If we now should
reverse the motion and turn the arc _l_ backward, the tooth at _e_
would, in turn, be released and the tooth following after _D_ (but not
shown) would engage _l_ at _f_. By supplying motive to revolve the
escape wheel (_E_) represented by the circle _n_, and causing the arc
_l_ to oscillate back and forth in exact intervals of time, we should
have, in effect, a perfect escapement. To accomplish automatically such
oscillations is the problem we have now on hand.
HOW MOTION IS OBTAINED.
In clocks, the back-and-forth movement, or oscillating motion, is
obtained by employing a pendulum; in a movable timepiece we make use of
an equally-poised wheel of some weight on a pivoted axle, which device
we term a balance; the vibrations or oscillations being obtained by
applying a coiled spring, which was first called a "pendulum spring,"
then a "balance spring," and finally, from its diminutive size and coil
form, a "hairspring." We are all aware that for the motive power for
keeping up the oscillations of the escaping circle _l_ we must contrive
to employ power derived from the teeth _D_ of the escape wheel.
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