102 Prof. Challis on a Theory of Magnetic Force. 
The terrestrial streams probably circulate, those which issue 
nearly perpendicularly to the earth’s surface in the antarctic 
regions, after rising to great heights; turning back in curved 
courses distant from the earth, so as to enter again nearly per- 
pendicularly in the arctic regions. To this circulation must be 
added that spoken of in art. 27, which conspires with the ant- 
arctic streams and opposes the arctic, and thus probably accounts 
for the observed excess of antarctic magnetic intensity. 
30. If the earth had been symmetrical with respect to its axis, 
but not with respect to its equator, the magnetic streams would 
also have been symmetrical with respect to the axis. But the 
actual irregular distribution of land and sea is opposed to this 
law, and it is found in fact that there is an approximation to 
two magnetic poles as well in the arctic as the antarctic regions. 
Also it is established by observation that these poles have not 
fixed positions on the earth’s surface. Taking account of this 
circumstance, and of the character of the irregularities to which 
the theory ascribes the generation of the currents, we might 
infer that the magnetic poles would be unsymmetrically situated 
with respect to the earth’s poles, that the streams would be 
disposed unsymmetrically about them, and that in their move- 
ment they would not retain the same relative positions. These 
inferences observation appears to countenance. 
31. The arches and streamers of the Aurora Borealis and the 
Aurora Australis are portions of terrestrial magnetic streams 
made visible by particular disturbances. According to my view 
of the undulatory theory of light, I should have no difficulty in 
admitting that etherial streams that are steady, and on that 
account emit no light, become luminous upon being disturbed. 
32. We have now to consider the disturbances which the 
steady terrestrial currents may undergo from cosmical influences. 
And in the first place it may be stated, as a deduction from 
hydrodynamical principles, that the steady streams to which 
magnetic force has been attributed, and the vibrations which 
were assumed to exist in the theory of the force of gravity, may 
coexist without mutual interference, so that the two kinds of force 
act independently of each other. 
33. The effect which any motion of translation which the 
earth’s atoms may have relatively to the ether, may be presumed 
to be the same, in respect to generation of secondary currents, 
as that of a current within the earth at rest. In the mvestiga- 
tion of such effect, the motion of translation of the solar system 
would have to be taken into account, together with the motion 
of the earth in its orbit. As observation has not detected a 
variation of terrestrial magnetism corresponding to the law of 
the variation of velocity which would result from the combina- 
