THE CYLINDER ESCAPEMENT
is seen in Fig. 204. The escape-wheel has sharp teeth set on stalks. (One tooth is removed to show the stalk.) The balance-wheel is mounted on a small steel cylinder, with part of the circumference cut away at the level of the teeth, so that if seen from above it would appear like a in our illustration. A tooth is just beginning to shove its point under the nearer edge of the opening. As it is forced forwards, b is revolved in a clockwise direction, winding up the hairspring. When the tooth has passed the nearer edge it flies forward, striking the inside of the further wall of the cylinder, which holds it while the spring uncoils. The tooth now pushes its way past the other edge, accelerating the unwinding, and, as it escapes, the next tooth jumps forward and is arrested by the outside of the cylinder. The balance now reverses its motion, is helped by the tooth, is wound up, locks the tooth, and so on.
THE LEVER ESCAPEMENT
is somewhat more complicated. The escape-wheel teeth are locked and unlocked by the pallets P P1 projecting from a lever which moves on a pivot (Fig. 205). The end of the lever is forked, and has a square notch in it. On the arbor of the balance-wheel is a roller, or plate, R, which carries a small pin, I. Two pins, B B, projecting from the plate of the watch prevent the lever moving too far. We must further notice the little pin C on the lever, and a notch in the edge of the roller.
Fig. 205.—"Lever" watch escapement.
In the illustration a tooth has just passed under the "impulse face" b of P1. The lever has been moved upwards at the right end; and its forked end has given an impulse to R, and through it to the balance-wheel. The spring winds up. The pin C prevents the lever dropping, because it no longer has the notch opposite to it, but presses on the circumference of R. As the spring unwinds it strikes the lever at the moment when the notch and C are opposite. The lever is knocked downwards, and the tooth, which had been arrested by the locking-face a of pallet P, now presses on the impulse face b, forcing the left end of the lever up. The impulse pin I receives a blow, assisting the unwinding of the spring, and C again locks the lever. The same thing is repeated in alternate directions over and over again.
COMPENSATING BALANCE-WHEELS.
The watchmaker has had to overcome the same difficulty as the clockmaker with regard to the expansion of the metal in the controlling agent. When a metal wheel is heated its spokes lengthen, and the rim recedes from the centre. Now, let us suppose that we have two rods of equal weight, one three feet long, the other six feet long. To an end of each we fasten a 2-lb. weight. We shall find it much easier to wave the shorter rod backwards and forwards quickly than the other. Why? Because the weight of the longer rod has more leverage over the hand than has that of the shorter rod. Similarly, if, while the mass of the rim of a wheel remains constant, the length of the spokes varies, the effort needed to rotate the wheel to and fro at a constant rate must vary also. Graham got over the difficulty with a rod by means of the compensating pendulum. Thomas Earnshaw mastered it in wheels by means of the compensating balance, using the same principle—namely, the unequal expansion of different metals. Any one who owns a compensated watch will see, on stopping the tiny fly-wheel, that it has two spokes (Fig. 206), each carrying an almost complete semicircle of rim attached to it. A close examination shows that the rim is compounded of an outer strip of brass welded to an inner lining of steel. The brass element expands more with heat and contracts more with cold than steel; so that when the spokes become elongated by a rise of temperature, the pieces bend inwards at their free ends (Fig. 207); if the temperature falls, the spokes are shortened, and the rim pieces bend outwards (Fig. 208).[39] This ingenious contrivance keeps the leverage of the rim constant within very fine limits. The screws S S are inserted in the rim to balance it correctly, and very fine adjustment is made by means of the four tiny weights W W. In ships' chronometers,[40] the rim pieces are sub-compensated towards their free ends to counteract slight errors in the primary compensation. So delicate is the compensation that a daily loss or gain of only half a second is often the limit of error.