e of our escape-wheel tooth, Fig. 129.

There is no reason why we should take twenty-degrees for the angle _k e
l_ except that the practical construction of the larger sizes of
cylinder watches has established the fact that this is about the right
angle to employ, while in smaller watches it frequently runs up as high
as twenty-five. Although the cylinder is seemingly a very simple
escapement, it is really a very abstruce one to follow out so as to
become familiar with all of its actions.

THE CYLINDER PROPER CONSIDERED.

[Illustration: Fig. 131]

We will now proceed and consider the cylinder proper, and to aid us in
understanding the position and relation of the parts we refer to Fig.
131, where we repeat the circles _d_ and _h_, shown in Fig. 130, which
represents the inside and outside of the cylinder. We have here also
repeated the line _f_ of Fig. 130 as it cuts the cylinder in half, that
is, divides it into two segments of 180 degrees each. If we conceive of
a cylinder in which just one-half is cut away, that is, the lips are
bounded by straight radial lines, we can also conceive of the relation
and position of the parts shown in Fig. 130. The first position of which
we should take cognizance is, the tooth _D_ is moved back to the left so
as to rest on the outside of our cylinder. The cylinder is also supposed
to stand so that the lips correspond to the line _f_. On pressing the
tooth _D_ forward the incline of the tooth would attack the entrance
lip of the cylinder at just about the center of the curved impulse face,
imparting to the cylinder twenty degrees of angular motion, but the
point of the tooth at _d_ would exactly encounter the inner angle of the
exit lip, and of course the cylinder would afford no rest for the tooth;
hence, we see the importance of not cutting away too much of the half
shell of the cylinder.

But before we further consider the action of the tooth _D_ in its action
as it passes the exit lip of the cylinder we must finish with the action
of the tooth on the entrance lip. A very little thought and study of
Fig. 130 will convince us that the incline of the tooth as it enters the
cylinder will commence at _t_, Fig. 130, but at the close of the action
the tooth parts from the lip on the inner angle. Now it is evident that
it would require greater force to propel the cylinder by its inner angle
than by the outer one. To compensate for this we round the edge of the
entrance lip so that t