Prof. Challis on a Theory of Magnetie Force. 105 
a set of maxima prevailing in hours of the night, and another 
set in hours of the day, and that the directions of the deviations 
of the needle in the two cases would be opposite, the directions 
of the gyratory streams of the superior and inferior planets 
being opposite for their positions of maximum disturbance. 
These laws General Sabine has in fact detected by a discussion of 
numerous observations taken at Kew and Hobarton. (See 
‘ Proceedings of the Royal Society,’ vol. x. No. 41, p. 632.) 
41. The facts relating to the movements of the earth’s mag 
netic poles do not appear to be sufficiently well ascertained to 
allow of applying the theory to explain them. Conceive, for 
the sake of illustration, that there are two north magnetic poles 
at the same distance from the earth’s pole, and 180° from each 
other, that the currents converging to them are of the same 
intensity, and that they have the same uniform movement from 
east to west. On these supnositions it is evident that when the 
poles are on the great circle passing through the pole of the 
earth and a given place, the Inclination is at a maximum, and 
the Declination is changing from east to west. When they 
have passed over 90° the Declination will have gone through a 
western maximum, and be on the point of changing from west 
to east, because the needle, being equally acted upon by the two 
poles in the new positions, must point exactly between them. 
At the same time the Inclination will be at a minzmum. After 
the poles have advanced 90° further, the needle will have gone 
through a maximum eastern Declination, and returned to its 
original position, and the Inclination will again be at a maximum: 
and soon. ‘These hypothetical circumstances are, perhaps, not 
so far different from the actual as to prevent our concluding 
from them that neither the direction nor the amount of the 
motion of the magnetic poles can be inferred with any certainty 
from mere observations of “pomts of convergence” of the 
horizontal needle—at any rate not with so much certainty as 
the direction of the movement may be inferred from the direc- 
tion in which the Declination is changing when it is passing 
through zero at any place, as London or Paris, ¢f at the same 
time the Inclination is near its maximum at the same place. 
Now early observations at Paris show that the Declination was 
zero there about the year 1663, and was changing from east to 
west, and observations both at Paris and London indicate a 
constant diminution of Inclination from that date, while, accord- 
ing to Hansteen’s Isoclinal lines, the Inclination was less in 
1600 than in 1700. It may therefore be inferred that a 
maximum of Inclination occurred between those two epochs, 
and consequently, from the above argument, that the motion of 
one of the north magnetic poles is from east to west, If this 
