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rarely passes through the magnetic pole, and seldom passes through 

 the same point, for three different places. I have, however, felt even 

 more persuaded that the aurora is, partly at least, an optical phe- 

 nomenon, from a consideration of that phase of the aurora constituting 

 the corona borealis, a persuasion that I stated in the Literary Gazette 

 of the time, in giving an account of the beautiful corona of October 

 24, 1847. 



Mairan, and, more lately, Dalton, have explained this phase of 

 the aurora by a hypothesis of polar beams, long fiery rods of solar 

 atmosphere, according to the one, of red-hot ferruginous particles, 

 according to the other, seen in perspective, as they lie in the direc- 

 tion of the magnetic force. A little acquaintance with the pheno- 

 menon — the rushing and tilting of the beams against each other, 

 one beam occasionally rising from the horizon, passing through the 

 centre of the crown and beyond it — would shew the improbability of 

 this hypothesis, I am persuaded, that the phenomena of the corona 

 borealis is produced in a narrow horizontal stratum of the earth's 

 atmosphere. Thanks to the discoveries of Dr Faraday, we do not 

 require a ferruginous sea, in order to have polarized particles ; the 

 watery crystals that inhabit the upper regions of the atmosphere can 

 themselves assume a polar state, determined by the passage of elec- 

 tric currents ; and we have only to complete this fact by a hypothesis 

 of luminous electric discharges seen refracted by these crystals, the 

 position of visibility of the refracted rays depending on the angles of 

 the crystals, and the deflections from the direction of the magnetic 

 force which they suffer, by the electric currents. Such a hypothesis, 

 which occurs at once when an optical phenomenon has to be ac- 

 counted for, would explain these remarkable auroral clouds, so often 

 seen in connection with the aurora itself; it would also serve to explain 

 the appearance of the arch at certain altitudes, lower for lower alti- 

 tudes, determined by the position of the source of light, direction of 

 the magnetic force at the place, and the effect of the electric current 

 in deflecting the crystals. The crystals successively deflected by 

 electric currents would also exhibit the rushing pencils or beams. 



It need scarcely be remarked, that differently formed crystals 

 micrht give rise to different phases of the phenomenon ; while reflec- 

 tion might be combined with refraction in certain cases, especially in 

 the case of arches seen south of the anti-dip. Such a hypothesis 

 evidently assumes a source of light, independent of these optical re- 



