and Directive Force of the Magnetic Needle. 197 
Next suppose Toronto (N. lat. 435°) to be the station of the 
needle, and represent it by P’ in Fig. 5. P/E will be the prime 
vertical on the photosphere; and P/EQ= 434°, PE= SU? 
and NE= SE = 233°. From which we have, NP’= 74° 5, 
NP’E =17° 30’, SP’-E=17° 30’, SP’=105° 55’. Current NP’ 
impels the north end of the needle toward the East, and SP’ 
toward the West. The meridional component of current NP’= 
to 2 NP sin 235°.* Going through with the calculation we find 
vg 
a 
'_ Same for the months that precede and follow the solstices. This 
fact as the result of observation is exhibited to the eye in Fig. 6, 
Which is a transcript of Fig. 1, p. 20, Vol. II, of ‘Toronto Ob- 
servations), From the cause now under consideration the same 
effect should occur at the hours following 7 to 8 a.M.; but it 
should be less in amount from hour to hour, because the angle 
P'S is less and less as the arc NS is carried farther to the west 
by the diurnal revolution. ‘This fact is also shown in the dia- 
aay tte ares NP’, SP’, NP, is meant the force of the currents proceeding from 
NandStoPandp? ’ 
it The effect of the currents proceeding from 
sin NP’ : Br e's 
.) Point, as N, varies with the distance by reason of the divergence of the individ- 
* Currents, which follow arcs of great circles, because of the effect of resistances 
individual currents, and oe also from the very nature of the prop- 
; intensity of a galvanic current varies inversely as the square root of 
- fhe length of the wire traversed. 
