inder half
shell is 196 degrees. We now set our dividers to the radius of any
convenient arc which we have divided into degrees, and from _g_ as a
center sweep the short arc _l l_, and from the intersection of this arc
with the line _g k'_ we lay off sixteen degrees on the said arc _l_ and
establish the point _n_, from _g_ as a center draw the radial line _g
n'_. Take ten degrees from the same parent arc and establish the point
_m_, then draw the line _g m'_. Now the arc on the circles _h j_ between
the lines _g n'_ and _g m_ limits the extent of the exit lip of the
cylinder and the arc between the lines _g k'_ and _g m'_ represents the
locking surface of the cylinder shell.

[Illustration: Fig. 134]

To delineate the U arms we refer to Fig. 135. Here, again, we draw the
arc _a b c_ and delineate a tooth as before. From the point _e_ located
at the heel of the tooth we draw the radial line _e e'_. From the point
_e_ we lay off on the arc _a_ five degrees and establish the point _p_;
we halve this space and draw the short radial line _p' s'_ and _p s_.
From the point _e_ on the arc _A_ we lay off twenty-four degrees and
establish the point _t_, which locates the heel of the next tooth in
advance of _A_. At two and a half degrees to the right of the point _t_
we locate the point _r_ and draw the short radial line _r s_. On the arc
_b_ and half way between the lines _p s_ and _r s_, we establish the
point _u_, and from it as a center we sweep the arc _v_ defining the
curve of the U arms.

We have now given minute instructions for drawing a cylinder escapement
in all its details except the extent of the banking slot of the
cylinder, which is usually made to embrace an angular extent of 270
degrees; consequently, the pillar of the cylinder will not measure more
than ninety degrees of angular extent.

There is no escapement constructed where carefully-made drawings tend
more to perfect knowledge of the action than the cylinder. But it is
necessary with the pupil to institute a careful analysis of the actions
involved. In writing on a subject of this kind it is extremely
perplexing to know when to stop; not that there is so much danger of
saying too much as there is not having the words read with attention.

As an illustration, let us consider the subject of depth between the
cylinder and the escape wheel. As previously stated, 196 degrees of
cylinder shell should be employed; but suppose we find a watch in which
the hal