We commence the
drawing at Fig. 57, as before, by drawing the line _A B_ and the arcs
_a_ and _b_ to represent the pitch circles. We also sweep the arc _f_ to
enable us to delineate the line _A g'_. Next in order we draw our jewel
pin as shown at _D_. In drawing the jewel pin we proceed as at Fig. 56,
except we let the line _A g'_, Fig. 57, assume the same relations to the
jewel pin as _A B_ in Fig. 56; that is, we delineate the jewel pin as if
extending on the arc _a_ six degrees on each side of the line _A g'_,
Fig. 57.
THE THEORY OF THE FORK ACTION.
To aid us in reasoning, we establish the point _m_, as in Fig. 55, at
_m_, Fig. 57, and proceed to delineate another and imaginary jewel pin
at _D'_ (as we show in dotted outline). A brief reasoning will show that
in allowing thirty degrees of contact of the fork with the jewel pin,
the center of the jewel pin will pass through an arc of thirty degrees,
as shown on the arcs _a_ and _f_. Now here is an excellent opportunity
to impress on our minds the true value of angular motion, inasmuch as
thirty degrees on the arc _f_ is of more than twice the linear extent as
on the arc _a_.
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