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THE AURORA BOREALIS. 



the electricity and magnetism of the earth and its atmosphere must now be 

 regarded as its source. 



In his treatise on these meteors, Mairan describes their appearance and the 

 succession of changes to which they are subject with great minuteness and 

 precision. The more conspicuous auroras commence to be formed soon after 

 the close of twilight. At first a dark mist or foggy cloud is perceived in the 

 north, and a little more brightness toward the west than in the other parts of the 

 heavens. The mist gradually takes the form of a circular segment, resting at 

 each corner on the horizon. The visible part of the arc soon becomes sur- 

 rounded with a pale light, which is followed by the formation of one or several 

 luminous arcs. Then come jets and rays of light variously colored, which 

 issue from the dark part of the segment, the continuity of which is broken by 

 bright emanations, which indicate a movement of the mass, which seems agi- 

 tated by internal shocks, during the formation of these luminous radiations, 

 which issue from it as flames do from a conflagration. When this species of 

 fire has ceased, and the aurora has become extended, a crown is formed at the 

 zenith, to which these rays converge. From this time the phenomenon dimin- 

 ishes in its intensity, exhibiting, nevertheless, from time to time sometimes on 

 one side of the heavens and sometimes on another jets of light, a crown and 

 colors more or less vivid. Finally the motion ceases, the light approaches 

 gradually to the horizon ; the cloud, quitting the other parts of the firmament, 

 settles in the north. The dark part of the segment becomes luminous, its 

 brightness being greatest near the horizon, and becoming more feeble as the 

 altitude augments, until it loses its light altogether. 



The aurora is sometimes composed of two luminous segments, which are 

 concentric, and separated from each other by one dark space, and from the 

 earth by another. Sometimes, though rarely, there is only one dark segment, 

 which is symmetrically pierced round its border by openings, through which 

 light or fire is seen, as represented in fig. 1. A meteor of this kind was ob- 



served by Mairan himself at Breuille-Pont, on the 19th of October, 1726. 

 This meteor was seen at the same time in distant parts of Europe, such as 

 Warsaw, Moscow, St. Petersburg, Rome, Naples, Lisbon, and Cadiz. The 

 least height which is compatible with its observed position in these places 

 would be about fifty leagues above the surface of the earth. 



In the year 1817, M. Biot made a voyage to the Shetland isles, where he 

 had frequent and favorable opportunities of observing these phenomena ; and 

 the known habits of accuracy and skill in experimental investigation of that 

 philosopher must confer great value on the results of his observations. A re- 

 markable aurora was seen by him on the 27th of August, 1817. 



Several thin jets of light were first seen to rise at the northeast to a small 

 height. Having played for some time, they were extinguished ; but, after an 

 hour and a half, they reappeared, with increased extent and brilliancy, in the 

 same part of the sky. They soon began to form above the horizon a regular 



