acts upon the bolt: The application therefore of any force to the tumbler, which is superior to the force of the spring, will cause it to quit its hold, and set the bolt at liberty." In the common method of applying these machines, however, it matters nothing how far the tumbler is lifted above the point at which it ceases to control the bolt; but it is otherwise in those of Mr Baron's construction. The action of his tumblers is circumscribed by a certain space cut in the centre of the bolt, of dimensions sufficient only to answer the purpose intended. The space in which the tumbler moves is an oblong square; and is not only furnished with niches on the under side, into which the hooks of the tumblers are forced by the spring as in other locks, but is provided with correspondent niches on the other side, into which the hooks are driven, if any greater force be applied to the tumblers than what is just sufficient to disengage them from the bolt. Hence it becomes absolutely necessary, in the making of a false key, to construct it in such a manner, that it may with the greatest exactness give the requisite degree of prelure and no more.

Mr Bramah allows that this is a very great improvement, but objects that it is still possible to frame a key which will open it as well as its own; nor will the addition of any number of tumblers preclude the possibility of opening it. "By giving (says he) an uniform motion to the tumblers, and presenting them with a face which exactly tallies with the key, they still partake, in a very great degree, of the nature of fixed wards; and the security of this lock is thereby rendered in a proportionable degree defective." Thus, suppose the false key to have passed the wards, and to be in contact with the most prominent of the tumblers, the impression, which the slightest touch will leave on the key, will direct the application of the file till sufficient space is prepared to give it a free passage. The key will then bear upon a more remote tumbler; which difficulty being in like manner got over, the lock will be as easily opened by the false as by the true key."

This seemingly insuperable objection to the perfection of lock-making, however, our author removes with the greatest ease imaginable, by causing the tumblers which project unequally to present a plane surface: whence they would require a separate and unequal motion to disengage them; of consequence no distinct impression could be made by them upon the plane surface of the web that would give any idea of their positions with regard to one another, and the construction of a false key would be altogether impossible.

But though the principal difficulty with regard to Mr Baron's lock be now overcome, others still occur, viz. the difficulty of making locks which are constructed with tumblers sufficiently durable. The tumblers themselves, he observes, must be but slightly made; and being exposed to perpetual friction by the key and their own proper motion, they must soon decay; and the keys of Mr Baron's locks, he also observes, are much less durable than those of any other locks he ever saw.

With regard to the lock which Mr Bramah presents to the public as absolutely perfect, he informs us, that the idea of constructing it was first suggested by the alarming increase of house robberies, which may reasonably be supposed to be perpetrated in a great measure by perfidious servants, or accomplished by their connivance. Thus it is evident, that the locks which might exclude ordinary housebreakers could be no security against faithless servants, who having constant access to the locks, might easily get false keys fabricated at their leisure. In considering the subject, our author was convinced, that his hope of success depended entirely upon his using means as dissimilar as possible to those by which the old locks were constructed; as these, however varied, had been found insufficient for the purpose. "As nothing (says he) can be more opposite in principle to fixed words than a lock which derives its properties from the motion of all its parts, I determined that the construction of such a lock should be the subject of my experiment." In the prosecution of this experiment he had the satisfaction to find, that the least perfect of all his models fully ascertained the truth and certainty of his principle. The exclusion of words made it necessary to cut off all communication between the key and the bolt; as the same passage, which (in a lock simply constructed) would admit the key, might give admission likewise to other instruments. The office, therefore, which in other locks is performed by the extreme point of the key, is here assigned to a lever, which cannot approach the bolt till every part of the lock has undergone a change of position. The necessity of this change to the purposes of the lock, and the absolute impossibility of effecting it otherwise than with the proper key, are the points to be ascertained; and this our author does in the following manner.

Fig. 1. Shows Mr Bramah's first attempt to construct a lock upon this principle: which, to his surprise, turned out complete and perfect. A represents a common axis on which the fix levers, crossing the face of the lock, are united as on 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. B represents a frame through which the levers pass by separate grooves, exactly fitted to their width, but of sufficient depth to allow them a free motion in a perpendicular direction. The part which projects from the opposite side of the joint A, and is inserted in the bolt C, is a lever to which two offices are assigned; one to keep the bolt in a fixed position, in the absence of the key; the other, to give it its proper motion upon the application of the key. D is a circular platform turning upon a centre. On this the joint or carriage of the levers, and the springs on which they rest, are fixed; and the motion of this platform impels the bolt, in either direction, by means of the lever which is projected from the joint A. The inviolable restraint upon this lock, by which means it is subjected only to the action of the key, is lodged in the part E, which is 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 blocks 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 fix notches which receive the ends of the levers projecting beyond the frame B; 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 overcomes these obstacles, it must be observed, that each lever has a notch on its extremity, and that these 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 E, which the notches are exactly fitted to receive. The least motion of the machine, while 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 E will admit, and which is sufficient to give the machine a power of acting on the bolt. The impossibility of thus bringing the notches on the points of the levers into a direct line, so as to tally with the edge of the plate E by any other means than the motion and impulse of the key, is that which constitutes the principal excellency of this lock.

The key (fig. 2.) 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; "and (lays our author) as one stroke of a file is sufficient to cause such a disproportion as will prove an insurmountable impediment to their motion, I may safely assert, that it is not in art to produce a key or other instrument, by which a lock, constructed upon this principle, can be opened."

On this principle it would even 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 B, 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 on 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. This process must be rendered extremely troublesome by means of the springs; and 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 to a lock of this kind.

These difficulties render it necessary in making locks of this kind not to fit the key to the lock, as is usual in other locks, but to fit the lock to the key. In this kind of lock, therefore, the key must 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. 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 B, and sink their points to unequal depths into the space beneath the plate E. While the levers are in this position, the edge of the plate E 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 B, and the inequality of the recesses on the bit of the key, will appear as a seal and its corresponding impression.

The following is a lock contrived upon the same principle, but more curious; and, in our author's opinion, more extensively useful. Fig. 3. represents a circular block of metal divided from the centre into eight compartments, each containing a cell which forms a passage through the block, as is represented by the small circles described on the flat surface A. In each of these cells two grooves are cut at opposite points, which open a communication with the centre at one point, and with the spherical surface of the block or barrel at the other. The small circle, which marks the centre of the flat surface A, is the key-hole, which likewise forms a passage through the barrel in a parallel line with the cells which surround it. This figure represents the frame in which the active parts of the lock are deposited.

Fig. 4. shows a spiral spring lodged in the bottom of each cell, and occupying one half of the space, the other being filled with a slider resting upon the spring, and represented by fig. 5. the office of these sliders exactly corresponding with that of the levers in the lock already described. Thus, when lodged in their respective cells, they are sustained, like the levers, by the elasticity of the springs upon which they rest, till a superior power be applied; and they are again restored to their stations by the reaction of the springs when the weight is removed. The side B of each slider is projected beyond the circular surface, as represented fig. 6. in a manner similar to the projection of the levers in the former lock beyond the curved frame in which they move. The point C is projected through the interior groove into the space which forms the centre or key-hole, expressed on the flat surface A.

Fig. 7. represents the key. When this is applied, it must of course encounter these interior projections; and when pressed forward, the indented spaces on its point being unequal, will force the sliders to unequal distances from their bearers; bringing the notches expressed on their exterior projections in a direct line with each other, in a manner similar to that by which the effect is produced upon the levers in the former lock. When the key is withdrawn, and the sliders resume their stations by the pressure of the springs, the disposition of the notches must be irregular in the same proportion that the indentations on the point of the key are unequal; and they must necessarily fall again into a straight line when acted upon by the key.

Fig. 6. shows the barrel completely fitted for action. Its interior end is capped with a plate, which unites its compartments, and confines the springs and sliders within the cells to which they belong. From that plate proceeds the point A, which represents the lever by which the bolt is projected or withdrawn, according to the direction in which the machine performs its revolution.

Fig. 8. shows the flat surface of a thin plate, corresponding in its office with the part C of the former lock. The space cut in its centre is exactly fitted to the spherical surface of the barrel; the circle describing its circumference, and the notches cut on its edge coinciding with the projections of the sliders. The barrel, when encircled with this plate at the middle of its spherical surface, has its motion totally suspended till the notches on the projections of the sliders are forced, by the pressure of the key, into a line with each other: a groove being thus formed on the spherical surface of the barrel parallel to, and coinciding with, the edge of the plate, the machine is at liberty to perform a revolution in any direction, but returns to its confined state when the key is withdrawn.

The parts of the movement being thus united, the interior end of the barrel is deposited in a bed represented fig. 9. To this it is fastened at the angles of the plate represented fig. 8. by which the barrel is encircled. The station of the bolt is at A; the lever which acts upon it being projected on the other side. Fig. 10. is a cap or mask which covers the face of the movement, and completes the lock.

On this lock our author observes, that it is excellent for street doors: "For no method of robbery (says he) is more practised, than gaining admittance into houses by those keys, which, as is well known, may be procured at the old iron shops to fit almost any lock in use. Such robberies are generally committed where the servants are allowed to take the key with them when sent on errands, it being impracticable while the key is fixed in the lock. The variations, by which the production of correspondent keys is avoided, have two sources; the one arising from the changes that may be made in the disposition of the levers; the other from the number of points contained on the projected surface of each lever; by which the position of its notch may, in the smallest degree, be varied.

"The variations producible in the dispositions of fix figures only, are 720: these, being progressively multiplied by additional figures, will increase by astonishing degrees; and eventually show, that a lock containing twelve levers will admit of 479,001,500 changes; which, with the addition of another lever, will increase to 6,227,020,800. These being again multiplied by the number of changes which the projected surface of the levers will admit in the disposition of the notches, their amount will exceed numeration, and may therefore be properly said to be infinite. The slightest inspection will at once show, that their construction precludes all possibility of obtaining an impression of their internal parts, which is necessary for the fabrication of a false key; for it will be easily seen, that the positions into which the levers are forced by the pressure of the key in opening the lock, can no more be ascertained when the key is withdrawn, than the seal can be copied from its impression on a fluid, or the course of a ship be discovered by tracing it on the surface of the waves. But inviolable security is not the only excellence they possess; the simplicity of their principle gives them likewise a great advantage over locks that are more complicated, in point of duration; for their essential parts being subject to no friction, nor exposed to any possible accident from without, they will be less affected by use, and less liable to stand in need of repair."

Weir, in inland navigations, the general name for all those works of wood or stone made to confine and raise the water of a river: the banks also which are made to divert the course of a river, are called by these names in some places. But the term lock is more particularly appropriated to express a kind of canal enclosed between two gates; the upper called by workmen the sluice gate, and the lower called the flood gate. These serve in artificial navigations to confine the water, and render the passage of boats easy in passing up and down the stream. See CANAL.