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.
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