Under the new conditions the escape wheel would commence to move and get sufficient velocity on it to act on the impulse jewel as soon as it was inside of the periphery of the escape wheel.  If the balance was turned slowly now the tooth of the escape wheel would not encounter the impulse jewel at all, but fall into the passing hollow n; but if we give the balance a high velocity, the tooth would again encounter and act upon the jewel in the proper manner.  Experienced adjusters of chronometers can tell by listening if the escape-wheel tooth attacks the impulse jewel properly, i.e., when both 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]