We have already instructed the pupil how to delineate
a cylinder escape wheel tooth and we will next describe
how to draw a cylinder. As already stated, the
center of the cylinder is placed to coincide with the
center of the chord of the arc which defines the impulse
face of the tooth. Consequently, if we design
a cylinder escape wheel tooth as previously described,
and setting one leg of our compasses at the point *e*
which is situated at the center of the chord of the
arc which defines the impulse face of the tooth and
through the points *d* and *b* we define
the inside of our cylinder. We next divide the
chord *d b* into eight parts and set our dividers
to five of these parts, and from *e* as a center
sweep the circle *h* and define the outside of
our cylinder. From *A* as a center we draw
the radial line *A e’*. At right angles
to the line *A e’* and through the point
*e* we draw the line from *e* as a center,
and with our dividers set to the radius of any of the
convenient arcs which we have divided into sixty degrees,
we sweep the arc *i*. Where this arc intersects
the line *f* we term the point *k*, and from
this point we lay off on the arc *i* 220 degrees,
and draw the line *l e l’*, which we see
coincides with the chord of the impulse face of the
tooth. We set our dividers to the same radius
by which we sweep the arc *i* and set one leg
at the point *b* for a center and sweep the arc
*j’*. If we measure this arc from the
point *j’* to intersection of said arc
*j’* with the line *l* we will find
it to be sixty-four degrees, which accounts for our
taking this number of degrees when we defined the
face 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