Prof. Challis on a Theory of Magnetic Force. 101 
due to the earth’s rotation, it may be asserted that the gyratory 
motion is transferred without alteration through space with the 
earth. It may not be that the motion of a given particle of the 
fluid remains the same; but the motion is constantly the same 
im positions which in successive instants are the same relative to 
the earth’s centre and equatorial plane. Coexistent with the 
gyrations of the «ther, motions of translation must be impressed 
upon it, having constant relations to the earth’s motion. With 
respect to that portion of ether which has sensibly the same 
rotation as the earth, the most probable supposition is that it 
has also very nearly the same motion of translation, and that 
consequently the relative motion of the earth’s atoms and the 
ether in its interior is very small. 
These conclusions apply to the other rotatory bodies of the 
solar system. All are consequently centres of gyratory motions, 
which, according to the reasoning in art. 15, may coexist in the 
cosmical spaces without interfering with each other. 
29. But it is not true, as supposed, that a earth is symme- 
trical with respect to its axisand equatorial plane, the superficial 
distribution of land and water showing that this is not the case. 
On the supposition cf symmetry, the centrifugal force of the 
earth’s rotation would induce a variation of its internal density, 
proper, according to the theory, for generating secondary streams 
under the influence of a primary current. But as by reason of 
the symmetry equal and opposite effects would be produced on 
every diameter of the earth, no streams would become sensible. 
For the same reascn no streams result from the increase of the 
earth’s density towards the centre by the effect of ee In 
the actual case of deviation from symmetry, the motion of the 
ether cannot be wholly in symmetrical gyrations as above 
inferred, although it will be approximately such. There must 
be motion relative both to the equator and the axis, steady in its 
character, as depending on constant causes, and constituting a 
primary stream, which flowing through the earth, put into an 
unsymmetrical state of restraint by the centrifugal force, will 
give rise to secondary streams. The primary current : may be of 
feeble intensity, and yet by originating accelerative forces which 
act through large spaces, may generate streams of great intensity, 
The earth’s magnetic streams may be regarded as resulting from 
a combination of the primary and secondary streams. According 
to these views the deviations of the earth’s form and matter from 
symmetry determine the directions of the magnetic streams, which 
appear from experiment to enter the earth on the north side of the 
magnetic equator, and to zssue from it on the south side. The earth 
is thus a vast magnet, the streams of which are of constant intensity, 
excepting so far as they may be disturbed by cosmisal influences: 
