released and passed through an arc of twelve degrees. A tooth resting on
a flat surface, as imagined above, would also rest dead. As stated
previously, the pallets we are considering have equidistant locking
faces and correspond to the arc _l l_, Fig. 6.

In order to realize any power from our escape-wheel tooth, we must
provide an impulse face to the pallets faced at _f e_; and the problem
before us is to delineate these pallets so that the lever will be
propelled through an arc of eight and one-half degrees, while the escape
wheel is moving through an arc of ten and one-half degrees. We make the
arc of fork action eight and one-half degrees for two reasons--(1)
because most text-books have selected ten degrees of fork-and-pallet
action; (2) because most of the finer lever escapements of recent
construction have a lever action of less than ten degrees.


LAYING OUT ESCAPE-WHEEL TEETH.

To "lay out" or delineate our escape-wheel teeth, we continue our
drawing shown at Fig. 6, and reproduce this cut very nearly at Fig. 8.
With our dividers set at five inches, we sweep the short arc _a a'_ from
_f_ as a center. It is to be borne in mind that at the point _f_ is
located the extreme point of an escape-wheel tooth. On the arc _a a_ we
lay off from _p_ twenty-four degrees, and establish the point _b_; at
twelve degrees beyond _b_ we establish the point _c_. From _f_ we draw
the lines _f b_ and _f c_; these lines establishing the form and
thickness of the tooth _D_. To get the length of the tooth, we take in
our dividers one-half a tooth space, and on the radial line _p f_
establish the point _d_ and draw circle _d' d'_.