The astronomer's principal clock--the one he uses in all his calculations--marks what is known as sidereal, not ordinary, time. The revolution of the earth in its orbit sets the sun back in its place in the heavens at the rate of about four minutes a day, or one whole day in a year, so that this clock, indicating star time, gains this amount and is only with ordinary clocks once a year. After it is once adjusted, no attempt is made to regulate it exactly, as the astronomer would better calculate its differences than disturb its regulation, always provided its rate is very uniform and accurately known. One or more of the other clocks, however, are made to show ordinary time, and corrected by observations taken every few days. It is from this clock that the standard time is sent out. It is possible to connect any of these clocks telegraphically with an instrument in the observatory, known as a chronograph. It consists of a cylinder with a sheet of paper around it, on which rests a pen connected with the telegraphic instrument which follows the beats of the clock. The cylinder is turned slowly by clockwork, and the pen, carried slowly along by a screw, describes a spiral on the paper with jogs or teeth in it about a quarter of an inch apart, caused by the beats of the clock. In this way the astronomer secures a visible record of the beating of his clock, or rather of the movements of his telegraphic recorder. Thus, if he has another key on the same circuit with the clock, connected with his chronograph recorder, and should touch it between the beats of his clock, it would put in an extra jog or tooth on his record, and it will show, what he could not have told in any other way, in just what part of the second he touched this key, whether in the first or last part of the second, and precisely how far from either end--that is, he can determine fractions of a second with great nicety. [Illustration: A SHORT SECTION FROM THE PAPER BAND OF THE CHRONOGRAPH CYLINDER, SHOWING TRACINGS OF PEN CONNECTED WITH CLOCKS: 1, seconds of sidereal clock; 2, both sidereal and common clocks; 3-10, the tracings of the mean-time clock fall steadily behind the other; 11, sidereal only; 12, connected with observer's key. The extra teeth show when a star passed each of the five spider lines. At the extreme right is a "rattle," put in to show where the observation is on the cylinder.]