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 half shell has had too much cut away, so the tooth on entering the half shell after parting with the entrance lip does not strike dead on the inside of the shell, but encounters the edge of the exit lip.  In this case the impulse of the balance would cause the tooth to slightly retrograde and the watch would go but would lack a good motion.  In such an instance a very slight advance of the chariot would remedy the fault—­not perfectly remedy it, but patch up, so to speak—­and the watch would run.