CHAP, vi.] ELECTltIC HOROLOGY. G45 



III. ELECTRIC TIME SIGNALS. 



A very important application of electricity is that of the accurate 

 determination of longitude between British and the more important 

 Continental observatories, and the indication at distant stations of 

 Greenwich mean time. The spread of railways throughout the 

 length and breadth of the United Kingdom necessitates the punc- 

 tual departure of trains according to published time-tables. In 

 Great Britain, Greenwich mean time is employed at all stations 

 from Penzance in Cornwall to Lerwick in Shetland. In Ireland 

 Dublin time is taken, the constant difference between Dublin time 

 and Greenwich time being allowed. The indication of true time 

 by an audible signal, by means of the isochronism of controlled 

 electric clocks was first practically carried out at Edinburgh, by Pro- 

 fessor Piazzi Smyth, the Astronomer-Eoyal for Scotland, between 

 the Eoyal Observatory, Calton Hill, and the Castle. The daily 

 discharge of the gun at 1 P.M. from the castle ramparts in this 

 instance is effected by means of two clocks connected by a wire, the 

 one at the observatory, the other upon the castle ramparts adjacent 

 to the gun, and their isochronous action is ensured by magneto-electric 

 controlled pendulums. At the precise moment of time the castle 

 clock liberates a weighted trigger which mechanically effects the 

 discharge of the gun. There is no city in the world in which time 

 is generally so accurately kept and observed as Edinburgh, and the 

 chronometrical arrangements of the Eoyal Scottish Observatory are 

 fully appreciated. The indication of true time at a distant station of 

 an audible signal by the direct action of an electric spark was first 

 carried out by Mr. Nath. J. Holmes, at the Meeting of the British 

 Association at Newcastle-upon-Tyne, in 1863. In that year a gun was 

 discharged from the ramparts of the old Eoman Tower, the ignition 

 being effected by the direct action of a magneto-electric spark trans- 

 mitted through the telegraph wire between Edinburgh and Newcastle, 

 One end of the wire was connected with a magneto machine, and the 

 circuit automatically closed at the precise interval of time by the 

 Observatory clock, while the Newcastle end of the wire was in con 

 nection with a detonating fuse inserted into the touch-hole of the 



