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.
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