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.
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