THUNDER AND LIGHTNING. 



hemispheres. The term Aurora Borealis, or Northern Lights, has 

 been applied to it, because the opportunities of witnessing it are, 

 from the geographical character of the globe, much more frequent 

 in the northern than in the southern hemisphere. The term 

 aurora polaris would be a more proper designation. 



This phenomenon consists of luminous rays of various colours, 

 issuing from every direction, but converging to the same point, 

 which appear after sunset generally toward the north, occasionally 

 toward the west, and* sometimes, but rarely, toward the south. 

 It frequently appears near the horizon, as a vague and diffused 

 light, something like the faint streaks which harbinger the rising 

 sun and form the dawn. Hence the phenomenon has derived its 

 name, which signifies northern morning. Sometimes, however, it 

 is presented under the form of a sombre cloud, from which luminous 

 jets issue, which are often variously coloured, and illuminate the 

 entire atmosphere. 



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 towards 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 surrounded 

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

 several luminous arcs. Then come jets and rays of light variously 

 coloured, which issue from the dark part of the segment, the 

 continuity of which is broken by bright emanations, indicating a 

 movement of the mass, which seems agitated by internal shocks, 

 during the formation of these luminous radiations, that 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 diminishes 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 

 colours more or less vivid. Finally the motion ceases, the light 

 approaches gradually to the horizon ; and 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. 

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