16 Prof. Challis's Theory of the 



In arts. 120-122 and 130-132 of this publication, theoretical 

 reasons are given for concluding that little or no magnetic 

 effect is attributable to the earth's motion of translation, and 

 that terrestrial magnetism is almost wholly due to streams gene- 

 rated by the rotation about its axis. For the sake of conveni- 

 ence, I propose to reproduce here the reasoning by which these 

 conclusions are there arrived at, previously to adding other con- 

 siderations confirmatory of them. As the writings above enu- 

 merated sufficiently indicate the way in which the hydrodyna- 

 mical theory of magnetism accounts generally for magnetic force 

 by the action of steady streams of the aether, it will only be ne- 

 cessary for the present purpose to investigate the mode of gene- 

 ration of those steady streams by the agency of which the prin- 

 cipal facts of the proper magnetism of the earth may admit of 

 explanation. 



It must first be mentioned that the philosophical principles I 

 have so long advocated forbid making any other hypotheses 

 respecting atoms and the aether than those I have frequently had 

 occasion to enunciate in this Journal. Accordingly it will be 

 supposed that the earth consists wholly of inert spherical atoms 

 of constant form and magnitude — that they are held in positions 

 of equilibrium by atomic and molecular forces, which are only 

 modes of action of the aether itself, which is assumed to be the 

 source of all active force, and to press always and everywhere 

 proportionally to its density — and that consequently the aether 

 at rest has the same density within the earth as in the space 

 outside. Moreover it is supposed that the occupation of space 

 by the finite atoms is very small compared with the intervening 

 spaces, even at the earth's centre, where the atomic density is 

 greatest. 



Considering, first, the motions which the atoms impress on the 

 aether by reason of the earth's orbital motion, we may at once 

 infer from Poisson's solution of the problem of the simultaneous 

 movements of a ball-pendulum and the surrounding air, the 

 motioDs of the sether which will be produced by any single atom ; 

 and since the motions due to different atoms, as being steady 

 relatively to the positions of their centres, may coexist, we may 

 conclude, on the hypothesis that the proportion of space occu- 

 pied by the atoms is very small, that the total motion is quam 

 proxime the sum of all the motions caused by individual atoms. 

 Also since, according to that solution, just as much fluid flows 

 backwards as the atom displaces by its forward motion, it fol- 

 lows that during the time the earth is moving over a portion of 

 its orbit equal to its diameter, a quantity of fluid crosses the 

 transverse plane which is tangential to its surface in the first and 

 last positions, equal to the quantity which would just fill the 



