gth, it has the good points of both the equidistant
and circular pallets, as the unlocking can be performed on the tangent
and the lifting arms are of equal length. The wheel, however, is so much
heavier as to considerably increase the inertia; also, we have a metal
surface of quite an extent sliding over a thin jewel. For practical
reasons, therefore, it has been slightly altered in form and is only
used in cheap work, being easily made.

[Illustration: Fig. 4.]

We will now consider the drop, which is a clear loss of power, and, if
excessive, is the cause of much irregularity. It should be as small as
possible consistent with perfect freedom of action.

In so far as _angular_ measurements are concerned, no hard and fast rule
can be applied to it, the larger the escape wheel the smaller should be
the angle allowed for drop. Authorities on the subject allow 1 1/2deg. drop
for the club and 2deg. for the ratchet tooth. It is a fact that escape
wheels are not cut perfectly true; the teeth are apt to bend slightly
from the action of the cutters. The truest wheel can be made of steel,
as each tooth can be successively ground after being hardened and
tempered. Such a wheel would require less drop than one of any other
metal. Supposing we have a wheel with a primitive diameter of 7.5 mm.,
what is the amount of drop, allowing 1 1/2deg. by angular measurement?
7.5 x 3.1416 / 360 x 1.5 = .0983 mm., which is sufficient; a hair could
get between the pallet and tooth, and would not stop the watch. Even
after allowing for imperfectly divided teeth, we require no greater
freedom even if the wheel is larger. Now suppose we take a wheel
with a primitive diameter of 8.5 mm. and find the amount of drop;
8.5 x 3.1416 / 360 x 1.5 = .1413 mm., or .1413 - .0983 = .043 mm.,
more drop than the smaller wheel, if we take the same angle. This is a
waste of force. The angular drop should, therefore, be proportioned
according to the size of the wheel. We wish it to be understood that
common sense must always be our guide. When the horological student once
arrives at this standpoint, he can _intelligently_ apply himself to his
calling.

_The Draw._--The draw or draft angle was added to the pallets in order
to draw the fork back against the bankings and the guard point from the
roller whenever the safety action had performed its function.

[Illustration: Fig. 5.]

Pallets with draw are more difficult to unlock than those without it,
this is in