164 THE POPULAR SCIENCE MONTHLY. 



a correction of one second in about ten seconds. The correction is 

 made as follows : Between the sidereal chronometer (b, Fig. 5) and the 

 mean-time chronometer (c) there is a commutator (d). By moving its 

 handle toward the right, a current is sent through the " accelerating or 

 retarding coil " which accelerates the mean solar standard ; by moving 

 the handle toward the left, the current goes through the coil in the 

 opposite direction, and retards the mean solar standard ; in the inter- 

 mediate position (shown in the figure) no action takes place. The 

 operator, having ascertained the error of the sidereal standard and its 

 sympathetic chronometer, by astronomical observation as described, 

 applies this error to the face-reading of the sidereal chronometer, and 

 gets the exact sidereal time ; by simple reduction he finds the corre- 

 sponding mean solar time, and, by comparison, the error of the mean- 

 time chronometer ; he then moves the handle of the commutator, and 

 corrects the error of the mean solar standard, and of all the clocks 

 controlled by it, without leaving his position in the computing-room. 

 This correction can be made at any instant when the exact time is 

 desired ; it is usually made at 10 a. m. and 1 p. m., because at those 

 hours a general distribution of time-signals takes place. 



The mean solar standard serves for the distribution of accurate 

 time in the following ways : 



Nearly all the mean-time clocks in the Royal Observatory are driven 

 by the standard clock ; they are, in fact, simply dials whose hands are 

 moved in the same way and by the same battery as the hands of the 

 standard itself. These clocks are placed in the various rooms of the 

 observatory, so that the astronomers have the exact time close to any 

 of their instruments. One of them is in the wall surrounding the 

 grounds, and will be familiar to every one who has visited the observa- 

 tory ; several are placed in the chronometer-room, where the navy and 

 other chronometers are corrected and regulated. 



The seconds-relay (a, Fig. 5), already referred to, is also driven by 

 the mean solar standard. 



Until 1880 the standard clock controlled, by seconds-beats, a num- 

 ber of clocks on a private wire in London, which were made to beat 

 synchronously with the standard by an application of the Jones sys- 

 tem,* in which the electric current is used, not as a driver, but as a 

 regulator of clocks already running with small error and by means 

 of their own motive powers. This plan, though still used within the 

 observatory, has been abandoned in London. 



With the standard clock is connected another electric circuit, open 

 in two places. These are both automatically closed by the clock, one 

 at the end of each minute, but the other only for some seconds on 

 either side of the end of each hour ; so that they are both closed only 

 at the end of each hour, and then only can the current pass. 



* For an illustration of the Jones system for regulating clocks at a distance, see article 

 on " Time-keeping in Paris," "Popular Science Monthly," January, 1882. 



