his construction.
The point in which the anchor escapement was superior to all that had
gone before, was that it would work well with a small arc of swing of
the pendulum. The balance escapement, even when adapted to a pendulum,
necessitated a swing of some 20 deg., and hence the circular error, that
is to say, the deviation of the path from a true cycloid, was
considerable. But with an anchor escapement the pendulum swing need be
only 3 deg. or 4 deg. On the other hand, it violates the conditions above
laid down for a perfect escapement, inasmuch as the pendulum is never
free, but at the end of its swing is still operated on by the
escapement, which it causes to recoil.

[Illustration: FIG. 9.--Dead Escapement.]

Dead escapements.

To get rid of this defect the dead escapement, or, as the French call
it, _l'echappement a repos_, was invented by G. Graham. It is
represented in fig. 9. It will be observed that the teeth of the
scape-wheel have their points set the opposite way to those of the
recoil escapement. The tooth B is here represented in the act of
dropping on to the right-hand pallet as the tooth A escapes from the
left pallet. But instead of the pallet having a continuous face as in
the recoil escapement, it is divided into two, of which BE on the
right pallet, and FA on the left, are called the impulse faces, and
BD, FG, the dead faces. The dead faces are portions of circles (not
necessarily of the same circle), having the axis of the pallets C for
their centre; and the consequence evidently is, that as the pendulum
goes on, carrying the pallet still nearer to the wheel than the
position in which a tooth falls on to the corner A or B of the impulse
and the dead faces, the tooth still rests on the dead faces without
any recoil, until the pendulum returns and lets the tooth slide down
the impulse face, giving the impulse to the pendulum as it goes. In
order to diminish the friction and the necessity for using oil as far
as possible, the best clocks are made with jewels (sapphires are the
best for the purpose) let into the pallets.

The pallets are generally made to embrace about one-third of the
circumference of the wheel, and it is not at all desirable that they
should embrace more; for the longer they are, the longer is the run of
the teeth upon them, and the greater the friction. In some clocks the
seconds hand moves very slowly