and rests a very short time; this shows
that the impulse is long in proportion to the arc of swing. In others
the contrary is the case. A not uncommon proportion is that out of a
total arc of swing of 3 deg., 2 deg., or about one degree on each side
of the vertical, are occupied in receiving the impulse. In other
words, the points F and A should subtend an angle of 2 deg. at the
centre C. It is not to be forgotten that the scape-wheel tooth does
not overtake the face of the pallet immediately, on account of the
moment of inertia of the wheel. The wheels of astronomical clocks, and
indeed of all English house clocks, are generally made too heavy,
especially the scape-wheel, which, by increasing the moment of
inertia, causes a part of the work to be lost in giving blows, instead
of being all used up in gentle pushes.

[Illustration: FIG. 10.--Pin-Wheel Escapement.]

A very useful form of the dead escapement, which is adopted in many of
the best turret clocks, is called the "pin-wheel escapement." Fig. 10
will sufficiently explain its action and construction. Its advantages
are--that it does not require so much accuracy as the other; if a pin
gets broken it is easily replaced, whereas in the other the wheel is
ruined if the point of a tooth is injured; a wheel of given size will
work with more pins than teeth, and therefore a train of less velocity
will do, and that sometimes amounts to a saving of one wheel in the
train, and a good deal of friction; and the blow on both pallets being
downwards, instead of one up and the other down, the action is more
steady; all which things are of more consequence in the heavy and
rough work of a turret clock than in an astronomical one. It has been
found expedient to make the dead faces not quite dead, but with a very
slight recoil, which rather tends to check the variations of arc, and
also the general disposition to lose time if the arc is increased;
when so made the escapement is generally called "half-dead."

In the dead escapement, during each excursion of the pendulum the
repose surface of the pallets rubs against the points of the teeth of
the scape-wheel. Thus the pendulum is subject to a constant
retardation by friction. Curiously enough, this friction, which at
first sight might appear a defect, is an advantage, and to a large
extent accounts for the excellence of the escapement. For if the
driving forc