THE BRAMAH LOCK.
The lock which was invented by the late Mr. Bramah deservedly occupies a
high place among this class of contrivances. It differs very materially
from all which has gone before it; its mechanical construction is
accurate and beautiful; its key is remarkable for smallness of size; and
the invention was introduced by the publication of an essay containing
much sensible observation on locks generally. The full title of this
essay runs thus: “A dissertation on the Construction of Locks.
Containing, first, reasons and observations, demonstrating all locks
which depend upon fixed wards to be erroneous in principle, and
defective in point of security. Secondly, a specification of a lock,
constructed on a new and infallible principle, which, possessing all the
properties essential to security, will prevent the most ruinous
consequences of house-robberies, and be a certain protection against
thieves of all descriptions.” A second edition of this _Dissertation_
was published in 1815; but the work is now extremely scarce, and hardly
attainable.
It is remarkable to observe the boldness and self-relying confidence
with which Mr. Bramah, some sixty years ago, declared that _all_ locks
were, up to that time, violable; he felt that this was strictly true,
and he hesitated not to give expression to his conviction. The following
is from his _Dissertation_:--
“It is observable that those who are taken in the desperate occupation
of house-breaking are always furnished with a number and variety of keys
or other instruments adapted to the purpose of picking or opening locks;
and it needs no argument to prove that these implements must be
essential to the execution of their intentions. For unless they can
secure access to the portable and most valuable part of the effects,
which in most families are deposited under the imaginary security of
locks, the plunder would seldom recompense the difficulty and hazard of
the enterprise; and till some method of security be adopted by which
such keys and instruments may be rendered useless, no effectual check or
opposition can be given to the excessive and alarming practice of
house-breaking.
“Being confident that I have contrived a security which no instrument
but its proper key can reach; and which may be so applied as not only to
defy the art and ingenuity of the most skilful workman, but to render
the utmost force ineffectual, and thereby to secure what is most valued
as well from dishonest servants as from the midnight ruffian, I think
myself at liberty to declare (what nothing but the discovery of an
infallible remedy would justify my disclosing), that all dependence on
the inviolable security of locks, even of those which are constructed on
the best principle of any in general use, is fallacious. To demonstrate
this bold and alarming proposition, I shall first state the common
principles which are applied in the art of lock-making; and by
describing their operation in instruments differently constructed, prove
to my intelligent readers that the best-constructed locks are liable to
be secretly opened with great facility; and that the locks in common use
are calculated only to induce a false confidence in their effect, and to
throw temptation to dishonesty in the way of those who are acquainted
with their imperfections, and know their inefficacy to the purpose of
security” (p. 5).
Tumblers had been so little thought of and used at the time Bramah
wrote, that his attention was almost exclusively directed to _warded_
locks. The mysterious clefts in a key, connected with some kind of
secret mechanism in the lock, had given the warded locks a great hold on
the public mind, as models of puzzlement and security; and it was to
shew that this confidence rested on a false basis, that he to a great
extent laboured. The following is his exposition of the principle and
the defects of the warded lock.
“Locks have been constructed, and are at present much used and held in
great esteem, from which the picklock is effectually 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. For if a
lock of any given size be furnished with wards in as curious and
complete a manner as it can be, those wards being necessarily expressed
on what is termed by locksmiths the bit or web of the key, do not admit
of a greater number of variations than can be expressed on that bit or
web; when, therefore, as many locks have been completed of the given
size as will include all the variations which the surface of the bit
will contain, every future lock must be the counterpart of some former
one, and the same key which opens the one will of course unlock the
other. It hence follows that every lock which shall be fabricated on
this given scale, beyond the number at which the capability of variation
ends, must be as subject to the key of some other lock as to its own;
and both become less secure as their counterparts become more numerous.
This objection is confirmed by a reference to the locks commonly fixed
on drawers and bureaus, in which the variations are few, and these so
frequently repeated, from the infinite demand for such locks, that, even
if it were formed to resist the picklock, they would be liable to be
opened by ten thousand correspondent keys. And the same observation
applies in a greater or less degree to every lock in which the
variations are not endless.
“But if the variation of locks in which the bolt is guarded only by
fixed wards could be multiplied to infinity, they would afford no
security against the efforts of an ingenious locksmith; for though an
artful and judicious arrangement of the wards, or other impediments, may
render the passage to the bolt so intricate and perplexed as to exclude
every instrument but its proper key, a skilful workman having access to
the entrance will be at no loss to fabricate a key which shall tally as
perfectly with the wards as if the lock had been open to his inspection.
And this operation may not only be performed to the highest degree of
certainty and exactness, but is conducted likewise with the utmost ease.
For the block or bit, which is intended to receive the impression of the
wards, being fitted to the keyhole, and the shank of the key bored to a
sufficient depth to receive the pipe, nothing remains but to cover the
bit with a preparation which, by a gentle pressure against the
introductory ward, may receive its impression, and thus furnish a
certain direction for the application of the file. The block or bit
being thus prepared with a tally to the first ward, gains admission to
the second; and a repetition of the means by which the first impression
was obtained, enables the workman to proceed, till by the dexterous use
of his file he has effected a free passage to the bolt. And in this
operation he is directed by an infallible guide; for, the pipe being a
fixed centre on which the key revolves without any variation, and the
wards being fixed likewise, their position must be accurately described
on the surface of the bit which is prepared to receive their impression.
The key therefore may be formed and perfectly fitted to the lock without
any extraordinary degree of genius or mechanical skill. It is from hence
evident that endless variations in the disposition of fixed wards are
not alone sufficient to the purpose of perfect security. I do not mean
to subtract from the merit of such inventions, nor to dispute their
utility or importance. Every approach towards perfection in the art of
lock-making may be productive of much good, and is at least deserving of
commendation; for if no higher benefit were to result from it, than the
rendering difficult or impossible to many that which is still
practicable and easy to a few, it furnishes a material security against
those from whom the greatest mischiefs and dangers are to be
apprehended.”
There can be little doubt, in the present day, that Bramah did not
over-rate the fallacies embodied in the system of wards for locks. He
was sufficiently a machinist to detect the weak points in the ordinary
locks; and, whatever may have been his over-estimate of his own lock
(presently to be described), he was certainly guilty of no injustice to
those who had preceded him; for their locks were substantially as he has
described them. To understand the true bearings of his Dissertation too,
we must remember that housebreaking had risen to a most daring height in
London at the time he wrote (about the middle of the reign of George
III.); and men’s minds were more than usually absorbed by considerations
relating to their doors and locks.
Mr. Bramah, after doing due justice to the ingenuity of Barron’s lock,
in which, if the tumbler be either _over_ lifted or _under_ lifted the
lock cannot be opened, pointed out very clearly the defective principle
which still governed the lock. “Greatly as the art is indebted to the
ingenuity of Mr. Barron, he has not yet attained that point of
excellence in the construction of his lock which is essential to perfect
security. His improvement has greatly increased the difficulty but not
precluded the possibility of opening his lock by a key made and
obtained as above described (by a wax impression on a blank key); for an
impression of the tumblers may be taken by the same method, and the key
be made to act upon them as accurately as it may be made to tally with
the wards. Nor will the practicability of obtaining such a key be
prevented, however complicated the principle or construction of the lock
may be, whilst the disposition of its parts may be ascertained and their
impression correctly taken from without. I apprehend the use of
additional tumblers to have been applied by Mr. Barron as a remedy for
this imperfection.” Mr. Bramah thought that Barron had a perception of a
higher degree of security, but had failed to realise it; because, by
giving a uniform motion to the tumblers, and presenting them with a face
which tallies exactly with the key, they still partake in a very great
degree of the nature of fixed wards, and the security of the lock is
thereby rendered in a proportionate degree defective and liable to
doubt.
To shew how this insecurity arises, Mr. Bramah illustrates the matter in
the following way: “Suppose the key with which the workman is making his
way to the bolt 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. This being
accomplished, the key will of course bear upon the tumbler which is most
remote; and being formed by this process to tally with the face which
the tumblers present, will acquire as perfect a command of the lock as
if it had been originally made for the purpose. And the key, being thus
brought to a bearing on all the tumblers at once, the benefit arising
from the increase of their number, if multiplied by fifty, must
inevitably be lost; for, having but one motion, they act only with the
effect of one instrument.”
It is worthy of notice, that even while thus shewing the weak points of
the Barron lock, Mr. Bramah seems to have had in his mind some
conception of infallibility or inviolability attainable by the lock in
question. After speaking of the defect arising from the bad arrangement
of the tumblers, he says: “But nothing is more easy than to remove this
objection, and to obtain perfect security from the application of Mr.
Barron’s principle. If the tumblers, which project unequally and form a
fixed tally to the key, were made to present a plane surface, it would
require a separate and unequal motion to disengage them from the bolt;
and consequently no impression could be obtained from without that would
give any idea of their positions with respect to each other, or be of
any use even to the most skilful and experienced workman in the
formation of a false key. The correction of this defect would rescue the
principle of Mr. Barron’s lock, as far as I am capable of judging, from
every imputation of error or imperfection; and, as long as it could be
kept unimpaired, would be a perfect security. But the tumblers, on which
its security depends, being of slight substance, exposed to perpetual
friction--as well from the application of the key as from their own
proper motion--and their office being such as to render the most
trifling loss of metal fatal to their operation, they would need a
further exertion of Mr. Barron’s ingenuity to make them durable.”
It may perhaps be doubted whether the principle of Bramah’s lock is not
more clearly shewn in the original constructed by him than in that of
later date. In appearance it is totally different, but the same
pervading principle is observable in both; and the cylinder lock can
certainly be better understood when this original flat lock has been
studied. The annexed woodcut is taken from the first and very scarce
edition of Mr. Bramah’s _Dissertation_; the description is somewhat more
condensed, but perhaps sufficient for the purpose.
[Illustration: fig. 33. Bramah’s first model.]
The lock is supposed to be lying flat, with the bolt B half-shot. Ranged
somewhat diagonally are six levers, turning on a horizontal joint or
pivot at A, each lever having a slight extent of vertical motion
independent of the others. Each lever rests on a separate spring of
sufficient strength to sustain its weight, or, if depressed by a
superior force, to restore it to its proper position when the force is
withdrawn. F is a curved piece of metal, pierced with six grooves or
passages; these grooves are exactly equal in width to the thickness of
the levers, but are of sufficient depth to allow the levers a free
motion in a perpendicular direction. The ends of the levers are inserted
in these grooves, and have this freedom of motion, whether lifted by the
elastic power of the springs or depressed by a weight from above. In the
bolt B is a notch to receive a peculiarly-shaped lever, which shoots or
withdraws the bolt according as it traverses to the right or the left.
This lever, the six long levers, the springs beneath them, the bent
piece F, and the pivot, all alike are fixed to a circular platform P,
which turns on a centre; so that if any force can make this platform
turn partially round, the bolt must be shot or unshot by the lever which
works in the notch. The six long levers are the contrivances whereby
the platform shall _not_ be allowed to turn until the proper moving
agent (the key) shall have been applied, the plate _p_ being one of the
assistants in this obstruction. This plate, which is hollow underneath,
has six notches in one of its edges; the points of the levers catch into
these notches; and while so caught, the levers cannot move horizontally,
and all the machinery is at a stand-still. To enable the key to set the
mechanism in action, other contrivances are necessary. Each lever has a
notch at its extreme end, and the six are notched very irregularly in
respect one to another. These notches must be brought all into one
plane, to enable the levers to pass horizontally out of the notches in
the plate, in the same way as the two prongs of a fork might traverse
one above and the other below the blade of a knife; and when the
lever-notches are in this position, all in one plane and in the plane of
the plate, the levers can be moved, and with it the stump which shoots
the bolt. To ensure this due pressing down of the levers, a key is used
such as is shewn in the cut, having six steps or bits to correspond with
the six levers; this key, put upon the pin K, presses down all the
levers to the exact distance necessary for bringing their notches into
one plane, viz. the plane of the plate; the key then being turned round
turns the movable platform P, and shoots the bolt. It is evident at a
glance, that unless the various steps of the key are so cut, that each
shall press down its own lever to the proper extent, the ends of the
levers cannot pass the notches in the plate, and the bolt can neither be
locked nor unlocked.
It may be well to give Bramah’s own words in relation to this lock: “I
may safely assert that it is not in art to produce a key or other
instrument by which a lock constructed on this principle can be opened.
It will be a task, indeed, of great difficulty, even to a skilful
workman, to fit a key to this species of lock, though its interior face
were open to his inspection; for the levers being raised by the
subjacent springs to an equal height 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 proper key. Unless, therefore, a method be contrived
to bring the notches on the ends 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 in vain attempt to fit a key to the
lock, or by any effort of art to move the bolt. And when it is
considered that this process will be greatly impeded, and may perhaps be
entirely frustrated, by the action of the springs, it must appear that
great patience and perseverance, as well as great ingenuity, will be
required to give any chance of succeeding in the attempt. I do not state
this circumstance as a point essential or of any importance to the
purpose of the lock, but to prove more clearly what I have before
observed upon its principle and properties; for if such difficulties
occur to a skilled workman, as to render it almost, if not altogether
impracticable to form a key when the lock is open to his inspection and
its parts accessible to his hand, it pretty clearly demonstrates the
impossibility of accomplishing it when no part of the movement can be
touched or seen.”
It is evident that Mr. Bramah had his thoughts directed to that mode of
picking locks which depends on taking impressions of the moving parts,
rather than to the _mechanical_ or _pressure_ method which has been
developed in later times. There can be little doubt that a lock was, to
his mind, a beautiful and admirable machine, far elevated above the
level of mere blacksmith’s work; and his name will ever be associated
with what may be termed the philosophy of lock-making.
After the model-lock, which has just been described, was constructed,
and found to corroborate the idea which was working in Mr. Bramah’s
mind, he proceeded to the construction of his barrel or cylinder-lock,
embracing similar elements placed in more convenient juxta-position. In
his Essay he gives an engraving to illustrate the principle on which his
lock acts, rather in the manner of a diagram than as depicting any lock
actually made; his main object being to impart a clear notion of the
action of the slides which form such a distinguishing feature in his
lock.
[Illustration: fig. 34. Diagram to illustrate the Bramah lock.]
Viewed in this sense, therefore, simply as an illustrative diagram, the
annexed cut may represent the action of the safety slides. B is a
sliding bar or bolt, having a power of longitudinal motion in the frame
F. This frame has six notches cut on each of its long sides, the two
series being exactly opposite each other; and there are six similar
notches cut in the bolt B. The concurrent effect of all these eighteen
notches is, that the six slides _a b c d e f_ can move freely up and
down across the bolt. When the slides are thus placed, the bolt cannot
move, and may in this case be considered to be locked. There are six
clefts or notches in the six slides, one to each (1, 2, 3, 4, 5, 6); and
until all these are brought in a right line, the bolt cannot move
through them. If a tally or key be prepared, as shewn at T in the lower
part of the cut, with six projections, and if these projections thrust
up the six slides till their clefts rise to the plane of the bolt, then
can the bolt be withdrawn or the lock opened. This serves to illustrate
the relation between the slides and the key, as carried out in the way
now to be described.
One peculiarity of the Bramah lock is, that from the essential part of
the apparatus being a barrel or cylinder, much of the working can be
conducted in the lathe; and this has given a beauty to the details
generally and deservedly admired. Mr. Bramah, when he worked out the
theory of his lock, resolved to discard altogether the use of fixed
wards, and also the use of tumblers working on a pivot at one end;
substituting in their stead a system of slides, working in a very novel
way. The body of a Bramah lock may be considered as formed of two
concentric brass barrels, the outer one fixed, and the inner rotating
within it. The inner barrel has a projecting stud, which, while the
barrel is rotating, comes in contact with the bolt in such a way as to
shoot or lock it; and thus the stud serves the same purpose as the bit
of an ordinary key, rendering the construction of a bit to the Bramah
key unnecessary. If the barrel can be made to rotate to the right or
left, the bolt can be locked or unlocked; and the problem is, therefore,
how to ensure the rotation of the barrel. The key, which has a pipe or
hollow shaft, is inserted in the keyhole upon the pin, and is then
turned round; but there must be a very nice adjustment of the mechanism
of the barrel before this turning round of the key and the barrel can be
ensured. The barrel has an external circular groove at right angles to
the axis, penetrating to a certain depth; and it has also several
_internal_ longitudinal grooves, from end to end. In these internal
grooves thin pieces of steel are able to slide, in a direction parallel
with the axis of the barrel. A thin plate of steel, called the
locking-plate, is screwed in two portions to the outer barrel,
concentric with the inner barrel; and at the same time occupying the
external circular groove of the inner barrel; this plate has notches,
fitted in number and size to receive the edges of the slides which work
in the internal longitudinal grooves of the barrel. If this were all,
the barrel could not revolve, because the slides are catching in the
grooves of the locking-plate; but each slide has also a groove,
corresponding in depth to the extent of this entanglement; and if this
groove be brought to the plane of the locking-plate, the barrel can be
turned, so far as respects that individual slide. All the slides must,
however, be so adjusted that their grooves shall come to the same plane;
but as the notch is cut at different points in the lengths of the
several slides, the slides have to be pushed in to different distances
in the barrel, in order that this juxta-position of notches may be
ensured. This is effected by the key, which has notches or clefts at the
end of the pipe equal in number to the slides, and made to fit the ends
of the slides when the key is inserted; the key presses each slide, and
pushes it so far as the depth of its cleft will permit; and all these
depths are such that all the slides are pushed to the exact position
where their notches all lie in the same plane; this is the plane of the
locking-plate, and the barrel can be then turned.
[Illustration: fig. 35. Exterior of a Bramah lock.]
This is the principle which Mr. Bramah adopted; and we have now to trace
it, step by step, by means of illustrative details. Fig. 35 represents
the exterior of a box or desk lock, one among many varieties which the
Bramah lock presents. A A shews the bolt, formed something like two
hooks rising out of a bar of metal, which bar has a backward and forward
motion upon the plate B B. The upper edge of this plate is turned over
at right angles, forming a small horizontal surface through which two
openings are cut to receive the two hooked portions of the bolt. The
movements of the bolt are otherwise guided by the edges of square holes
through which it works; the holes being made in the edge-pieces of the
lock, riveted to the main plate. The bolt is further prevented from
rising out of its place by means of a plate of metal C, which is secured
to the edge-pieces by two screws 1, 1, and by two steadying pieces. This
plate has on its surface a cylindrical projection D, which contains in
effect all the working mechanism of the lock. The pins 4 4 are employed
for securing a plate, which we shall have to describe presently. When
such a lock is fixed upon a desk or box, the portion _D_ projects to a
small distance through a hole in the wood-work, forming in itself a very
neat escutcheon, with a key-hole in the centre.
[Illustration: fig. 36. Details of the Bramah lock.]
[Illustration: fig. 37. The slides.]
So much for the exterior. We must now proceed to examine the interior of
the lock, especially the part contained within the cylinder. In fig. 36,
for convenience of arrangement, the several parts are exhibited
separately, and as if the plane of the lock were horizontal, with the
key acting vertically. The essential part of the mechanism is a barrel
or cylinder _E_, pierced or bored with a cylindrical hole down its
centre. The inside of the bore has six narrow grooves, cut parallel with
the axis, and in the direction of radii; the grooves are not cut through
the thickness of the cylinder, but leave sufficient substance of metal
for strength. In every groove is fitted a steel slide of peculiar form,
such as is shewn at _a´ a´_ in fig. 37. Each slide is split in its
thickness (seen in section), so that it may move up and down in its
groove with a slight friction, and thereby not fall simply by its own
weight. Each slide has three small notches (3, 2, 3´), the use of which
will presently appear. Reverting to fig. 36, the lower part of the
opening through the cylinder _E_ is closed by a circular plate of metal,
fixed to it by two screws; this plate is represented at _F_, in the
lower part of the figure. This plate has a vertical pin rising from its
centre (also seen at _b_, fig. 39), and serving as a key-pin on which
the pipe of the key may work or slide; and it has also a short circular
stud _c_ projecting from its under side, and fitted to enter into a
curved opening in the bolt presently to be described.
The point to be now borne in mind is this, that if the cylinder _E_
turns round, the plate _F_ will also turn round, and with it the stud
_c_; and as this stud works into the peculiarly formed cavity _d_ in a
portion of the bolt (fig. 38), it causes the bolt to be shot backwards
or forwards. Now, in order to prevent this rotating of the cylinder
unless the proper key be employed, the following mechanism is
introduced: the cylinder has a groove cut round its circumference at _e
e_, extending sufficiently near to the internal bore to produce the
desired effect without too much weakening the metal. Into this notch is
introduced the thin circular plate of metal _f f_, it being divided into
two halves for this purpose; and when so placed, it occupies the
position shewn by the dotted portion _e e_. When this plate is screwed
to the case of the lock by the screws 4, 4, it cannot of course turn
round; but the cylinder itself will or will not turn round according to
the position of the slides. The plate _f f_ has six notches, 5, 5, 5,
&c. in the inner edge or circle; so adjusted that, when the plate is in
its place, the slides _a a_ can move up and down. The cylinder cannot
move round in a circle without carrying the slides with it; and these
cannot so move unless they are all depressed to such exact distances in
their respective grooves, that the deep notch of each slider (shewn at 2
in fig. 37) shall come into the plane of the circular plate: when all
are so brought, the cylinder can be turned. If any one of the slides be
pressed down either too low or not low enough, this turning of the
cylinder cannot be effected, because the slides will be intersected by
the edges of the notches 5, 5; and it is the office of the key,
therefore, to press all the six slides down to the exact distances
required. When the slides are not pressed upon by the key, they are
forced upwards to the top of the cylinder by a spiral spring 6, coiled
loosely round the pin _b_; this pressure forces up a small collet, 7, on
which the upper part of the slides rest by a sort of step.
[Illustration: fig. 38. The bolt.]
The first locks were made with a separate and independent spring to each
slide; but it is a very great improvement, the introduction of one
common spring to raise up the whole number; because if a person attempts
to pick the lock by depressing the slides separately by means of any
small pointed instruments, and by chance brings two or more of them to
the proper depth for turning round, should he press any one too low, it
is difficult to raise it again without relieving the spring 6, which
immediately throws the whole number of slides up to the top, and
destroys all that had been done towards picking the lock. Another
improvement of this lock, and one which very much increased the
difficulty of picking, and its consequent security, was the introduction
of false and deceptive notches cut in the sliders, as seen at 3, 3. It
was found that in the attempt to pick this lock, an instrument was
introduced by the keyhole to force the cylinder round. At the same time
that the slides were depressed by separate instruments, those slides
which were not at the proper level for moving round were held fast by
the notches 5, 5 in the plate _f f_ bearing against their sides; but
when pressed down to the proper level, or till the notch 2 came opposite
_f f_, they were not held fast, but were relieved. This furnished the
depredator with the means of ascertaining which slides were pressed low
enough, or to the point for unlocking. The notches 3, 3 in the slides
are sometimes cut above the true notch 2, sometimes below, and at other
times one on each side (one above and one below); they are not of
sufficient depth to allow the cylinder to turn round, but are intended
to mislead any one who attempts to pick, by his not knowing whether it
is the true notch or otherwise, or even whether the slider be higher or
lower than the true notch.
We have not yet sufficiently described the key of the Bramah lock. One
merit of the lock is the remarkable smallness of the key, which renders
it so conveniently portable. The key, as shewn in the upper part of the
figure, has six notches or clefts at the end of its pipe or barrel;
these clefts are cut to different depths, to accord with the proper
extent of movement in the slides. There is a small projection, 10, near
the end of the pipe, fitted to enter the notch D in the cylinder; this
forces the cylinder round when the parts are all properly adjusted. The
bolt of the lock, when properly shot or locked, is prevented from being
forced back by the stud _c_ on the bottom, F, of the cylinder coming
into a direct line with its centre of motion, as shewn in fig. 39; in
this position no force, applied to drive the bolt back, would have any
tendency to turn the cylinder round.
[Illustration: fig. 39. Section of the Bramah cylinder.]
To facilitate the comprehension of this very curious and beautiful
mechanism, the cylinder is shewn in section in the annexed fig. 39, the
same letters and figures of reference being used as before. In the whole
of this description we have spoken of six slides, and six only; but
Bramah locks may be, and have been, constructed with a much larger
number.
There have been several attempts made to modify the action of Bramah’s
lock, or to combine this action with that of some other inventor. It
will suffice to describe one of these. The lock invented by Mr. Kemp of
Cork, and for which a patent was obtained in 1816, is called by him the
_Union_ lock, as combining the principles of Barron’s and Bramah’s
locks. It contains two, three, or more sliders or tumblers, operated
upon by two, three, or more concentric tubes. These concentric tubes are
of different lengths, and are placed inside the barrel of the key; so
that the barrel may, in fact, be conceived to consist of a series of
concentric tubes. These tubes are made of such respective lengths as to
push back the tumblers, sliders, or pins which detain the bolt; and this
to the precise extent that will bring certain notches in all the sliders
to the position which will allow the bolt to pass. The inventor gives
this lock its distinctive appellation because it combines something of
the pushing motion which Bramah gives to his key, with something of the
tumbler-motion observable in Barron’s locks. The principle of safety is
considered here to rest chiefly on the extreme difficulty of imitating
the key.
Mr. Bramah calculates the number of changes of position which the slides
of his lock are capable of assuming before the right one would be
attained. “Let us suppose the number of levers, slides, 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 maybe thus attempted. Let the
denomination of these slides be represented by twelve arithmetical
progressionals; we find that the ultimate number of changes that may be
made in their place or situation is 479,001,600; and by adding one more
to that number of slides, they would then be capable of receiving a
number of changes equal to 6,227,020,800; 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 sliders,
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 all its variations.”