ce prior to this jewel passing through an arc of five degrees inside
the periphery of the escape wheel. It will be evident on thought that if
an escape-wheel tooth engaged the impulse stone before the five-degrees
angle had passed, the contact would not be on its flat face, but the
tooth would strike the impulse jewel on its outer angle. A continued
inspection will also reveal the fact that in order to have the point of
the tooth engage the flat surface of the impulse pallet the impulse
jewel must coincide with the radial line _c g_. If we seek to remedy
this condition by setting the impulse jewel so the face is not radial,
but inclined backward, we encounter a bad engaging friction, because,
during the first part of the impulse action, the tooth has to slide up
the face of the impulse jewel. All things considered, the best action is
obtained with the impulse jewel set so the acting face is radial to the
balance staff and the engagement takes place between the tooth and the
impulse jewel when both are moving with equal velocities, i.e., when
the balance is performing with an arc (or motion) of 11/4 revolutions
or 225 degrees each way from a point of rest. Under such conditions the
actual contact will not take place before some little time after the
impulse jewel has passed the five-degree arc between the lines _c e_ and
_c g_.

THE DROP AND DRAW CONSIDERED.

Exactly how much drop must be allowed from the time the tooth leaves the
impulse jewel before the locking tooth engages the locking jewel will
depend in a great measure on the perfection of workmanship, but should
in no instance be more than what is absolutely required to make the
escapement safe. The amount of draw given to the locking stone _c_ is
usually about twelve degrees to the radial line _k a_. Much of the
perfection of the chronometer escapement will always depend on the skill
of the escapement adjuster and not on the mechanical perfection of the
parts.

The jewels all have to be set by hand after they are made, and the
distance to which the impulse jewel protrudes beyond the periphery of
the impulse roller is entirely a matter for hand and eye, but should
never exceed 2/1000". After the locking jewel _c_ is set, we can set the
foot _F_ of the detent _D_ forward or back, to perfect and correct the
engagement of the escape-wheel teeth with the impulse roller _B_. If we
set this too far forward, the tooth _A^3_ will encounter the roller
while the tooth