288 Prof. Challis on a Theory of the Force of Electricity* 



tricity are found to act upon each other so as to produce motions 

 of translation. But these motions cannot be due to the attrac- 

 tions and repulsions which have hitherto been under considera- 

 tion ; for if so, an electrified body, according as its electricity is 

 negative or positive, would attract or repel another on which it 

 induces electricity. Whereas the fact is, attraction takes place 

 in both cases, and it is only after the bodies have been in con- 

 tact, and their electricities have become of the same kind, that 

 they mutually repel. Also the attractions and repulsions hitherto 

 considered act directly upon only an extremely small portion of 

 the whole mass, and for this reason must have a feeble translating 

 power. If they are at all operative in producing motions of 

 translation, such motions must be veiled by the action of more 

 powerful forces of a different kind. It is clear that forces by 

 which two bodies at one instant attract, and at the next repel 

 each other, cannot be intrinsic or resident in the bodies them- 

 selves, but must be attributed to the dynamic action under dif- 

 ferent conditions of the agent to which the force of electricity is 

 ascribed. Now we have just seen that the dynamic action which 

 the setherial medium — the agent assumed in this theory — exerts 

 by means of waves, fails to give the requisite explanation, and we 

 are thus restricted to the supposition that it acts by currents. 

 This, I think, will be found to be the true explanation. That 

 there exist steady setherial currents, is indicated by magnetic 

 phsenomena, as I hope at some time to show. Assuming the 

 existence of such currents, in whatever direction they traverse 

 oodies, whose atoms are so arranged that the density increases 

 with the distance from a fixed plane, secondary currents must 

 be generated which flow from the rarer to the denser parts with 

 a velocity depending on the magnitudes of the atoms, and the 

 number in a given space. The reason for this assertion is derived 

 from the hydrodynamical equation generally applicable to cases 

 of steady motion, which is of this form, 



Pressure =/ (/) — ^-j 



showing that where the velocity is greater the pressure is less. 

 But the velocity of a steady current is greater through the 

 denser parts of a medium than through the rarer, because in the 

 former the space the aether fills is less than in the latter, on 

 account of the greater occupation of space by the atoms of the 

 medium. Hence there is a constant accelerative force from 

 the rarer towards the denser parts, generating a current in that 

 direction. These secondary currents may be regarded as instances 

 of steady motion to which the equation above is applicable. 

 19. Now it has been shown (in arts, 13 and 14) that in a 



