Now let us determine at what point on the inner face of the
prong _E'_ the jewel pin parts from the fork, or engages on its return.
To do this we draw a line from the center _r_ (Fig. 59) of the jewel
pin, so as to meet the line _e_ at right angles, and the point _t_ so
established on the line _e_ is where contact will take place between the
jewel pin and fork.
It will be seen this point (_t_) of contact is some distance back of the
angle _u_ which terminates the inner face of the prong _E'_;
consequently, it will be seen the prongs _E E'_ of the fork can with
safety be shortened enough to afford a safe ingress or egress to the
jewel pin to the slot in the fork. As regards the length of the outer
face of the prong of the fork, a good rule is to make it one and a half
times the diameter of the jewel pin. The depth of the slot need be no
more than to free the jewel in its passage across the ten degrees of
fork action. A convenient rule as to the depth of the slot in a fork is
to draw the line _k_, which, it will be seen, coincides with the circle
which defines the jewel pin.
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