646 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



gun. Thus Greenwich mean time was daily announced at Newcastle 

 from Edinburgh during the Meeting of the Association, by the 

 discharge of a gun at 1 P.M., electrically fired from the Observatory 

 125 miles distant. This Newcastle electric time-gun experiment by 

 Holmes is historically of interest as having formed the initiatory 

 experiment which resulted in the establishment of the electric-torpedo 

 system of defence, so successfully employed by the Confederate States 

 in their naval operations during the continuance of the American 

 civil war raging at that date ; a system which has been the basis of 

 the present more highly developed torpedo defence, in use by the 

 various European powers. By a modification in the arrangements for 

 the transmission of the spark, Holmes afterwards successfully estab- 

 lished a chain of electric time-gun signals, fired daily at 1 P.M., in 

 connection with the Edinburgh Eoyal Observatory south, at New- 

 castle, North Shields, and Sunderland, and west, at Glasgow, where 

 three guns were planted, on the high ground of port Dundas for the 

 city, at the Exchange for the merchants, and at the Broomielaw for the 

 shipping, and a fourth gun at the Albert Quay, Greenock, for the docks 

 and vessels lying at the tail of the Bank. The Astronomer-Eoyal for 

 Scotland, Professor Piazzi Smyth, in 1863, had therefore eight time- 

 gun signals daily discharged at 1 P.M. from the Observatory, Edinburgh. 

 Seven of these guns are now historical, the castle gun alone being- 

 daily discharged in connection with the Edinburgh Koyal Observatory. 



The acquisition of the telegraph wires by the Post Office has 

 introduced a modified system of electric time-signal currents dissemi- 

 nated over the kingdom from the Greenwich Eoyal Observatory. 



These electric Greenwich time-currents may be classified into two 

 groups. First, the metropolitan, and second, the provincial currents. 

 By the first group Greenwich time is given by special wires every 

 hour to London ; by the second group, Greenwich time is given to the 

 country by means of the telegraph lines twice a day, at 10 A.M. and 

 at 1 P.M. The necessary electrical contacts arid currents are auto- 

 matically controlled and distributed by means of an apparatus termed 

 a " chronopher." The indication of true time is variously registered ; 

 at times it is by the dropping of a time-ball placed upon a -roof or 

 tower in a conspicuous situation, or by the beat of an ordinary 

 galvanometer needle, the stroke of a bell, or, as already noticed, by the 

 discharge of a powerful cannon. This last method is the most 



