TERRESTRIAL MAGNETISM. 229 



artificial magnets and in a locality sufficiently distant from iron, 



never showed the slightest deviation, (which we should rather 



expect,) the equality of the two fluids might with the highest 



degree of probability be inferred for the whole earth ; though 



without wholly excluding the possibility of some inequality. 



The only difference which the existence of such an inequality 



would occasion in our theory would be, that P° (Art. 1 7) would 



no longer be = 0. The consequence of this would be, that for 



all external space it would be necessary to add to the expression 



R- P" 

 for Z the member — 5— ; so that on the surface of the earth the 

 v 



(constant) member P" must be added, but X and Y would be 



in no respect affected. When there shall exist in future times 



a much more extensive collection of accurate observations than 



we at present possess, it may be examined whether a vanishing 



value of P^ is or is not required for their accurate representation. 



With our present data such an undertaking would be wholly 



useless. 



36. 



Another part of our theory on which there may exist a doubt 

 is, the supposition that the agents of the terrestrial magnetic force 

 are situated exclusively in the interior of the earth. If we seek 

 for their immediate causes, partly or wholly, without the earth, 

 and confine ourselves to known scientific gi'ounds, we can only 

 think of galvanic currents. But the atmosphere is no conductor 

 of such currents, neither is vacant space ; thus, in seeking in 

 the upper regions for a vehicle of galvanic currents we go be- 

 yond our knowledge. -But our ignorance gives us no right ab- 

 solutely to deny the possibility of such currents; we are for- 

 bidden to do so by the enigmatical phenomena of the Aurora 

 BoreaUs, in which there is every appearance that electricity in 

 motion performs a principal part. It will therefore still be in- 

 teresting to examine what form magnetic action arising from 

 such currents would assume on the surface of the earth. 



37. 

 Let us, then, assume the existence of constant galvanic cur- 

 rents in a concave sphere, S, surrounding the earth, and call S^ 

 all the space included by S, and S" all the space external to S. 

 Whatever may be configuration of the galvanic currents, we 

 can always substitute for them a fictitious distribution of the 



