100 Prof. Challis on a Theory of Magnetic Force. 
We are to assume that the mass of the earth consists of dis- 
erete spherical atoms, and that the whole of its interior, excepting 
the space occupied by atoms, is filled with ether in the same 
state of density as in the external regions of space, with only 
such variations of density as are produced by its motion. The. 
same assumption is to be extended to the sun, the moon, and 
all the planets. In the first place, let us endeavour to ascertain 
what kind of movement is impressed on the ether by the earth’s 
rotation ; and for the sake of distinctness, I shall, at first, suppose 
that the earth has no motion of translation, and that it is per- 
fectly symmetrical with respect to its axis and its equatorial 
plane. Then as the terrestrial atoms will impress a part of their 
velocity on the ether, a rotatory motion of the latter will be 
produced, which by its centrifugal force will tend to draw the 
fluid from the earth’s axis. This tendency cannot give rise to a 
stream from one pole to the other, because there is no reason 
why it should flow in one direction rather than the other. 
Neither can streams be generated setting from the equaterial 
parts towards the poles, because as the velocity will be greatest 
in and near the plane of the equator, the fluid would rather be 
urged towards this plane, and circulate by return currents along 
the axis towards the poles. The tendency of the centrifugal 
force to draw off the fluid from the axis of rotation, will be 
counteracted by accelerative forces due to increments of den- 
sity of the ether which result from the resistance of its mertia 
to the centrifugal movement. These forces, acting always 
towards the axis, cause the circular motion immediately im- 
pressed by the atoms to extend into the ether beyond the 
earth’s surface. But it is evident that so long as the terrestrial 
atoms have greater velocity than the ether contiguous to them, 
the centrifugal force is on the increase, and the permanent state 
is reached only when there is no motion of the atoms relative 
to the ether. The opposite forces then maintain a steady 
motion at all points, there bemg no reason why the motion 
should not be steady at one point rather than at another. Con- 
sequently in the earth’s interior, and at the surface, and sensibly 
to considerable heights above, the ether, like the atmosphere, 
partakes of the earth’s rotation. This gyratory motion must 
spread to remote distances, the velocity decreasing with the di- 
stance according to a law which eventually may be found to 
admit of mathematical investigation on hydrodynamical prin- 
ciples. As the motion of rotation must be combined with the 
circulating currents above mentioned, the total motion is spiral. 
28. Now let the earth be supposed to have a motion of trans- 
lation, either uniform or variable. Then on the principle that 
there can be neither gain nor loss of the momentum of the ether 
