Causes of the Aurora Burealls. 239 



rising from the southern horizon towards the north ; and the opinion 

 of Halley, that the magnetic effluvia became luminous in their progress. 

 from the north to find the south pole, is equally nugatory ; for it 

 would be an absurdity to suppose, that those forces were flowing 

 over from the north to seek a south pole, when at the same time the 

 aurora is seen ascending from the antarctic regions.* 



Nor can its various phases be accounted for by the reflection of 

 the solar light from the polar ices. 



Reflected light falls on those spaces enlightened by direct rays 

 from the illuminating source ; hence, if the direct rays of the sun 

 were reflected from the polar ices, there should be no polar night ; a 

 constant uniform light should be diffused throughout the arctic re- 

 gions. If the reflection upon the ice does not proceed from the inci- 

 dent rays, but from the light refracted by the atmosphere, the unifor- 

 mity of the effect must be the same, although it should differ in the 

 degree of intensity. If the rotation of the earth, or change of sea- 

 sons, cause a difference, still the efiJect must be as uniform as the 

 cause, and the phenomenon should be regulated by periodical laws. 

 This is not the case with the aurora, which is signally irregular in the 

 times of its appearance. Nor could the solar hght refracted by the 

 atmosphere, and subsequently reflected from ice within the polar cir- 

 cle, be sufficiently intense to illuminate the atmosphere at such an 

 altitude as would render its second refraction visible in the tem- 

 perate zone, a distance of more than two thousand miles. If the re- 

 flection from the ice were the cause, and its effects were uniformly 

 diffused light, how could the fleeting forms of the aurora rise more 

 conspicuous near the pole, than in other latitudes, shooting its spires 

 and crescents perpendicularly to those very ices which reflect the rays 

 to supply its light ? Nor could light be atmospherically refracted 

 from an altitude above the pole, which would not be intercepted by 

 the spherical figure of the earth, before it could fall on the lower 

 latitudes of the temperate zone. If the reflection of light from the 

 ices cannot at the same place and same point of time, produce an 

 equally diffused light, and the uncertain forms of the aurora, the phe- 

 nomenon must be referred to some other agency. 



If electricity and magnetism are inadequate to produce the aurora 

 by any known action or combination of their powers; and if the hy- 



' Seen by Forster, who accompanied Capt. Cook, in 1773, in south lat. 58°. 



