on, let us imagine that a real guard
point rests on the circle _c_ at _h_. Suppose we make a notch in the
guard roller represented by the circle _c_, to admit such imaginary
guard point, and then commence to revolve the circle _c_ in the
direction of the arrow _j_, letting the guard point rest constantly in
such notch. When the notch _n_ in _c_ has been carried through thirty
degrees of arc, counting from _B_ as a center, the guard point, as
relates to _A_ as a center, would only have passed through an arc of
five degrees. We show such a guard point and notch at _o n_. In fact, if
a jewel pin was set to engage the fork on the pitch circle _b a_, the
escapement would lock. To obviate such lock we widen the notch _n_ to
the extent indicated by the dotted lines _n'_, allowing the guard point
to fall back, so to speak, into the notch _n_, which really represents
the passing hollow. It is not to be understood that the extended notch
at _n_ is correctly drawn as regards position, because when the guard
point was on the line _A f_ the point _o_ would be in the center of the
extended notch, or passing hollow. We shall next give the details of
drawing the double roller, but before doing so we deemed it important to
explain the action of such guard points more fully than has been done
heretofore.

HOW TO DESIGN A DOUBLE-ROLLER ESCAPEMENT.

We have already given very desirable forms for the parts of a
double-roller escapement, consequently we shall now deal chiefly with
acting principles as regards the rollers, but will give, at Fig. 82, a
very well proportioned and practical form of fork. The pitch circle of
the jewel pin is indicated by the dotted circle _a_, and the jewel pin
of the usual cylindrical form, with two-fifths cut away. The safety
roller is three-fifths of the diameter of the pitch diameter of the
jewel-pin action, as indicated by the dotted circle _a_.

The safety roller is shown in full outline at _B'_, and the passing
hollow at _E_. It will be seen that the arc of intersection embraced
between the radial lines _B c_ and _B d_ is about sixty-one and a half
degrees for the roller, but the angular extent of the passing hollow is
only a little over thirty-two degrees. The passing hollow _E_ is located
and defined by drawing the radial line _B c_ from the center _B_ through
the intersection of radial line _A i_ with the dotted arc _b_, which
represents the pitch circle of the safety roller. We will name this
inte