The escapement is a general term for the manner of communicating the motion of the wheels to the pendulum of clocks or balance of watches. One of the 'most ancient escapements is that which is now applied in almost all common pocket watches : it is represented in Fig. 1, and is best suited to the long vibrations of the balance, which was invented earlier than the pendulum. a b denotes the rim of a contrate wheel, called the crown wheel, having its teeth pointed and sloped on one side only, so that the points advance before any other part of the teeth during the motion. c and d are two pallets or flaps, proceeding downwards from the verge e f. The pallets are nearly at right angles to each other ; and when the balance f g, fixed on the verge, is at rest, the pallets remain inclined to the plane of the wheel, in an angle of about forty five degrees; but when it is made to vibrate, one of the pallets is brought nearer to the perpendicular position, while the other becomes more nearly parallel. The wheel must be supposed to have one of its teeth resting against a pallet by virtue of the maintaining power. This tooth will slip off or escape as the pallet rises toward the horizontal position, at which instant a tooth on the opposite side of the wheel will strike against the other pallet which is down. The returning vibration, by raising this last pallet, will suffer that tooth to escape, and another tooth will apply itself to the first-mentioned pallet. By this alternation, the crown wheel will advance the quantity of half a tooth each vibration, and the balance or pendulum will be prevented from coming to rest, because the impulse of the teeth against the pallets will be equal to the resistances from friction, and the reaction of the air. This escapement not being adapted to such vibrations as are performed through arcs of a few degrees only, another construction has been made, which has been in constant use in clocks for this century past, with a long pendulum beating seconds. Fig. 2, on the next page, a b represents a vertical wheel, called the swing wheel, having thirty teeth. c d represents a pair of pellets connected together, and movable, in conjunction with the pendulum, on the centre of axis j'. One tooth of the wheel in the present position rests on the inclined surface of the inner part of the pallet c, upon which its disposition to slide tends to throw the point of the pallet further from the centre of the wheel, and consequently assists the vibration in that direction. While the pallet c moves outwards, and the wheel advances, the point of the pallet d, of course, approaches towards the centre, in the opening ' between the two nearest teeth ; and when the acting tooth of the wheel slips off, or escapes from the pallet c, another tooth on the opposite side immediately falls on the exterior inclined face of cl, and by a similar operation tends to push that pallet from the centre. The returning vibration is thus assisted by the wheel, while the pallet moves towards the centre, and receives the succeeding tooth of the wheel after the escape from the point of d In this manner the alteration may be con ceived to go on without limit. The celebrated George Graham improved this escapement very much, by taking off part of the slope furthest from the points of the pallets ; instead of which part he formed a circular or cylindrical face, having its axis in the centre of motion. Pallets of this kind are seen on the opposite side of the wheel at e and g, having A for their centre or axis. A tooth of the wheel is seen resting upon the circular inner surface of the pallet g, which is not therefore affected by the wheel, excepting so far as its motion, arising from any other cause, may be affected by the friction of the tooth. If the vibration of the pendulum be supposed to carry g outwards, the sloped surface will be brought to the point of the tooth, which will slide along it and urge the pallet outwards during this sliding action. When the tooth has fallen from the point of this pallet, an opposite tooth will be received on the circular surface of e, and will not affect the vibration, excepting when the slope surface of e is carried out so as to suffer the tooth to slide along it. In the two former escapements, there is always a certain portion of vibration takes place after the drop which drives the pallet back, and causes the index also to recede through a small arc: this has been distinguished by the name of a recoil. The escapement of Graham, and all such as have no recoil, have been called dead beat escapements, because the index for seconds falls directly through its arc, and remains motionless on the line of division till the next vibration. It may be observed, that the main taining power in Graham's escapement, may be applied during a small portion only of the vibration ; and that an increase of the maintaining power tends to enlarge the arc of vibration, but scarcely interferes with its velocity. In the escapements just described, the escape wheel is in continual contact with the pallets belonging to the axis of the balance, and the friction arising from this circumstance may be considered as a principal cause of the irregularity in the going of watches. If we suppose a regulator to be made so perfect as to be perfectly isochronal, while vibrating in a free position, that advantage would be diminished, or lost, as soon as it was placed in connexion with it train of wheels; and the errors would be more or less, according to the nature and quantity of friction in the escapement. It would be, therefore, extremely useful to secure to the regulator a perfect liberty of vibration, except during the short intervals of time which may be necessary for the action of the escape wheel, to give it a new impulse. This ingenious idea was first started, and also carried into execution by P. Leroy, who in 1748 presented to the Academy of Sciences in Paris a model of a detached escapement, the effect or action of which may be briefly described as follows: an escape wheel is kept in repose by a lever decent ; the balance unlocks the detest, and receives an impulse or stroke on a pallet through a part of every second vibra tion, and during great part of its course it is free and detached. A great variety of escapements have been contrived by various ingenious men; those in which springs are used in the locking pieces instead of pivots are at present generally preferred; we shall, therefore, proceed to describe the escapement of Mr. Arnold, who we believe is the author of this improvement. The engraving
on the next page is a representation of this escapement. The teeth of the escape wheel are of a cycloidal shape, in the face part, which is intended for action, the sections of which, with those of the two other sides, form a sort of mixed triangle. b b d represents the detent, which is formed of a flexible piece or spring, bending between c and n; and in the part n b d, which is stronger than the other, is fixed the locking pallet a, opposite an adjusting screw! The pallet projecting below the spring detent locks upon the interior angle of the tooth, suspending the motion of the escape wheel, and leaving the balance to vibrate free, as pointed out in the preceding escapements. The action of the spring detent (for the joint of the detent is itself a spring) presses the locking pallet against the screw f, except at the time of unlocking the wheel. A ve delicate spring n e, called the disc "ng, or unlocking spring, (and also e tender spring,) is attached by one end n to the spring detent cbnb a; and passing under the adjusting screw f, extends a little beyond the extremity d, by the detent itself; h k h is a circular piece attached to the axis of the balance, and o the discharging pallet.
This pallet when the balance is in motion from s to d presses against the end of the discharging spring n e; and carrying it together with the locking spring 6 b d disengages the locking piece a out of the internal angle of the tooth, with which it was in contact ; and the escape wheel then communicates a new power to the balance by its impulse on a pallet fn, which is fixed or set in the aperture of the circular piece. As soon as this is done, the spring detent, or locking spring, fans back to its position against the adjusting screw f ; and the pallet, by receiving or intercepting the next tooth, stops the motion of the escape wheel. When the balance returns from d to e, the unlocking pallet acts again on the extremity of the discharging spring, but this being very delicate, gives way without disturbing the detent, or locking spring; and the balance, after suffering a trifling degree of resistance by that contact, continues its free vibrations. At the next vibration the unlocking takes place, and the action of the escapement proceeds successively, as explained before. The detached escapement was applied first to chronometers or time-pieces, but is now also used for astronomical clocks ; and various excellent constructions have been invented by different artists, amongst which we may mention those of Hardy and Reid. In 1812, Mr. Prior, jun. was rewarded by the Society of Arts for the construction of a detached remontoire escapement for clocks, which pos sesses considerable merit. The advantages of this escapement consist in the freedom of its parts from friction ; in the exact and equal impulse which it will continue to give to the pendulum, unaffected by the clogging of oil and increased friction of the train ; and in the small power required for restoring the tension of the remontoire spring, which does not require to be wound up quick, or to be pushed beyond any catch or spring to keep it in its proper situation. The engravings on page 696, with the following description extracted from the Transactions of the Society, will explain the construction. The swing-wheel, Irsgs.1 and 3, has thirty teeth cut in its periphery, and is constantly urged forwards by the maintaining power which is supplied by the small weight; two spring detente are used to catch the teeth of the wheel alternately ; these are, at the proper intervals, unlocked by the parts marked 1 and 3 upon the pendulum rod, intercepting two small pins projecting from the detente, as it vibrates towards the one or the other; the renovating or remontoire spring is fixed to the same stud as the detents ; it is wound up by the highest tooth of the wheel, as seen in dig. 1, (its position when unwound, being shown by the dotted lines.) This being the case, suppose a tooth of the wheel is caught by one of the detente, this prevents the wheel from moving any further, and keeps the renovating spring from escaping off the point of the tooth ; in this posi tion, the pendulum is quite detached from the wheel ; now, if the pendulum be caused to vibrate to the right, the part of it marked 2 comes against the upper pin, seen in Fig. 2, projecting from the renovating spring, and pushes this spring from the point of the wheels tooth ; on vibrating a little further, the pendulum removes the detent which detained the wheel, by the part 3 striking the lower pin, Fig. 2, which projects from the detent; the maintaining power of the clock causes the wheel, thus unlocked, to advance until detained by a tooth resting upon the end of the other detent on the opposite side ; by this means the renovating spring will be clear of the tooth of the wheel as it returns with the pendulum, and gives it an impulse with its pin pressing against the part 2 of the pendulum, until the spring comes to the position shown by the dotted line, in which position it is unwound, and restst a pin fixed against a cross bar of the plate ; the pendulum continues vibrating to the left, nearly to the extent of its vibra tion, when the part 1 meets the pin in the same detent, and removes it from the wheel, and unlocks it; the maintaining power now carries it forward, pushing the renovating spring before it until another tooth is caught by the first detent, which detains the wheel in the position first described, the renovating spring being wound up ready to give another impulse to the pendulum. The pin is not fixed to the renovating spring itself; but is part of a piece of brass, which is screwed fast to the renovating spring, and is made very slender near the screw which fastens it ; this permits the renovating spring to give way, if by the weight being taken off the dock, or any other accident, the escape wheel should be wound backwards, so as to catch on the detente improperly. The weight in the preceding figures merely represents the means by which the escape wheel was put in motion in the model presented to the Society, which consisted merely of the escapement ; but when attached to a clock, the remon toire spring is wound up by the maintaining power of the clock transmitted to the escape wheel by means of the train.