HOW STAND ABB TIME IS OBTAINED. 215 



place in the heavens at the rate of abont 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 ex- 

 actly, 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 



u 1 r— 



A Short Section from the Paper Band of the Chronograph Cylinder, showing Tra- 

 cings OF 1'en connected with Clocks: 1, seconds of .sidereal clock; 2. Loth sidereal 

 and common clocks ; 3-10, the traciny:s 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. 



rests a pen connected with the telegraphic instrument which fol- 

 lows 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, con- 

 nected 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 



