the escape wheel requires an
appreciable time to move forward and attack the impulse jewel, and
during this appreciable time the impulse jewel has been moving forward
inside of the arc _A A_, which represents the periphery of the escape
wheel. The proper consideration of this problem is of more importance in
chronometer making than we might at first thought have imagined,
consequently, we shall dwell upon it at some length.

HOW TO SET THE DISCHARGING JEWEL.

[Illustration: Fig. 140]

Theoretically, the escape-wheel tooth should encounter the impulse jewel
at the time--instant--both are moving with the same velocity. It is
evident then that there can be no special rule given for this, i.e.,
how to set the discharging jewel so it will free the tooth at exactly
the proper instant, from the fact that one chronometer train may be much
slower in getting to move forward from said train being heavy and clumsy
in construction. Let us make an experiment with a real chronometer in
illustration of our problem. To do so we remove our balance spring and
place the balance in position. If we start the balance revolving in the
direction of the arrow _y_, Fig. 140, it will cause the escapement to be
unlocked and the balance to turn rapidly in one direction and with
increasing velocity until, in fact, the escape wheel has but very little
effect on the impulse jewel; in fact, we could, by applying some outside
source of power--like blowing with a blow pipe on the balance--cause the
impulse jewel to pass in advance of the escape wheel; that is, the
escape-wheel tooth would not be able to catch the impulse jewel during
the entire impulse arc. 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