his arc intersects the line _w_ (which represents the impulse
face of the tooth) is where the outer angle _t_ of the entrance pallet
_C_ will touch the impulse face of the tooth. To prove this we draw the
radial line _A v_ through the point where the short arc _t t'_ passes
through the impulse face _w_ of the tooth _D_. Then we continue the line
_w_ to _n_, to represent the impulse face of the tooth, and then measure
the angle _A w n_ between the lines _w n_ and _v A_, and find it to be
approximately sixty-four degrees. We then, by a similar process, measure
the angle _A t s'_ and find it to be approximately sixty-six degrees.
When contact ensues between the tooth _D_ and pallet _C_ the tooth _D_
will attack the pallet at the point where the radial line _A v_ crosses
the tooth face. We have now explained how we can delineate a tooth or
pallet at any point of its angular motion, and will next explain how to
apply this knowledge in actual practice.

PRACTICAL PROBLEMS IN THE LEVER ESCAPEMENT.

To delineate our entrance pallet after one-half of the engaged tooth has
passed the inner angle of the entrance pallet, we proceed, as in former
illustrations, to establish the escape-wheel center at _A_, and from it
sweep the arc _b_, to represent the pitch circle. We next sweep the
short arcs _p s_, to represent the arcs through which the inner and
outer angles of the entrance pallet move. Now, to comply with our
statement as above, we must draw the tooth as if half of it has passed
the arc _s_.

To do this we draw from _A_ as a center the radial line _A j_, passing
through the point _s_, said point _s_ being located at the intersection
of the arcs _s_ and _b_. The tooth _D_ is to be shown as if one half of
it has passed the point _s_; and, consequently, if we lay off three
degrees on each side of the point _s_ and establish the points _d m_, we
have located on the arc _b_ the angular extent of the tooth to be drawn.
To aid in our delineations we draw from the center _A_ the radial lines
_A d'_ and _A m'_, passing through the points _d m_. The arc _a_ is next
drawn as in former instructions and establishes the length of the
addendum of the escape-wheel teeth, the outer angle of our escape-wheel
tooth being located at the intersection of the arc _a_ with the radial
line _A d'_.

As shown in Fig. 92, the impulse planes of the tooth _D_ and pallet _C_
are in contact and, consequently, in parallel planes, as mentioned on
page 91