We show in Fig. 140 the impulse jewel D just passing inside the circle of the periphery of the escape wheel.  Now the attendant conditions are these:  The escape wheel is locked fast and perfectly dead, and in the effort of unlocking it has to first turn backward against the effort of the mainspring; the power of force required for this effort is derived from the balance in which is stored up, so to speak, power from impulses imparted to the balance by former efforts of the escape wheel.  In actual fact, the balance at the time the unlocking takes place is moving with nearly its greatest peripheral velocity and, as stated above, the escape wheel is at rest.

Here comes a very delicate problem as regards setting the unlocking or discharging jewel.  Let us first suppose we set the discharging jewel so the locking jewel frees its tooth at the exact instant the impulse jewel is inside the periphery of the escape wheel.  As just stated, the escape wheel is not only dead but actually moving back at the time the release takes place.  Now, it is evident that 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.