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.

HOW TO DELINEATE THE SAFETY ACTION.

[Illustration: Fig. 60]

We will next consider a safety action of the single roller type. The
active or necessary parts of such safety action consist of a roller or
disk of metal, usually steel, shaped as shown in plan at _A_, Fig. 60.
In the edge of this disk is cut in front of the jewel pin a circular
recess shown at _a_ called the passing hollow. The remaining part of the
safety action is the guard pin shown at _N_ Figs. 61 and 62, which is
placed in the lever. Now it is to be understood that the sole function
performed by the guard pin is to strike the edge of the roller _A_ at
any time when the fork starts to unlock the engaged pallet, except when
the jewel pin is in the slot of the fork. To avoid extreme care in
fitting up the passing hollow, the horns of the fork are arranged to
strike the jewel pin and prevent unlocking in case the passing hollow is
made too wide. To delineate the safety action we first draw the fork and
jewel pin as previously directed and as shown at Fig. 63. The position
of the guard pin should be as close to the bottom of the slot of the
fork as possible and be safe. As to the size of the guard pin, it is
usual to make it about one-third or half the diameter of the jewel pin.
The size and position of the guard pin decided on and the small circle
_N_ drawn, to define the size and position of the roller we set our
dividers so that a circle drawn from the center _A_ will just touch the
edge of the small circle _N_, and thus define the outer boundary of our
roller, or roller table, as it is frequently called.

[Illustration: Fig. 61]

[Illustration: Fig. 62]

For deciding the angular extent of the passing hollow we have no fixed
rule, bu