18 Prof. Challis's Theory of the 



the aether by the revolution of the atoms about the earth's axis 

 are circular, there will be no such refluent streams due to the 

 inertia of the general mass of the aether as those produced by 

 the earth's motion in its orbit. In fact, so long as there is any 

 relative circular motion between the aether and the atoms, the 

 latter will continually impel the a?ther, till the relative motion is 

 reduced to zero. It may therefore be inferred that in the ulti- 

 mate or steady state of the motion, the circular motion of the 

 aether within the earth is the same as that of the atoms by which 

 it is immediately impressed. This circular motion will, by 

 reason of the centrifugal force, operate upon the aether external 

 to the earth ; and the condition of steadiness requires that the 

 external aether should partake of a certain amount of circular 

 motion decreasing with the distance from the axis of revolution, 

 in order that the whole tendency of the centrifugal force trans- 

 versely to the axis may be counteracted by the inertia of the sur- 

 rounding mass of the aether. In short, if we conceive to be 

 impressed on the earth and the aether a motion equal and con- 

 trary to the earth's orbital motion, the result of combining this 

 motion of progression with the circular motion would be a com- 

 posite spiral motion of the aether having always at the same 

 points the same relations to the fixed centre of the earth. Hence, 

 the earth being now supposed to move, this spiral form of steady 

 motion would be capable of travelling uniformly through space 

 in company with the earth. (See a discussion of this kind of 

 motion in the article on the theory of magnetism in the Phi- 

 losophical Magazine for June 1872.) 



But it is evident that this rotatory motion will tend to draw 

 the aether from the axis equally in all directions in any given 

 transverse plane, and thus to cause the external aether to rush in 

 at the polar parts to supply any partial vacancy which the cen- 

 trifugal force might produce. If, however, the circumstances of 

 the earth were in all respects symmetrical with respect to the 

 equatorial plane, these tendencies would just counteract each 

 other, and no currents, other than the circular movements, 

 would be generated ; for the circulating currents which the influx 

 at one pole would produce of itself would be just neutralized by 

 those due to the influx at the other, and in consequence the 

 influx at each would be stopped. But because the circumstances 

 of the solid and fluid parts of the earth are not symmetrical with 

 respect to the equatorial plane, there will be a differential action 

 due to the centrifugal force, whereby streams will be produced 

 which, as satisfying the necessary condition of circulating, will 

 be permanent relatively to the earth. The direction of the flow 

 will be determined by the circumstances which violate the con- 

 dition of symmetry ; and it is therefore reasonable to suppose, 



