This very useful article needs little description; but an explanation of the principles on which its security depends will probably be new and interesting. The most essential portion of a lock is a bolt which can be thrown out or withdrawn only by its proper key; and the best lock is that which most perfectly prevents any other instrument from effecting this purpose.

There are two distinct modes by which this excellence has been sought; and a clear understanding of their principles and differences will greatly promote a due appreciation of the many ingenious contrivances to which the pursuit of this object has given rise.

The first mode consists in the insertion of fixed obstacles, called wards, to the entrance of other instruments than the proper key. They are placed between the keyhole and the bolt; and in good locks they are made numerous and intricate, for the double purpose of preventing access to the bolt, and of varying the patterns of the keys.

The second mode consists in the insertion of obstacles to the motion of the bolt; not fixed, therefore, like the wards, but moveable, by means of the proper key, into positions which give freedom to the bolt. They are frequently called tumblers; and we shall here apply that very expressive term to all moveable impediments to the retraction of the bolt, retaining the equally appropriate designation of wards to describe the fixed impediments to the introduction of surreptitious instruments.

These two means of protection, namely, wards and tumblers, may be applied separately or combined.

Till the middle of the last century, the locks used in Europe depended principally on the number and intricacy of their wards. But very little skill was exhibited even in this respect, though a great deal of misplaced labour and ingenuity was sometimes expended in fanciful decorations.

One very serious objection to the use of wards alone is, that they do not require to have exact counterparts in the key. It is true that the proper key is usually cut in perfect correspondence with the wards in the lock; but such accuracy is not indispensable, as the diagram figures 1 and 2 in Plate CCCXXIV. will render evident.

Let figure 1 represent the bit or tee of a key curiously cut. The essential parts of such a key are the pipe a, which fits on to the pin of the lock, and the extremity b, which acts against the bolt. The intermediate portions are required merely to connect these two; the greater portion of the metal is therefore superfluous, and might clearly be removed, as shown in figure 2, which retains all that is wanted for connection and strength. But this facility of fabrication from an impression of the key is not the only evil. There is the chance that out of many variously formed skeletons, one will be found to pass almost any lock guarded by wards alone; to which may be added, the fact that an impression of almost any fixed wards may be obtained by introducing into the lock a blank key of proper size, having its bit covered with wax or tallow.

Whilst we are on the subject of keys, it may be noticed that some, instead of the small piercings, such as are shown in figures 1 and 3, have considerable portions of the bit cut away, more nearly resembling figure 2. In the former cases the wards are formed of thin iron plate; in the latter, of solid brass, and hence such locks are called solid warded. This peculiarity, however, adds nothing to the difficulty of picking, or of making false keys. The best sort of wards are those which intertwine from opposite sides of the bit, as in figure 3, which, it is obvious, would require to be more nearly imitated than the more numerous cuttings of figure 1. The bits of keys, as viewed endways, are variously shaped; the ordinary form is figure 4; figure 5 is called a taper bit, and figure 6 an S-shaped bit. No effect besides variety of pattern is obtained from these devices.

In reference to the operation called picking, the number and variety of the wards afford no security whatever. There must always be a clear passage in the lock for the revolution of the portion b of the key, and through this passage the pick-lock introduces his pick, which is made semicircular for the purpose of passing round the exterior circle of the wards.

Locks destitute of tumblers may be generally recognized by a smart snapping noise in the act of locking, which arises from the pressure of a spring formed in the back of the bolt, and from which they are called back-spring locks. Figure 7 shows one of these locks, in which a, a are two notches on the underside of the bolt, connected by a curved part; b is the back-spring, which is here compressed by the passage of the curve through a limited aperture in the rim cc of the lock. The bolt is in the state called half-shot; when it is wholly shot, or wholly withdrawn, the notches a, a rest upon the rim cc, and the force with which they fall into this position, urged by the spring b, gives rise to the noise before mentioned. The back-spring lock is further objectionable from the facility with which the bolt may be forced back by any pressure applied to its end; an application which, on wooden doors and furniture, it is frequently possible to make.

The first improvement on the lock guarded by wards consisted in the addition of a single tumbler, as seen in figure 8. The bolt is here also supposed to be half shot, and it has two notches on its upper edge, which receive the stud a of the tumbler respectively, when the bolt is at one or other end of its course. The tumbler turns on a pin at its other extremity; it is forced downwards by a spring, and it is wide enough, lying behind the bolt, as shown by the dotted lines, to be elevated by the bit of the key.

This construction, in addition to a more or less perfect box of wards, forms what is called the common-tumbler lock, and which, though better than the back-spring lock, is easily picked, for this reason, that the tumbler merely requires to be lifted sufficiently high; if it be raised more than is necessary, the bolt is equally set free. An expert smith therefore finds little difficulty in throwing out the tumbler, and at the same time moving the bolt.

In this state the art of lock-making remained, when, in the year 1778, Mr Barron obtained his patent for what may be called the first scientific attempt to improve this very important instrument. Mr Barron increased the number of tumblers to two or more, and he so applied them, that mere elevation was no longer sufficient to release the bolt; they required a precise degree of elevation, neither more nor less, and each tumbler a peculiar quantity. Figure 9 will explain this ingenious and effectual change; the tumblers are shown by dotted lines behind the bolt, which latter has in its middle a groove or slit, notched on both edges, through which are seen projecting the studs of the tumblers. The bolt, as in the former cases, is supposed to be half shot; but it will be easily perceived, that if the studs of the tumblers rested in the lower notches, they would require to be elevated to the level of the groove before the bolt could be moved; and it is equally clear, that if lifted too high, they would be engaged in the upper notches, and would equally detain the bolt. The tumblers were made unequally wide, and demanded therefore similar inequalities in the bit of the key, which was stepped or notched, as shown in figure 3. The relative positions of the bolt and tumblers are seen in the section of the lock, figure 10. It must, however, be understood, that this and the former figures are not portraits of locks, but rather diagrams, to explain the theory of their action.

Barron's lock, when well made, with two or three tumblers, and a good box of wards, is extremely difficult, if not impossible, to pick; for the wards, which are useless as preventives to the operation of throwing up a single tumbler, greatly impede so delicate a proceeding as the eleva- tion of two or more to a precise height; and this required delicacy of manipulation has formed the basis of all the subsequent attempts at improvement.

The first of these in time and ingenuity is that of Mr. Bramah, who procured his patent in 1784, but afterwards very materially modified his invention. Mr. Bramah's lock, though presenting little that is novel in principle, is singularly ingenious and compact in its arrangement; and it has the advantage of being chiefly formed in the lathe, by which means the most perfect accuracy of form is made compatible with very moderate cost.

Mr. Bramah rejected entirely the use of fixed wards, employing only moveable guards, which we shall call tumblers, because they are analogous to the parts so named in Barron's and other locks. Of these he introduced never less than four, and frequently six or eight. Having no wards, he required no large bit on which to cut their counterparts; and thus he effected the double purpose of giving great portability to the key, and of so reducing the size of the key-hole as in some measure to compensate for the absence of wards as checks to any fraudulent attempts upon the tumblers.

The general features of this lock will be best understood from an examination of figures 11 to 16. It may be described as consisting of two brass barrels, the outer one, \textit{aa}, being screwed to the plate or body of the lock, which is, however, not shown, and the inner one, \textit{cc}, revolving within it. The inner barrel is shown detached in figures 14 and 15: it has a pin or stud \textit{b}, which moves the bolt in the act of locking or unlocking, performing therefore the office of the bit of the ordinary key. The security of this lock depends, then, on certain contrivances, which prevent the revolution of this inner barrel, except on the application of its proper key. For this purpose, it has an external circular groove penetrating to the depth shown by the dotted circle in its section, figure 15; and it has four internal longitudinal grooves, as seen also in figure 15, but here seen occupied by what are in fact double pieces of steel-spring, formed like figure 16. These pieces of steel are the moveable guards or tumblers; and they prevent, except under certain circumstances, the revolution of the inner barrel. To do this, a thin plate of steel, called the locking-plate, \textit{d d}, figures 11, 12, and 13, is screwed in two portions to the outer barrel, and at the same time occupies the external circular groove of the inner barrel, having four notches to receive the edges of the tumblers, figure 16, which lie in the internal longitudinal grooves. It will be seen now that the inner barrel is locked to the outer, because the tumblers of the inner barrel are detained in the notches of the locking-plate \textit{d d}, which is screwed to the outer barrel. If, however, the tumblers be also notched as much as the locking-plate projects into their grooves, and if these notches be brought into the plane of the locking-plate, it is obvious that the barrel would then revolve, and the notches so formed would traverse the projection of the locking-plate into the longitudinal grooves. Such a notch is therefore formed in each tumbler; they are represented by the deepest of the three notches shown in the several figures, and are at various distances from the end of the tumbler. The smaller notches need not be alluded to at present. The tumblers have a projection at one end, by which they are pressed upwards by means of the spiral spring shown in figures 11 and 12. In the former figure, the tumblers occupy their usual position in the lock; in the latter they are unequally depressed by the unequal slits in the pipe of the key, so that their deep notches are now in the plane of the locking-plate, and the inner barrel is free to revolve. As the tumblers are unequally depressed, it is clear that only one of them (the lowest) can be supported by the spiral spring; the other three would therefore be left unsupported, but that they have sufficient friction in their grooves to retain them where they are placed, for the purposes of locking and unlocking. This friction is given by bending outwards the spring-plate of which they are made, as in figure 16; but it is not sufficient to prevent the spiral spring from elevating them all to the same level as soon as the key is withdrawn.

Such was Bramah's lock, as it first came extensively into use, and as it continued for many years, employed for the most important purposes, and by the most distinguished persons. At length an advertisement appeared in the public papers, requesting those who had lost keys of Bramah's locks, not, as had hitherto been done, to break open their doors or drawers, but to apply to the advertiser, who would undertake to save this destructive process by picking. And it appeared that an individual of great dexterity could perform this operation almost with certainty.

The effect of this discovery on the demand for the locks may easily be imagined; but the effect it had in stimulating ingenuity to provide a remedy, is one of the best illustrations of the proverb, that necessity is the mother of invention. Within a few days or weeks, Mr. Russell, who was at that time employed in Mr. Bramah's establishment, devised an alteration which at once, and without any expense, entirely overcame the difficulty, and converted the lock into one of perfect security. This contrivance is the most simple and extraordinary that ever effected so important an object; but before we describe it, we will endeavour to explain what has been called the tentative process of lock-picking, and which had been so successfully applied to Bramah's locks. To do this, we will refer to the diagram, figure 17, in which \textit{a} represents a bolt fitting loosely an outer case, and tending to move in the direction of the arrow. It is, however, prevented by three studs, which are moveable upwards and downwards by their knobs, 1, 2, and 3; but it would be free to move if these studs were elevated or depressed to correspond with the notches made in the end of the bolt. Now, if the case were closed, it would be perfectly easy to place these studs in the requisite position by the following process: Apply a pressure to the bolt, which can be done in any lock, tending to move it for unlocking. This will of course be resisted by the studs 1, 2, 3, but not by all equally; it will in fact be resisted by only one of them; the necessary imperfections of all workmanship would prevent more than one at a time from coming into sensible contact with the bolt. Having therefore applied pressure to the bolt with one hand, with the other the operator would ascertain which of the studs was most bound by the pressure. This would be easily done, and then he would move the stud gently up and down till he felt it catch in the notch to which it belonged. The bolt would move till the next most prominent stud received it, which would be tracked to its notch in the same manner, and so on till all were disposed of.

By a similar process the picking of Bramah's lock was effected. A tendency to revolve was given with some force to the barrel; then, by means of a pair of small forceps, the several tumblers were tried, and it was ascertained which was most detained by the pressure against the locking-plate. That which offered most resistance was gradually depressed till its notch was felt to hang itself upon the locking-plate, and so on till the whole were depressed in succession, exactly as they would have been depressed simultaneously by the key.

Returning to figure 17, it will be easily understood, that if, in addition to the three principal notches, a number of shallower notches, as indicated by dots, be made, equal however in depth to any possible inequality in the projection of the studs, the process we have described is entirely prevented, because it will be perfectly impossible for the picker to tell whether he has brought the stud to one of the deep notches, or to one of the shallow or false notches; and he will of consequence be entirely baffled.

This was the happy suggestion to which we have alluded. This will explain the two shallow or false notches which now appear in the edges of Bramah's tumblers, and which require corresponding false notches, or rather false widening in the notches, of the locking-plate, as shown by the dots in figure 13.

From this time the art of lock-making presents nothing very decidedly new, though almost every year produces something which claims the public attention for a time, and is forgotten. From this description must, however, be excepted the lock of Mr Chubb, which has come into extensive use. It consists of Barron's tumblers, more or less numerous in proportion to the price, with few or no fixed wards, and without the false notches, or any similar contrivance. But it has an ingenious piece of mechanism called a detector, consisting of a lever, which, if any undue elevation be given to the tumblers, catches into and detains the bolt, till, by a peculiar application of the proper key, it is again released. Whether this detector be a more or less perfect guard than the others we have described, and whether it be or be not an advantage to learn that a false key has accidentally or designedly been introduced into a lock, are questions which remain open to discussion. Certain, however, it is, that by thus varying the construction of locks, we obtain the benefit of greater variety in the keys, which would otherwise be so multiplied of similar makes, as to neutralize in a great measure the benefits of the skill which has been applied to the locks. On this point it may be mentioned, that Bramah's keys, as made by himself and by Mordan, are constructed by a beautiful tool, which secures a constant change in the pattern, so that duplicates are not likely to recur for a long series of years.

Of the locks called "Combination Locks," it will not be required to say much. The common letter padlock is generally known; and, from the description here given of other locks, it will be easily understood that the lettered rings of the padlock are in fact tumblers, exposed to the operation of the hands, instead of being subjected to the concealed application of a key. These locks are not difficult to pick, though they might be made quite secure by the use of the false notches. There is indeed no tumbler lock to which this most invaluable idea might not be advantageously applied.

We have reserved till now the notice of a very curious lock, which is said to have been used for four thousand years in Egypt and Turkey, and which was first made well known to Western Europe through the French invasion of Egypt at the beginning of the present century, though it had been previously described by Eton in his Survey of the Turkish Empire. It appears to be extensively, or rather universally, employed for gates of towns and of houses, as well as for the smallest articles of furniture. The evidence of its alleged antiquity is derived, according to M. Denon (in whose magnificent work on Egypt it was published), from the circumstance of finding one sculptured amongst the bas-reliefs which decorate the great temple at Karnak, from which also it was ascertained that during forty centuries the lock had undergone no sensible change.

However this may be, its construction is singular and interesting, and particularly so because it makes exclusive use of tumblers, which, it will have been seen, were the last means of security adopted in this country. It will be understood by an examination of figures 18 to 21. In figures 18 and 19, aa is the case screwed to the door, and bb is the bolt. In the case, above the bolt, are a number of small cells containing headed pins, arranged in any form chosen by the maker. The top of the bolt has a number of holes similarly arranged, so that when brought into the right position, the ends of the headed pins may drop into the corresponding holes in the bolt. In this state the bolt is fastened. In order to unlock it, a large hollow or cavity is made at the exposed end of the bolt, which extends under the holes occupied by the pins. The key consists of a piece of wood, figs. 20 and 21, having pins arranged like those in the lock, and as long as the thickness of the bolt above the cavity. When therefore the key is introduced and pressed upwards, its pins exactly fill the holes in the bolt; and of course dislodge those which had fallen from the upper part of the case. The bolt may in consequence be withdrawn, as in fig. 19, leaving the headed pins elevated in their cells, instead of occupying the position shown by the dotted lines in fig. 18. It should be observed, that in fig. 19 the key is shown in its place, diminishing therefore the apparent size of the cavity in the bolt, which of course must be high enough to receive the key and its pins before the latter are lifted into the holes.

The attention excited by this lock when it was first made generally known caused its merits to be greatly overrated. M. Denon described it as no less secure than any other lock, and fit to be applied on all rural occasions. There would, however, be little difficulty in picking it, or in obtaining a wax impression of the holes; and there is an obvious awkwardness in the mode of using the key as compared with those revolving round fixed centres, which quite disqualifies it for superseding the modern lock. In other respects, it admits of considerable variety in the number, arrangement, and length of the pins; and by grooves in the top of the bolt they might be varied in the same lock. By a slight modification, also, it might have the benefit of Barron's invention, and require a precise degree of elevation. In fact, it has this principle to some extent; for if the pins be raised too much, whatever raises them will hold the bolt by entering into the upper cells.

Numberless contrivances for the improvement of locks, and some of great merit and ingenuity, will be found in the Repertory of Patent and other Inventions, in the Transactions of the Society of Arts, and in other scientific journals. Many of these will be found more curious than useful; such, for instance, as the locks which ring bells or discharge fire-arms when an attempt is improperly made to open them. Such, also, are the propositions to enlist magnetism into the service of lock-making, by employing polarized tumblers, and magnetic keys to act upon them, through the agency of magnetic attractions and repulsions, instead of the vulgar modes of pulling and pushing; and such are the locks made to open only by a peculiar sleight of hand, as if it were possible to preserve such tricks from the knowledge of those who are interested in acquiring them. A more useful class of inventions are the shifting or combination keys, whose parts may be kept by the owner in such a state of derangement as to require the knowledge of a certain cipher or motto in order to use them.

The theory of what have been called master keys, which figure more perhaps in romances than in the real world, will be understood by considering figures 1 and 2 of the engraving. It is clear that a great variety of locks might be made, with keys somewhat resembling figure 1; differing, however, sufficiently to be opened only by their respective keys, yet all of them adapted to the skeleton, or, in this case, the master key, figure 2. In this respect, the greatest facilities are presented by Bramah's arrangement. Any number of these locks, of the same size, and having the same quantity of tumblers, may be made to open by one key, though each be proof against any other of the series. For this purpose, nothing is required, but, after the completion of the locks, to cut in each an additional set of notches adapted to the intended master key, leaving the other notches to open it by the application of its peculiar key. Lock, or 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 Navigation, Inland.