The space between the lines _B f'_ and _B i_ on the arc _g_ defines the
extent of the locking face of the entrance pallet _C_. The intersection
of the line _B f'_ with the arc _g_ we denominate the point _o_, and
from this point as a center sweep the short arc _p_ with a 5" radius;
and on this arc, from its intersection with the radial line _A b_, lay
off twelve degrees, and through the point so established, from _o_ as a
center, draw the radial line _o m_, said line defining the locking face
of the entrance pallet _C_.

[Illustration: Fig. 22]

It will be seen that this gives a positive "draw" of twelve degrees to
the entrance pallet; that is, counting to the line _B f'_. In this
escapement as delineated there is perfect tangential locking. If the
locking face of the entrance-pallet stone at _C_ was made to conform to
the radial line _A b_, the lock of the tooth _D_ at _o_ would be "dead";
that is, absolutely neutral. The tooth _D_ would press the pallet _C_ in
the direction of the arrow _x_, toward the center of the pallet staff
_B_, with no tendency on the part of the pallet to turn on its axis _B_.
Theoretically, the pallet with the locking face cut to coincide with the
line _A b_ would resist movement on the center _B_ in either direction
indicated by the double-headed arrow _y_.

A pallet at _C_ with a circular locking face made to conform to the arc
_g_, would permit movement in the direction of the double-headed arrow
_y_ with only mechanical effort enough to overcome friction. But it is
evident on inspection that a locking face on the line _A b_ would cause
a retrograde motion of the escape wheel, and consequent resistance, if
said pallet was moved in either direction indicated by the double-headed
arrow _y_. Precisely the same conditions obtain at the point _u_, which