Page:The American Cyclopædia (1879) Volume IV.djvu/710

 698 CLOCKS AND WATCHES clocks and watches, which is represented in fig. 16. The fork, moved by the pendulum p, takes the place of the balance in Vick's clock, and by its more isochronous vibrations produces a more accurate escapement. The pallets 1 1, carried by the verge, act upon the crown scape wheel k, in a similar manner as in Vick's clock. The arbor of the wheel g passes through the front plate, and carries a disk upon which there are 60 divisions. As this wheel turns around once in a minute, each division marks a second, and an index placed upon the dial, at the edge of an opening in it, measures the divisions as they pass. A seconds hand could have been placed upon the arbor in place of the disk, and caused to turn in front of a graduated circle in the manner now employed. The verge and crown wheel escapement which Huygens employed made it necessary to use short pallets/ in order to prevent too much re- coil, and consequently the use of a short pen- dulum vibrating in a large arc. Thus there was the unavoidable introduction of more or less error, which he sought to correct by the use of cycloidal cheeks against which a flexible pendulum was made to swing, and thus to carry the bob through a cycloidal curve. But although the theory was an illustration of Huygens's genius, it was found not to be ap- plicable in practice, and the solution of the difficulty was more easily found by making the pendulum longer and the bob heavier, and causing it to swing in a smaller arc. The ap- plication of an escapement by which the pen- dulum was only required to oscillate in a small arc was accomplished by Dr. Hooke, an English contemporary of Huygens, by means of a pair of anchor-shaped pallets moving in the plane of a spur wheel having ratchet teeth, instead of using a crown wheel. This escapement produces a recoil, and is usually called the recoil escape- ment. It is shown in fig. 3. When the pendulum swings to the left it lifts the pallet a from the upper face of the tooth, which has now passed by, while the pallet J has also moved to the left, meeting the tooth ', and by the momentum of the pendulum pro- ducing a recoil till it returns and al- lows the tooth to move on, giving at the same time the pallet an impulse, the pendulum swinging to the left until the pallet a is brought within reach of the tooth c, which strikes it before the pendulum has at- tained the limit of its vibration, thus producing another recoil of the scape wheel, which lasts Fio. 4. Dead-beat Escape- ment of Graham. Fio. 8. Recoil Escapement of Hooke. till the pendulum begins to return and lift the pallet away. The impulse faces of the pallets- are convex ; theoretically they should be con- cave, but on account of friction the convex form has been found to answer the purpose better. This recoil or anchor-pallet escape- ment was succeeded by what is called the dead- beat escapement, invented by Graham about 1720, and which is the one now in general use for clocks, and with but little modification for watches. It is repre- sented in fig. 4. When the pendulum swings to the right the tooth a escapes from the pal- let ft, while the tooth c is brought against the pallet d ; but a portion of the exterior surface of this pallet, and also the interior surface of J, are arcs drawn from the centre e ; and upon being struck by the teeth of the scape wheel in the direction of e no recoil is pro- duced, neither is there any impulse given to the pallet until the pendulum swings far enough to the left to bring the tooth upon its impulse face. When the pendulum attains the limit of its vibration to the left, the same dead beat is made upon the circular inner face of the pallet 6, to be followed by an impulse upon the im- I pulse face when the pendulum has again re- ! turned far enough to the right. For the pur- pose of avoiding friction, and for other reasons, of which the limits of this article exclude a statement, it has been found that the teeth should be made to fall upon the dead face of a pallet as near to the angle which divides it from the impulse face as possible. There is a tendency in the dead-beat escapement to gain time as the arc of vibration of the pendulum decreases, whereas the tendency in the recoil escapement is to lose. The further the pallets- are from the centre of motion, the greater will be the distance traversed over them by the teeth, and consequently the greater the fric- tion; therefore the best clock makers place them as near the centre of motion as circum- stances will allow, the usual practice being to have them describe an arc whose radius is- equal to that of the scape wheel, and to have the dead faces embrace 10J teeth, or one half tooth over one third the number in the wheel. There is one other form of escapement, often used in turret clocks, called the pin-wheel escapement, shown in fig. 5. The invention i ascribed to Lepaute of Paris in 1V55. The form usually described has the pallets placed on opposite sides of the scape wheel ; but that is unnecessary, and the construction represent- ed in the figure exhibits the plan better. In the relative position of the pallets and pen-