The radial lines _A d_ and _A d'_ represent the arc of angular motion of
fork action. The circle _b b_ represents the pitch circle of the jewel
pin, and the circle at _c c_ the periphery of the guard or safety
roller. The points established on the circle _c c_ by intersection of
the radial lines _A d_ and _A d'_ we will denominate the points _h_ and
_h'_. It is at these points the end of the guard point of the fork will
terminate. In construction, or in delineating for construction, we show
the guard enough short of the points _h h'_ to allow the fork an angular
motion of one degree, from _A_ as a center, before said point would come
in contact with the safety roller.
[Illustration: Fig. 81]
We draw through the points _h h'_, from _B_ as a center, the radial
lines _B g_ and _B g'_. We measure this angle by sweeping the short arc
_i_ with any of the radii we have used for arc measurement in former
delineations, and find it to be a trifle over sixty degrees. To give
ourselves a practical object lesson, let us imagine that a real guard
point rests on the circle _c_ at _h_.
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