Through the point _f_
we draw the tangent line _b' b b''_, and at the intersection of the line
_b_ with _i_ we establish the center of our pallet staff at _B_. At two
and a half degrees from the point _c_ we lay off two and a half degrees
to the right of said point and establish the point _n_, and draw the
radial line _A n n'_, which establishes the extent of the arc of angular
motion of the escape wheel utilized by the pallet arm.
[Illustration: Fig. 90]
We have now come to the point where we must exercise our reasoning
powers a little. We know the locking angle of the escape-wheel tooth
passes on the arc _a_, and if we utilize the impulse face of the tooth
for five degrees of pallet or lever motion we must shape it to this end.
We draw the short arc _k_ through the point _n_, knowing that the inner
angle of the pallet stone must rest on this arc wherever it is situated.
As, for instance, when the locking face of the pallet is engaged, the
inner angle of the pallet stone must rest somewhere on this arc (_k_)
inside of _a_, and the extreme outer angle of the impulse face of the
tooth must part with the pallet on this arc _k_.
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