The first problem in our consideration of the theoretical action of the
cylinder escapement, is to arrange the parts we have described so as to
have these two movements of the escape wheel of like angular values. To
explain what we mean by this, we must premise by saying, that as our
escape wheel has fifteen teeth and we make each tooth give two impulses
in alternate directions we must arrange to have these half-tooth
movements exactly alike, or, as stated above, of equal angular values;
and also each impulse must convey the same power or force to the
balance. All escape wheels of fifteen teeth acting by half impulses must
impel the balance during twelve degrees (minus the drop) of escape-wheel
action; or, in other words, when a tooth passes out of the cylinder from
the position shown at Fig. 126, the form of the impulse face of the
tooth and the shape of the exit lip of the cylinder must be such during
twelve degrees (less the drop) of the angular motion of the escape
wheel.
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