Lever Escapement.
horseback, as these instruments are liable to be affected by any sudden motion. Even the stepping quickly from a vehicle may stop them, and yet the escapement be as perfect as possible. They are only adapted for persons of very quiet habits. Thomas Mudge, in the year 1766, introduced an admirable invention, which, after many alterations and improvements, is now universally known as the 'Patent Detached Lever' escapement, represented by—a a the escapement-wheel, b b the ruby pallets, c the lever, d the balance. On the axis of the balance d, towards the lever c, is a small disc of steel, into which is inserted a small pin made of ruby. This pin fits with great nicety into a notch or opening in the end of the lever c, upon which are firmly fixed the two pallets b b, into which are secured rubies very finely polished. The balance in its vibration on either side, carrying with it the steel disc and ruby pin, causes that pin to enter the notch in the lever and carry the lever with it, and at the same time, to draw the pallet from the tooth of the escapement-wheel a. Power being exerted upon this wheel by the mainspring, the wheel tooth gets disengaged from the locking-face of the pallet, forces itself down the slopes of the pallet, and thus gives impulse to the balance. At each vibration the same unlocking takes place, but as soon as the wheel tooth falls from the slope, the opposite pallet is prepared to receive the advancing tooth of the escapement-wheel, and so on in succession beat after beat takes place. So excellent was this escapement considered a few years back, that chronometers were made upon the principle, and placed in the Royal Observatory for public trial. But since then many improvements have been made in it, so that makers are now enabled to produce a pocket watch, with the short angle lever escapement, which marks time at a steady rate of within four or five seconds weekly,—a rate which approaches so near to the time-keeping of a pocket chronometer, that unless the minutest exactness for some specific purpose is required, the last-named watch is all that can be wished for.
Chronometer Escapement.
About the year 1780 was invented the escapement which is now denominated the Detached or Chronometer Escapement (see opposite page), the principles of which are the nearest approach to perfection, the impulse to the balance being given at the centre of vibration. A is the escape-wheel, B the escape-wheel teeth, C the roller let on the verge, or axis of the balance. This roller is a circle of polished steel, with a notch cut out of it, into one side of which, D, a flat polished piece of ruby is inserted for the acting part. Below this steel roller, carried on the same verge, is a smaller roller of steel E), denominated the discharging pallet, having a sapphire fixed on its outer edge. F is a slender spring, which is screwed at I to the stouter one, having its fixture at the stud L, and polished away very thin at K, in order that it may bend readily, so as to cause very little resistance to the balance while forcing it on one side. G is a projecting piece, carrying an upright pin made of ruby, against which the wheel tooth B rests; at B is a small screw against which the spring L K G strikes, and thus prevents it from springing too far back. The action of these parts is as follows:—When at rest the circular edge of C is just clear of the two teeth of the wheel B, which cannot be set in motion while E and G remain quiescent; G rests against the screw at B, and the tooth resting against the locking pallet G, the escapement-wheel cannot turn. To set the chronometer going it is necessary to give it a rotary motion, which sets the balance in action. This causes the lower piece on the verge (called the lifting piece or discharging pallet) to strike against the end of the spring F, which, from its over-lapping the curved end of the prolonged spring K G, pushes it back, and thus releases the pin or locking stone G from before the tooth of the wheel: that is, it unlocks the escapement-wheel, which is immediately set in motion by the force of the mainspring. The same vibration given to balance and verge brings the ruby pallet D round before the tooth B, which strikes against it and carries it round. The recoil of the spring F has now brought the locking pallet G to catch the tooth B, the escapement-wheel is thus again stopped. But the stroke of the tooth upon the face of the ruby pallet D has driven the balance on in its vibration till it is counteracted by the tension of the balance spring, which brings it back again; in this return vibration the lifting pallet E, by its curved back, pushes the slender spring F before it, and passes it without affecting K, G, which is stiff enough to remain unmoved by F, even when this strikes and rests against it in recoiling. The wheel, therefore, continues locked on the upright pallet G, and the vibration proceeds uncontrolled till the great pallet is again brought round, and the balance spring again checks the vibration, the above process being repeated. In this escapement, consequently, part of one vibration in one direction, and the whole of that in the other, is performed without the balance being in any way under the influence of the motive power; while the parts are so contrived that the impulse given by the tooth of the escape-wheel, affects in a very slight degree the natural motion of the balance. It can be easily understood that the lifting pallet E can pass the spring F in one direction without moving K and G, while in the other it carries E and G with it.
Compensation Balance.
Several appliances have been from time to time introduced to correct the error in time-keeping caused by variations in the temperature, but none have come into such general use as that known by the term 'Compensation Balance,' invented by Thomas Earnshaw, of London, and for which he received a government reward. This balance, when properly adjusted, causes the watch to keep the same time whether the temperature be 32 deg. or 90 deg.; while without it a watch will show a considerable difference in time, on being merely transferred from the pocket to the dressing-table, where, probably, the temperature would not be so high. Our woodcut represents a balance of this kind; the divided rim A A, is composed of steel and brass run together by fusion, the more expansible metal, brass, being placed outwards, the result of which is as follows:—Heat elongates the pendulum spring, and thereby causes a slower vibration of the balance. The same amount of heat will also expand the metals composing the balance; but as the inner rim of steel does not expand so freely as the outer one of brass, the conflicting action of the two tends to draw the free end of the circular rim inwards towards its centre, and thus decreases in all but one direction the diameter of the balance. This decrease tends to quicken its vibration, and thus counteracts the effect of the elongation of the pendulum spring. In cold temperatures the pendulum spring is contracted, making the vibrations quicker, but the contraction of the brass rim draws the free end outwards, thus increasing its diameter, retarding its vibrations, and counteracting the effect of the contraction of the pendulum spring.
Many contrivances have been introduced to test the equality of compensation balances, but the majority have been abandoned from the circumstance that the heat was not equally distributed to the watches under trial. In pursuance of this object, an oven was invented, heated by hot water, which answers the desired end. It is an apparatus made of copper, two feet high, thirteen inches broad, and eight inches deep. From the top to the bottom, at the distance of fifteen inches, it is divided into two compartments. All around the upper one (except the front, which has a glass door through which the chronometers and watches are seen without opening it) is one inch of water. It has a chamber thirteen inches high, eleven inches broad, and seven inches deep for the reception of chronometers and watches. The water is introduced at the top in the same manner as a solar lamp is supplied with oil. The bottom compartment contains a jet of gas, which can at pleasure be regulated so as to keep the watch at any required temperature. The heat radiated from the inner surface of the chronometer chamber is thus equally distributed among the instruments under trial. A thermometer placed within the upper chamber indicates the temperature, and by this simple apparatus a watch can be regulated with the greatest nicety to suit the particular climate into which it may be taken.