216 ON MAGNETISM. 



changes of Magnetic Elements, described in the last 

 Section. 



The ordinates of these curves (considered as mea- 

 sures of the terrestrial galvanic currents passing through 

 their respective wires) were measured for correspond- 

 ing times. In order to determine experimentally the 

 sign to be given to a current, considered as positive 

 when its nature was that of a graphite current coming 

 from the distant station, a small battery was placed so 

 as to send graphite currents through the galvanometer, 

 and the nature of the movements produced by it was 

 noticed. Then it was conceived that each current 

 might be represented as the effect of one current from 

 the north and one from the west, the effect of each 

 upon one experimental wire being proportional to the 

 cosine of the angle made by that experimental wire 

 with the north and with the west respectively. Putting 

 a for the azimuth of Croydon from magnetic north 

 towards east, a for that of Dartford, G, D, N 9 W, the 

 currents from Croydon, from Dartford, to the North, 

 and to the West, respectively : 



C= Nx cos a + W x sin a, 

 D = JV x cos a' + If x sin a'; 

 from which 



_ _ r 



sm (a a ) sin (a - a ) 

 sina 



^y _ c . , D 



sin (a a') sin (a a') ' 



