drawing shown at Fig. 129. Here we establish
the point _A_ for the center of our escape wheel, and from this center
sweep the short arc _a a_ with a 10" radius, to represent the
circumference of our escape wheel. From _A_ we draw the vertical line
_A B_, and from the intersection of said line with the arc _a a_ we lay
off twelve degree spaces on each side of the line _A B_ on said arc _a_
and establish the points _b c_. From _A_ as a center we draw through
the points _b c_ the radial lines _b' c'_.

To define the face of the incline to the teeth we set our dividers to
the radius of any of the convenient arcs of sixty degrees which we have
provided, and sweep the arc _t t_. From the intersection of said arc
with the line _A b'_ we lay off on said arc sixty-four degrees and
establish the point _g_ and draw the line _b g_. Why we take sixty-four
degrees for the angle _A b g_ will be explained later on, when we are
discussing the angular motion of the cylinder. By dividing the eleventh
degree from the point _b_ on the arc _a a_ into thirds and taking two of
them, we establish the point _y_ and draw the radial line _A y'_. Where
this line _A y'_ intersects the line _b g_ we name the point _n_, and in
it is located the point of the escape-wheel tooth. That portion of the
line _b g_ which lies between the points _b_ and _n_ represents the
measure of the inner diameter of the cylinder, and also the length of
the chord of the arc which rounds the impulse face of the tooth. We
divide the space _b n_ into two equal portions and establish the point
_e_, which locates the position of the center of the cylinder. From _A_
as a center and through the point _e_ we sweep the arc _e' e'_, and it
is on this line that the points establishing the center of the cylinder
will in every instance be located. From _A_ as a center, through the
point _n_ we sweep the arc _k_, and on this line we locate the points of
the escape-wheel teeth. For delineating the curved impulse faces of the
escape-wheel teeth we draw from the point _e_ and at right angles to the
line _b g_ the line _e o_. We next take in our dividers the radius of
the arc _k_, and setting one leg at either of the points _b_ or _n_,
establish with the other leg the point _p'_ on the line _e o_, and from
the point _p'_ as a center we sweep the arc _b v n_, which defines the
curve of the impulse faces of the teeth. From _A_ as a center through
the point _p'_ we sweep the arc _p_, and in al