0) the diameter of drawn escape wheel is to our real escape wheel so
is the measured distance on our drawing to the real distance in the
chronometer we are constructing.

It is well to use great care in the large drawing to obtain great
accuracy, and make said large drawing on a sheet of metal. This course
is justified by the degree of perfection to which measuring tools have
arrived in this day. It will be found on measurement of the arc of the
circle _B_, embraced between the intersections _e e^2_, that it is
about forty-eight degrees. How much of this we can utilize in our
escapement will depend very much on the perfection and accuracy of
construction.

[Illustration: Fig. 139]

We show at Fig. 140 three teeth of an escape wheel, together with the
locking jewel _E_ and impulse jewel _D_. Now, while theoretically we
could commence the impulse as soon as the impulse jewel _D_ was inside
of the circle representing the periphery of the escape wheel, still, in
practical construction, we must allow for contingencies. Before it is
safe for the escape wheel to attack the impulse jewel, said jewel must
be safely inside of said escape wheel periphery, in order that the
attacking tooth shall act with certainty and its full effect. A good
deal of thought and study can be bestowed to great advantage on the
"action" of a chronometer escapement. Let us examine the conditions
involved. 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