We
will accept these proportions and conditions as best, from the fact that
they are now almost universally adopted by our best chronometer makers.
Although it would seem as if these proportions should have established
themselves earlier among practical men, we shall in these drawings
confine ourselves to the graphic plan, considering it preferable. In the
practical detail drawing we advise the employment of the scale given,
i.e., delineating an escape wheel 10" in diameter. The drawings which
accompany the description are one-fourth of this size, for the sake of
convenience in copying.
With an escape wheel of fifteen teeth the impulse arc is exactly
twenty-four degrees, and of course the periphery of the impulse roller
must intersect the periphery of the escape wheel for this arc (24 deg.).
The circles _A B_, Fig. 139, represent the peripheries of these two
mobiles, and the problem in hand is to locate and define the position of
the two centers _a c_. These, of course, are not separated, the sum of
the two radii, i.
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