Some makers bend the horn _k_ to a curve and
allow the end of the horn to arrest or stop the gold spring; but as it
is important the entire detent should be as light as possible, the
square end best answers this purpose. The banking placed at _j_ should
arrest the detent as thrown back by the spring _h_ at the "point of
percussion." This point of percussion is a certain point in a moving
mass where the greatest effort is produced and would be somewhere near
the point _x_, in a bar _G_ turning on a pivot at _z_, Fig. 138. It will
be evident, on inspection of this figure, if the bar _G_ was turning on
the center _z_ it would not give the hardest impact at the end _v_, as
parts of its force would be expended at the center _z_.
[Illustration: Fig. 138]
DECISIONS ARRIVED AT BY EXPERIENCE.
Experience has decided that the impulse roller should be about half the
diameter of the escape wheel, and experience has also decided that an
escape wheel of fifteen teeth has the greatest number of advantages;
also, that the balance should make 14,400 vibrations in one hour.
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