AURORA BOREALIS. 



traversing the heavens, and disappearing towards the south, 

 becoming rapidly feeble after passing the zenith. But sometimes, 

 also, when this zone extends over the summit of the firmament 

 from east to west, the mass of rays appears suddenly to come from 

 the south, and to form with those from the north the real boreal 

 corona, all the rays of which converge to the zenith. This 

 appearance of a crown, therefore, is doubtless the mere effect of 

 perspective ; and an observer, placed at the same instant at a 

 certain distance to the north or to the south, would perceive only 

 an arc. 



The total zone, measuring less in the direction north and south 

 than in the direction east and west, since it often leans upon the 

 earth, the corona would be expected to have an elliptical form ; 

 but that does not always happen : it has been seen circular, the 

 unequal rays not extending to a greater distance than from eight 

 to twelve degrees from the zenith, while at other times they reach 

 the horizon. 



Let it, then, be imagined, that all these vivid rays of light 

 issue forth with splendour, subject to continual and sudden 

 variations in their length and brightness; that these beautiful 

 red and green tints colour them at intervals ; that waves of light 

 undulate over them ; that currents of light succeed each other ; 

 and, in fine, that the vast firmament presents one immense and 

 magnificent dome of light, reposing on the snow-covered base 

 supplied by the ground which itself serves as a dazzling frame 

 for a sea, calm and black as a pitchy lake and some idea, though 

 an imperfect one, may be obtained of the splendid spectacle which 

 presents itself to him who witnesses the aurora from the Bay 

 of Alten. 



The corona, when it is formed, only lasts for some minutes : 

 it sometimes forms suddenly, without any previous bow. There 

 are rarely more than two on the same night ; and many of the 

 auroras are attended with no crown at all. 



The corona becomes gradually faint, the whole phenomenon 

 being to the south of the zenith, forming bows gradually paler, 

 and generally disappearing before they reach the southern 

 horizon. All this most commonly takes place in the first half of 

 the night, after which the aurora appears to have lost its intensity : 

 the pencils of rays, the bands, and the fragments of bows appear 

 and disappear at intervals ; then the rays become more and more 

 diffused, and ultimately merge into the vague and feeble light 

 which is spread over the heavens, grouped like little clouds, and 

 designated by the name of auroral plates (plaques aurorales}. 

 Their milky light frequently undergoes striking changes in its 

 brightness, like motions of dilatation and contraction, which are 



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