The escapement we next describe is that most generally used in chronometers. S S, Fig. 99, is the escape wheel which is kept from revolving by the detent D. On the axis of the balance are two discs, R_{1}, R_{2}, placed one under the other. As the balance revolves in the direction of the arrow, the pin P_{2} will come round and catch against the point of the detent, lifting it and releasing the escape-wheel, which will revolve, and the tooth T will hit against the stud P_{1}, giving the balance an impulse. The balance then swings on to the end of its course and returns, and the stud P_{2} passes the detent as follows: a light spring Y Y is fastened to the detent, projecting a little beyond it, and it is this spring, and not the detent itself, that the pin P_{2} touches: on the return of P_{2} it simply lifts the spring away from the detent and passes it, whereas in advancing the spring was supported by the point of the detent, and both were lifted together. [Illustration: FIG. 99.—Chronometer Escapement. ] [Illustration: FIG. 100.—The Fusee. ] In watches and chronometers and in small clocks a coiled spring is used instead of a weight, but its action is irregular, since when it is fully wound up it exercises greater force than when nearly down. In order to compensate for this the cord or chain which is wound round the barrel containing the spring passes round a conical barrel called a _fusee_ (Fig. 100): B is the barrel containing the spring and A A the fusee. One end of the spring is fixed to the axis of the barrel, which is prevented from turning round, and the other end to the barrel, so that on winding up the clock by turning the fusee the cord becomes coiled on the latter, and the more the spring is wound the nearer the cord approaches the small end of the fusee, and has therefore less power over it; while as the clock goes and the spring becomes unwound, its power over the axis becomes greater. The power, therefore, acting to turn the fusee remains pretty constant. ----- Footnote 9: By Messrs. E. Dent and Co. of the Strand. CHAPTER XIV. CIRCLE READING. One of the great advantages which astronomy has received from the invention of the telescope is the improved method of measuring space and determining positions by the use of the telescope in the place of pointers on the old instruments. The addition of modern appliances to the telescope to enable it to be used as an accurate pointer, has played a conspicuous part in the accurate measurement of space, and the results are of such importance, and they have increased so absolutely _pari passu_ with the telescope, that we must now say something of the means by which they have been brought about.