Parallel with the line _A i_ and at
the distance of half the diameter of the jewel pin _D_, we draw the
short lines _t t'_, which define the inner faces of the fork.
The intersection of the short line _t_ with the arc _n_ we will
designate the point _r_. With our dividers set to embrace the space
between the point _r_ and the point _f_, we sweep the arc which defines
the inner face of the prong of the fork. The space we just made use of
is practically the same as the radius of the circle _a_, and
consequently of the same curvature. Practically, the length of the guard
point _C'_ is made as long as will, with certainty, clear the safety
roller _B_ in all positions. While we set the point _f_ at two degrees
from the center _B_, still, in a well-constructed escapement, one and a
half degrees should be sufficient, but the extra half degree will do no
harm. If the roller _B'_ is accurately made and the guard point _C'_
properly fitted, the fork will not have half a degree of play.
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