Let us suppose, now, we set our unlocking or
discharging jewel in advance, that is, so the escapement is really
unlocked a little before the setting parts are in the positions and
relations shown in Fig. 141. Under the new conditions the escape wheel
would commence to move and get sufficient velocity on it to act on the
impulse jewel as soon as it was inside of the periphery of the escape
wheel. If the balance was turned slowly now the tooth of the escape
wheel would not encounter the impulse jewel at all, but fall into the
passing hollow _n_; but if we give the balance a high velocity, the
tooth would again encounter and act upon the jewel in the proper manner.
Experienced adjusters of chronometers can tell by listening if the
escape-wheel tooth attacks the impulse jewel properly, i.e., when both
are moving with similar velocities. The true sound indicating correct
action is only given when the balance has its maximum arc of vibration,
which should be about 11/4 revolutions, or perform an arc of 225
degrees on each excursion.
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