k.) 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 P^1 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.

[Illustration: FIG. 205.--"Lever" watch escapement.]

In the illustration a tooth has just passed under the "impulse face" _b_
of P^1. 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