ing influence. In
the designs we shall employ, it is our purpose to follow such
proportions as have been adopted by our best makers, in all respects,
including form, size and material. We would say, however, there has been
but little deviation with our principal manufacturers of marine
chronometers for the last twenty years as regards the general principle
on which they were constructed, the chief aim being to excel in the
perfection of the several parts and the care taken in the several
adjustments.

Before we proceed to take up the details of constructing a chronometer
escapement we had better master the names for the several parts. We show
at Fig. 136 a complete plan of a chronometer escapement as if seen from
the back, which is in reality the front or dial side of the "top plate."
The chronometer escapement consists of four chief or principal parts,
viz.: The escape wheel, a portion of which is shown at _A_; the impulse
roller _B_; unlocking or discharging roller _C_, and the detent _D_.
These principal parts are made up of sub-parts: thus, the escape wheel
is composed of arms, teeth, recess and collet, the recess being the
portion of the escape wheel sunk, to enable us to get wide teeth actions
on the impulse pallet. The collet is a brass bush on which the wheel is
set to afford better support to the escape wheel than could be obtained
by the thinned wheel if driven directly on the pinion arbor. The impulse
roller is composed of a cylindrical steel collet _B_, the impulse pallet
_d_ (some call it the impulse stone), the safety recess _b b_. The
diameter of the impulse collet is usually one-half that of the escape
wheel. This impulse roller is staked directly on the balance staff, and
its perfection of position assured by resting against the foot of the
shoulder to which the balance is secured. This will be understood by
inspecting Fig. 137, which is a vertical longitudinal section of a
chronometer balance staff, the lower side of the impulse roller being
cupped out at _c_ with a ball grinder and finished a ball polish.

[Illustration: Fig. 136]

[Illustration: Fig. 137]

It will be seen the impulse roller is staked flat against the hub _E_ of
the balance staff. The unlocking roller, or, as it is also called, the
discharging roller, _C_, is usually thinner than the impulse roller and
has a jewel similar to the impulse jewel _a_ shown at _f_. This roller
is fitted by friction to the lower part of the balance staf