So much for the eye and ear method of transit observation. There is another which has now to a very large extent superseded it. This is called the “chronographic method”; we owe it to Sir Charles Wheatstone, who made it possible about 1840. Figs. 124-7 are from drawings of the chronograph in use at Greenwich, and by their means we hope to make the principle of the instrument clear. In this chronograph, _g_ is a long conical pendulum which regulates the driving clock in the case below it, through the gearing of wheel-work, as it turns the cylinder, E, gently and regularly round. On the cylinder is placed paper to receive the mark registering the observations; along the side of the cylinder or roller run two long screws, K and N, Fig. 125, which are also turned by the clock, and on them are carried electro-magnets, A, B, Fig. 125, and prickers, 35, Fig. 126; as the screws turn, the magnets and prickers are moved along the roller, and, as the roller turns, the pointer, 36, Fig. 127, traces a fine line on the paper like the worm of a screw on the surface; and it is close to this line, which serves as a guide to the eye, that the prickers make a mark each time a current is sent through the electro-magnets; this turns each of them into a magnet, and they then attract a piece of iron which, in moving upwards, presses down its pricker by means of a lever, and registers the instant the current is sent. The different wires are brought, first from the transit circle to work one pricker, and then from the clock to work the other, the clock sending a current and producing a prick on the roller every second. [Illustration: FIG. 124.—The Greenwich chronograph. General view. ] The observer, instead of depending upon the eye and ear as he had to do before, has then the means of impressing a mark at any instant upon the same cylinder, in exactly the same way that the pendulum of the clock impresses the mark of any second, so that as each wire in the eyepiece of the transit instrument is passed by the star, he is able, by the same method as the clock, to record on this same revolving surface each observation, which can afterwards be compared with the marks representing the seconds, and so the exact time of each observation is read off more accurately and with less trouble than by the old method. Let us suppose we are making a transit observation: the clock will be diligently pricking sidereal seconds, while we, by a contact-maker held in the hand, are as diligently recording the moments at which the star passes each wire. [Illustration: FIG. 125.—Details of the travelling carriage which carries the magnets and prickers. Side view and view from above. ] [Illustration: FIG. 126.—Showing how on the passage of a current round the soft iron the pricker is made to make a mark on the spiral line on the cylinder. ] [Illustration: FIG. 127.—Side view of the carriage carrying the magnets and the pointer that draws the spiral. ]