The locking wheel turns from left to right; in the Figure one of its "legs" a' rests on the front block s. The tip of the armfof the front pallet is now in contact with one of the pins, which has lifted the arm into its present position. The pendulum moves in the direction of the arrow, and pushes the pallet a far enough to the right to release the leg a', which rests on the block s. The moment the leg a' escapes from the block, the leg e will drop against the block s' of the pallet a'; while simultaneously the lifting pins have moved also, and one of them has raised the pallet a' by carrying along the tip of its arm f', while at the same time the pin that rested against the tip of the arm f has moved away from it, thereby setting free the pallet a, the entire weight of which now presses against the pendulum, which soon begins to move in the opposite direction, receiv ing its impulse from the weight of the pallet. On its return it does not find the pallet a' leaning against the pin g', the position in which it parted from it, for the pallet has been lifted up a short distance by a lifting pin; while the pallet a still follows the pendulum until it is stopped by the checking pin g. The pendulum, in continuing its oscillations, pushes aside the pallet a', releases the locking wheel at s',and is locked again at s. The weight of the pallet a' will now give an impulse to the pendu lum, while the pallet a is lifted up and ready to impart another impulse as soon as a leg at s is again unlocked. Since the pallets are always lifted to the same height, it is evident that the impulses imparted to the pendulum must also remain unchanged independent of any change that may take place in the impelling power of the train. To prevent any jarring of the locking wheel by a too rapid movement of the same, there is placed on the arbor of the locking wheel a "fly," which is held in its place by the friction of a spring, by which the fly is permitted to continue its move ment for a moment when the locking wheel has stopped at its respective block.
Cylinder Escapenicat.—An escapement of good qualities, and specially adapted for watches subjected to hard usage, is the cylinder escapement, invented by Tompion or by Graham about 1700 (pl. 123, fig. 13). The es capement-wheel A varies greatly in form from the two preceding types; the wedge-shaped teeth are elevated above the surface of the wheel's body and supported by stalks, as represented in the engraving. The arrangement which produces the step-like movements of the escape-wheel is a cylindri cal steel shell, half of which is cut away to the extent shown in the engrav ing, and turns on its axis in bearings at dd. The outside diameter of the cylinder fits loosely between the spaces of the teeth, and the teeth them selves fit loosely into the inside diameter of the cylinder. At B the cylin der receives the balance and the hair-spring in a manner already described. We assume that the oscillating cylinder B is moving from the left to the right; we now perceive that the tooth just begins to press against the lip a of the cylinder; while progressing, the inclined tooth will glide along the lip a, thereby imparting the impulse for the vibrations of the balance. As soon as the tooth has left the lip a the tooth p will drop against the outside surface of the cylinder near the lip b', and will then rest against the cylin der until the tension of the hair-spring reverses its vibration; then the tooth will glide along the lip b', thereby imparting another impulse to the balance in the opposite direction. It will be seen that in this manner, alternately, either the inside of the cylinder encloses a tooth or the entire cylinder is placed between the space of two teeth, and that a tooth which comes into action first falls against the outside periphery of the cylinder on the right side, resting there until it can pass the entering lip b' of the cylinder, then remaining inside of the cylinder until it can pass also the exit-lip a. The inner and outer surfaces of the cylinder being parallel
circles oscillating around their centres, it is obvious that the escape-wheel while resting against either surface must come to a perfect standstill, which makes it a " dead-beat " escapement.
Lever Eseapcmen1.—An escapement far superior to the preceding is the lever escapement (fig. 14), which was invented by Thomas Mudge about 1770. The balance, after it has received its impulse from the escape ment, is no longer in contact with the escapement, but moves entirely free, and therefore it is called a " detached " escapement. It is of the greatest importance that the balance be relieved of friction as much as possible, in order to reduce to a minimum the elements that obstruct its vibrations, an advantage which is more or less gained by all detached escapements. Figure 14 shows the lever escapement of a Jurgen sen watch; the escapement exhibits a great similarity to the Graham " dead-beat " escapement (fig. 12); the escape-wheel C (fig. 14) moves in the direction of the arrow, its slender but blunt teeth being greatly inclined. The anchor A is fastened to its arbor at x, and its pallets 1 and e arc sections of a ring that has its centre at x; 1,11 is the lever, fastened to the anchor; at the left the lever terminates in a fork (le), and at the right in a counterpoise (11). The two pins, v and w, called '' banking pins," limit the extent of the lever's movements; the arbor of the balance carries a small steel wheel called the " roller," with an upright pin s, called the "impulse pin ;" when the balance is oscillating the roller i and the impulse pin s participate in its oscillations. The impulse pin enters the fork k , which follows the oscillations by moving to and fro. In the Figure the impulse pin has entered the fork to its deepest point, and a tooth of the escape-wheel has just passed to the end-surface, called the "driving face," of the pallet 2, which tooth consequently will glide over the face of the pallet and force it upward, thereby pressing the fork at i against the impulse phi, and imparting an impulse to the balance and moving it to the left. The pin s will be carried beyond the limited point of the fork's movement, and will leave the fork. At this moment the tooth leaves the pallet a', and the tooth y falls against the inner curved surface of the pal let 1, called the "locking surface ;" there the tooth remains until the balance reverses its vibrations and the impulse pin enters the fork again at i; then the anchor is moved in the opposite direction, and the tooth, after leaving the locking surface, enters the driving face b of the pallet / at a, glides over it, and presses the fork at / against the impulse pin, imparting an impulse to the balance while moving to the left. As soon as the tooth leaves the pallet 1, the tooth e drops against the locking surface of pallet .2, where it is detained until released to act upon the impulse face of the pallet. In this manner one tooth after the other exercises its pressure against the driving faces of the pallets and keep's tip the vibrations of the balance. The lever escapement has now become a favorite one for watches, and, no doubt, if made with sufficient care, is preferable to the cylinder escapement; but if made in a neglectful manner the defects are rarely as easily corrected as they are in the simple arrangement of the cylinder escanement.