ASTRONOMICAL OBSERVATORIES. 289 



erected a mast, upon which slides a ball, five feet in 

 diameter, consisting of a frame of wood covered with 

 leather. A little before one o'clock every day the ball is 

 slid up to the top of the mast, where it is held by in- 

 geniously contrived machinery. Precisely at one o'clock 

 an assistant, who is specially charged with this duty, 

 presses a spring, and the ball instantly descends. By this 

 means all persons in sight of the ball are enabled daily to 

 test the accuracy of their clocks. At the Washington 

 observatory a ball of smaller size than that at Greenwich 

 is elevated every day on a flag-staff, and is lowered at the 

 precise instant of twelve o'clock. 



Within the last two years the arrangements at Green- 

 wich for furnishing the public with an accurate knowl- 

 edge of the time have been very much improved. A 

 normal clock is furnished with a small apparatus, by 

 means of which, whenever its error is determined by 

 observations, its indications can be rendered perfectly 

 correct. This clock keeps in motion a sympathetic gal- 

 vanic clock at the entrance-gate of the observatory, and 

 also a clock at the terminus of the South-eastern Railway. 

 It sends galvanic signals every day along all the principal 

 railways diverging from London ; it drops the Greenwich 

 ball and the ball on the offices of the Electric Telegraph 

 Company in the Strand. The Lords Commissioners of 

 the Admiralty have also erected a time signal-ball at 

 Deal, for the use of the shipping in the Downs, which is 

 dropped every day by a galvanic current from the Royal 

 observatory. 



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