st 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'_.

To facilitate the drawing of the other teeth, we draw the circles _d' c'_,
to which the lines _f b_ and _f c_ are tangent, as shown. We divide
the circle _n n_, representing the periphery of our escape wheel, into
fifteen spaces, to represent teeth, commencing at _f_ and continued as
shown at _o o_ until the entire wheel is divided. We only show four
teeth complete, but the same methods as produced these will produce them
all. To briefly recapitulate the instructions for drawing the teeth for
the ratchet-tooth lever escapement: We draw the face of the teeth at an
angle of twenty-four degrees to a radial line; the back of the tooth at
an angle of thirty-six degrees to the same radial line; and make teeth
half a tooth-space deep or long.

[Illustration: Fig. 8]

We now come to the consideration of the pallets and how to delineate
them. To this we shall add a careful analysis of their action. Let us,
before proceeding further, "think a little" over some of the factors
involved. To aid in this thinking or reasoning on the matter, let us
draw the heavy arc _l_ extending from a little inside of the circle _n_
at _f_ to the circle _n_ at _e_. If now we imagine our escape wheel to
be pressed forward in the direction of the arrow _j_, the tooth _D_
would press on the arc _l_ and be held. If, however, we should revolve
the arc _l_ on the center _k_ in the direction of the arrow _i_, the
tooth _D_ would _escape_ from the edge of _l_ and the tooth _D''_ would
pass through an arc (rec