TERRESTRIAL GALVANIC CURRENTS. 215 



It was soon found that, when the wires are in this 

 state, galvanic currents sometimes pass through them 

 which are sufficiently strong to cause movement of the 

 galvanometer-needle: and (when a battery is placed 

 in the circuit for giving signals) sufficiently strong to 

 pervert the telegraph-signals. And it was at length 

 discovered that those currents, produced by the earth 

 only, occurred at the same times as magnetic storms. 

 In order to investigate the relation between the earth- 

 currents and the magnetic storms, two wires were 

 established in connexion with the Royal Observatory 

 of Greenwich ; one about 10 miles long, terminating 

 at Croydon, the other about 8 miles long, to Dartford : 

 each wire was carefully insulated in every part except 

 at both its extremities, which were plunged in earth, 

 and the two wires passed through two galvanometers, 

 one appropriated to each wire, in the Observatory. 

 Each of these wires might be expected to bring from 

 the earth at one end to the earth at the other end a 

 portion of the galvanism which is flowing through the 

 superficial strata of the earth. 



As it was soon found that currents, weak or strong, 

 were almost always perceptible in the movements of 

 the galvanometer, a self-registering apparatus was 

 prepared. To the needle of each galvanometer a small 

 plane mirror was attached, and the light of a lamp 

 shining upon the mirror was by lenses made to con- 

 verge, to form a spot upon a revolving barrel covered 

 with photographic paper. Thus two registers were 

 obtained similar in general character to those of the 



