escapement wheel, a tooth of which should thus
be enabled to leap upon the back of the pendulum, give it a short
push, and then be locked until the pendulum had returned and again
swung forward. An arrangement of this kind is shown in fig. 13. Let A
be a block of metal fixed on the lower end of a pendulum rod. On the
block let a small pall B be fastened, free to move round a centre C
and resting against a stop D. Let E be a 4-leaved scape-wheel, the
teeth of which as they come round rest against the bent pall GFL at G.
The pall is prevented from flying too far back by a pin H, and kept up
to position by a very delicate spring K. As soon as the pendulum rod,
moving from left to right, has arrived at the position shown in the
figure, the pall B will engage the arm FL, force it forwards, and by
raising G will liberate the scape-wheel, a tooth of which, M, will
thus close upon the heel N of the block A, and urge it forward. As
soon, however, as N has arrived at G the tooth M will slip off the
block A and rest on the pall G, and the impulse will cease. The
pendulum is now perfectly free or "detached," and can swing on
unimpeded as far as it chooses. On its return from right to left, the
pall B slips over the pall L without disturbing it, and the pendulum
is still free to make an excursion towards the left. On its return
journey from left to right the process is again repeated. Such an
escapement operates once every 2 seconds. One made on a somewhat
similar plan was applied to a clock by Robert-Houdin, about 1830, and
afterwards by Mr Haswell, and another by Sir George Airy. But the
principle was already an old one, as may be seen from fig. 14, which
was the work of an anonymous maker in the 18th century. A
consideration of this escapement will show that it is only the
application of the detached chronometer escapement to a clock.

[Illustration: FIG. 12.--Strasser's Escapement (Strasser & Rohde).]

[Illustration: FIG. 13.--Free Escapement.]

[Illustration: FIG. 14.--Free Escapement (old form).]

Even detached escapements, however, are not perfect. In order that an
escapement should be perfect, the impulse given to the pendulum should
be always exactly the same. It may be asked why, if the time of
oscillation of the pendulum be independent of the amplitude of the arc
of vibration, and hence of the impulse, it is necessary that the
impulse should be un