THE AURORA BOREALIS. 



Fig. 6. 



counted, forming as many bows, having their ends supported on the earth, and, 

 in their arrangement, resembling the short curtains suspended one behind the 

 other over the scene of a theatre, and intended to represent the sky. Some- 

 times the intervals between these bows diminish, and two or more of them 

 close upon each other, forming one large zone, traversing the heavens, and dis- 

 appearing toward 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 which have already passed beyond the mag- 

 netic zenith appear 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 

 splendor, subject to continual and sudden variations in their length and bright- 

 ness ; that these beautiful red and green tints color 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 splen- 

 did spectacle which presents itself to him whv> witnesses the aurora from the 

 bay of Alten. 



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

 forms suddenly, without any previous bow. There are rarely more than t\v > 

 on the same night ; and many of the auroras are attended with no crown at all. 



The corona becomes gradually faint, the whole pheaomenon 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 morge 

 into the vague and feeble light which is spread over the heavens grouped like j 



