are moving with similar velocities. The true sound indicating correct
action is only given when the balance has its maximum arc of vibration,
which should be about 11/4 revolutions, or perform an arc of 225
degrees on each excursion.

Fig. 142 is a side view of Fig. 141 seen in the direction of the arrow
_y_. We have mentioned a chariot to which the detent is attached, but we
shall make no attempt to show it in the accompanying drawings, as it
really has no relation to the problem in hand; i.e., explaining the
action of the chronometer escapement, as the chariot relates entirely to
the convenience of setting and adjusting the relation of the second
parts. The size, or better, say, the inside diameter of the pipe at _C_,
Fig. 143, which holds the locking jewel, should be about one-third of a
tooth space, and the jewel made to fit perfectly. Usually, jewelmakers
have a tendency to make this jewel too frail, cutting away the jewel
back of the releasing angle (_n_, Fig. 143) too much.

A GOOD FORM OF LOCKING STONE.

A very practical form for a locking stone is shown in transverse section
at Fig. 143. In construction it is a piece of ruby, or, better, sapphire
cut to coincide to its axis of crystallization, into first a solid
cylinder nicely fitting the pipe _C_ and finished with an
after-grinding, cutting away four-tenths of the cylinder, as shown at
_I_, Fig. 143. Here the line _m_ represents the locking face of the
jewel and the line _o_ the clearance to free the escaping tooth, the
angle at _n_ being about fifty-four degrees. This angle (_n_) should
leave the rounding of the stone intact, that is, the rounding of the
angle should be left and not made after the flat faces _m o_ are ground
and polished. The circular space at _I_ is filled with an aluminum
pin. The sizes shown are of about the right relative proportions; but
we feel it well to repeat the statement made previously, to the effect
that the detent to a chronometer cannot well be made too light.

[Illustration: Fig. 141]

[Illustration: Fig. 142]

[Illustration: Fig. 143]

The so-called gold spring shown at _H_, Figs. 141 and 142, should also
be as light as is consistent with due strength and can be made of the
composite metal used for gold filled goods, as the only real benefit to
be derived from employing gold is to avoid the necessity of applying oil
to any part of the escapement. If such gold metal is employed, after
hammering to obtain the grea