MAGNETIC DISTURBANCES WITH TERRESTRIAL GALVANIC CURRENTS. 469 
north in the direction N., E., S., W., is 122^°; and that of the Croydon station is 229 
Let C be the measure of a galvanic current from Croydon, D that of a current of the 
same kind (as referred to a battery-pole) from Dartford ; and assume these to be caused 
by a galvanic current N of the same kind towards the north, and a galvanic current W 
of the same kind towards the west ; and that these north and west currents admit of 
being resolved according to the rules of resolution of mechanical statical force. Put « 
for the azimuth of Croydon, a! for that of Dartford. Then it is easily found that 
W=C.-^^-D.- . c .° sa ,, ; 
sm (a— a') sin (a— a! ) 
N=C . . 7 . ^ -— D. . 7* . ; 
sm (a— a ) sin (a — a! ) 
which, converted into numbers, give 
W= -C X 0-55800+D x 0-68259, 
N=+Cx0 88437+Dx 0-79218. 
Now, upon interposing a battery in the course of each of the wires, with its graphite 
or copper pole towards Greenwich, it was found that, on the Croydon wire, it gave an 
ordinate upon the photographic sheet in the direction which we are accustomed to 
measure as positive ; and on the Dartford wire it gave an ordinate upon the photo- 
graphic sheet in the negative direction. Let C' and D' be the measured quantities ; 
then C'=+C, D'=— D; and 
Graphite current towards W=-C'x 0-55800 — D' x 0-68259, 
Graphite current towards N= + C'x 0-88437— D'x 0-79218. 
Conceiving this graphite current, if it is really a terrestrial galvanic current acting on 
our magnetometers, to be below them, it will deflect any part of a magnet possessing 
red or austral magnetism to the right, estimated with reference to the direction of the 
current’s course. Hence we obtain 
Magnetic tendency to N= — C'x 0-55800— D' x 0-68259, 
Magnetic tendency to W=— C'x 0-88437 +D'x 0-79218. 
It was found that when the ordinates as measured from the photographic sheet were 
multiplied by these factors, they gave results larger than those on the magnetometer 
sheets with which I desired to compare them ; and, in order to diminish their 
magnitude, they were multiplied by 0-5437. Hence, putting e=0-68259 X 0-5437. 
jf=0-55800 x 0-5437, ^=0-79218x0-5437, A= 0-88437 X 0-5437 (the order of C' and 
D' having been accidentally inverted), 
Magnetic tendency to N= — D'xe— C 'xf, 
Magnetic tendency to W = -j- D' X g — O X h. 
It now became important to consider how, from the photographic curves of the Earth- 
currents, these Magnetic Tendencies could be most conveniently obtained. For my own 
