204 Mr. \\ r . Ellis. On the Simultaneity of 



the different occasions, repeated at each station in a generally similar 

 manner, but with the individual difference peculiar to the station. 

 Disturbance having, however, once set in, it is found that, although 

 the magnetic irregularity will be practically similar for places not 

 geographically distant, as Greenwich and Kew, there is not the same 

 complete similarity in the movements at places widely separated. 

 The magnetic impulse at any moment during disturbance may, of 

 course, be simultaneously propagated through the earth, as in the case 

 of the initial movements ; but it is not usually possible to fix on any 

 phase of movement, and say to what phase it corresponds at another 

 distant station, unless the movement is sudden, when the initial type 

 again recurs, and such recurrence, occurring during disturbance at 

 Greenwich, has been found to correspond with recurrence at other 

 places. Generally it is only these cases of sudden, not necessarily 

 large, movement, occurring at the beginning or during a magnetic 

 disturbance, that we can readily identify as being really simultaneous 

 and corresponding movements. 



Reference has been made to the earth currents always so active 

 during magnetic disturbance. Mr. C. V. Walker, formerly telegraph 

 superintendent of the South Eastern Railway, in a paper read before 

 the Royal Society in the year 1861, came to the conclusion, from 

 observations made on the telegraph lines under his control, that earth 

 currents, at least at times of great magnetic disturbance, exercised a 

 direct action upon magnetometers, just as artificial currents confined 

 to a wire exercise a direct action upon a magnet. Two insulated lines 

 of wire were afterwards established in connexion with the Royal 

 Observatory, one passing from the Observatory to Dartford, the other 

 from the Observatory to Croydon, expressly for the study of earth 

 currents. The distance between the earth plates in the Dartford circuit 

 was 10 miles, and in the Croydon circuit 8 miles. Sir George Airy r 

 discussing the earth currents observed in these lines during the years 

 1865 to 1867 (' Phil. Trans.,' vol. 158, p. 471), confirmed generally Mr. 

 Walker's result, considering that " it is impossible to avoid the conclu- 

 sion that the magnetic disturbances are produced by terrestrial galvanic 

 currents below the magnets ;" but there were some anomalies, the earth 

 current appearing, in some cases, distinctly to follow the magnetic 

 motion, instead of being coincident with, or preceding, it, as, on the 

 supposition mentioned, should always happen, and in other cases pre- 

 ceding it by a longer interval of time than the conditions seemed to 

 require. 



In the year 1868 the earth current lines to Dartford and Croydon 

 were replaced by two others running, one from near Morden College, 

 Blackheath, to the North Kent East Junction of the South Eastern 

 Railway (distance between earth plates 2 miles), and the other from 

 Angerstein Wharf (on the bank of the River Thames, near Charlton) 



