i6 2 THE POPULAR SCIENCE MONTHLY. 



barometer. Counterpoises are added at/" to balance the magnet {b). 

 A plan of the lever on a smaller scale and a section at A are also 

 shown in the figure. The barometer-tube is made so much larger in 

 the shorter than in the longer leg that a change of one inch in the 

 barometer would move the float in the shorter leg only two tenths of 

 an inch. A rise or fall in the barometer causes a corresponding mo- 

 tion in the horseshoe magnet, and thus varies the intensity of its 

 attraction for the magnets on the pendulum-bob. By proper adjust- 

 ment this varying attraction is made to furnish the required compen- 

 sation. 



The small error which remains, notwithstanding the above-detailed 

 provisions for correction, is allowed to accumulate, but is determined 

 daily (unless clouds prevent) by transit observations,* so that the exact 

 sidereal time is always known. 



The standard sidereal clock registers its beats upon the chronograph 

 record ; controls, by electric connection, all the sidereal clocks in the 

 different rooms of the observatory ; and drives a sidereal chronometer 

 {b, Fig. 5), in agreement with itself, in the computing and time-dis- 

 tributing room. 



The secondary regulator of the time of England is the mean solar 

 standard clock at the Royal Observatory, which was specially erected 

 in 1852 for service in the time-signal system, of which it is now the 

 most important instrument. This clock has a seconds-pendulum, 

 which closes an electric circuit as it swings to the right. An electro- 

 magnet in the circuit lifts a small weight, which is discharged upon 

 the pendulum as it swings to the left, and gives it an impulse ; this 

 being repeated at each vibration is sufficient to keep it in motion. The 

 pendulum also closes other galvanic circuits one as it swings to the 

 right, another as it swings to the left which send currents alternately 

 positive and negative through electro-magnets, alternately attracting 

 and repelling bar-magnets fastened to an axis, which thus receives a 

 reciprocating motion. An arm projecting from this axis moves the 

 seconds-wheel one tooth forward each second ; proper gearing gives 

 motion to the minute and hour wheels. 



The mean solar standard, besides controlling other clocks, to be 

 enumerated later, drives a seconds-relay (a, Fig. 5), which controls a 

 mean-time chronometer (c). 



All the clocks controlled by the mean solar standard are required 

 to indicate exact Greenwich local time ; the error can not therefore be 

 allowed to accumulate, and the means of correction are provided. 

 Carried by an arm projecting from the pendulum-rod of the mean 

 solar standard is a magnet, five inches long, which swings just over a 



* The difference between the clock-time of the transit of a star over the meridian 

 (corrected for errors of position of the instrument, and for " personal equation") and the 

 right ascension of the star for the day, taken from the nautical almanac, is the error of 

 the clock. 



