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.
Fig. 125.—Details of the travelling carriage which carries the magnets and prickers. Side view and view from above.
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.
Fig. 127.—Side view of the carriage carrying the magnets and the pointer that draws the spiral.
This is done by pressing a stud, and sending a current at each transit; so that we shall have a dot in every other space between the clock dots, supposing the wires to be two seconds in time apart; supposing them to be three seconds apart, our dots will be in every third space; supposing them to be four seconds apart, our dots will be in every fourth space, and so on; and tenths and hundredths of seconds are estimated, by the position of each transit dot between those which record the seconds.