Page:The American Cyclopædia (1879) Volume IV.djvu/714

 702 CLOCKS AND WATCHES the angle g it will move the fork to the left, turning the lever upon its arbor A, and conse- quently lifting the pallet a sufficiently to allow the tooth c to fall upon the impulse face of the pallet. In this way the lever will receive a new impulse in addition to the one it received from the balance wheel, so that before the pin gets beyond reach of the side of the notch in the fork opposite g it will be struck by it, and thus an impulse will be given to the balance wheel by which its vibrations will be main- tained. It will be observed that the teeth of the scape wheel in this figure differ in shape from those in any scape wheel which has thus far been described. They are called club teeth, and their use will presently be noticed. Mudge used the old ratchet teeth. The alter- nate lifting and depressing of the pallets to liberate and to lock the teeth is the main object of escapements, and may be effected by three different methods: 1, by having the inclined planes on the pallets alone, and moving them by pointed teeth, as in fig. 9 ; 2, by having the inclined planes on the scape teeth which move against pallet edges ; 3, by having the inclined planes both on the pallets and on the scape teeth, as they are in the detached lever, with club teeth on the scape wheel. In this figure it will be seen that if the scape teeth were ratchet-shaped they would still raise the pallets, because their faces are inclined to the radius of the arc in which they move, although they would not be sufficiently raised. It will be noticed, however, that the toes of the scape teeth move in a larger arc than the heels, and therefore must increase the elevation of the pallets. In this lever it is necessary to make provision against the untimely unlocking of the pallets by irregularities produced in carry- ing. This is done by the employment of a guard pin and two banking pins. The guard pin k is placed at the junction of the fork with the lever, at such a distance from the lever arbor h that when those two points lie in the radius of the balance wheel, or nearly so, the guard pin will be nearer the balance arbor than is the circumference of the roller; there- fore a notch must be made in the periphery of the latter, on the side where the impulse pin is placed. This will allow it to pass only at cer- tain periods in the oscillations of the balance wheel. The banking pins, m m, are for the purpose of preventing the lever from being carried too far by any over-impulse of the bal- ance wheel, and their adjustment is a matter of considerable importance, and is secured with great nicety by placing them excentrically upon screws which pass into the pillar plate, as rep- resented in fig. 10. Before the perfection of the lever, the cylinder, or as it is sometimes termed the horizontal escapement (fig. 11), was introduced by Graham, who invented the dead- beat escapement in clocks. A section of a hol- low cylinder is cut out in such a way that its external and internal surfaces are made use of to receive the action of the teetli of the scape wheel, while the edges are cut at such angles as to form impulse pallet faces. The mode of action in this escapement is like that of the dead-beat escapement in clocks, the outer and FIG. 11. Cylinder Escapement of Graham. inner surfaces Of the cylinder forming long dead pallet surfaces, so that the teeth of the scape wheel may slide over them equally well, whe- ther the oscillations of the balance wheel are great or small. Indeed, the general principles upon which all clock and watch escapements are constructed are greatly similar ; the chro- nometer or detached escapement differing more from all the rest than they from each other. The devices and forms are various, but they secure the same desired result by making use of nearly the same mechanical forces and ap- pliances. The production of the chronometer or detached escapement is the work of many ingenious men, but principally of Le Roy, Berthoud, Earnshaw, and Harrison. The con- struction, as substantially given by Earnshaw, Fio. 12. Chronometer Escapement of Earnshaw. is represented in fig. 12. There has been but little change made in it since. The balance wheel B is shown turning in the di- rection of the arrows, the tooth T resting