We mentioned this matter previously, but venture on the repetition to
make everything clear and easily understood. We commence by drawing the
line _A B_ and dividing it in four equal parts, as on previous
occasions, and from _A_ and _B_ as centers draw the pitch circles _c d_.
By methods previously described, we draw the lines _A a_ and _A a'_,
also _B b_ and _B b'_ to represent the angular motion of the two
mobiles, viz., fork and roller action. As already shown, the roller
occupies twelve degrees of angular extent. To get at this conveniently,
we lay off on the arc by which we located the lines _A a_ and _A a'_ six
degrees above the line _A a_ and draw the line _A h_.
Now the angular extent on the arc _c_ between the lines _A a_ and _A h_
represents the radius of the circle defining the jewel pin. From the
intersection of the line _A a_ with the arc _c_ as a center, and with
the radius just named, we sweep the small circle _D_, Fig. 58, which
represents our jewel pin; we afterward cut away two-fifths and draw the
full line _D_, as shown.
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