AURORA BO RE AX IS. 



112 



AURORA BoREALls, an extraordinary lumi- 

 nous appearance or meteor, shewing itself in the 

 night-time, in northern latitudes, whence it has got 

 ,its name of northern limits, or northern dann. It is 

 also known among the vulgar by the name of Stream- 

 . or merry dancers. 



The aurora borealis may with propriety be distin- 

 guished into two kinds, the tranquil, and the varying. 

 The tranquil shines with a mild and steady light, very 

 much resembling the clearness of twilight ; and pre- 

 serves, for a considerable time, the form in which it 

 first appears, with little or no variation. Different 

 names have been given by the ancient philosophers to 

 this kind of aurora, according to the forms which it 

 assumes. They are thus enumerated by Muschen- 

 broek : Irabs, or the beam, an oblong luminous tract, 

 parallel to the horizon. Sagitla, the arroiv, the same 

 torm with the beam, but terminating in a cusp. Faces, 

 the torch, which has various positions in the heavens, 

 but always one extremity larger than the other. Ca- 

 pra saltans, the dancing goat, a luminous appearance 

 agitated by the wind, so as successively to appear 

 and disappear. Bothy not; the cave, aluminous cloud, 

 having the appearance of a recess or hollow in the 

 heavens, surrounded by a corona. Pithice, the tun, 

 an aurora resembling a large luminous cask. These 

 names, it is easy to perceive, are somewhat fanciful, 

 and do not serve greatly to illustrate the nature of 

 this singular meteor. 



The varying aurora is still more remarkable in its 

 appearance, and occasionally exhibits the most bril- 

 liant and rapidly diversified forms. It has been mi- 

 nutely described by Muschenbroek, who paid great 

 attention to its peculiarities ; and from whose de- 

 scription we select the following particulars. In that 

 region of the air which is directly towards the north, 

 or which stretches from the north towards the east 

 or west, there appears at first a cloud in the horizon, 

 which rarely rises to the height of 40 degrees. This 

 cloud is sometimes contiguous to the horizon, some- 

 times detached from it ; in which last case the inter- 

 mediate sky appears of a bright blue colour. The 

 cloud occupies a portion of the heavens extending in 

 length from 5 to 100 degrees, and sometimes still far- 

 ther. It is generally -white and shining, but some- 

 times black and thick. Its upper edge is parallel to 

 the horizon, bordered by a long train of light which 

 rises higher in some places than in others. It appears 

 also bent in the form of a bow, or like the segment 

 of a sphere which has its centre considerably beneath 

 the horizon ; and sometimes a large white or luminous 

 band is visible skirting the superior edge of the black 

 cloud. The dark part of the cloud becomes white 

 and luminous when the aurora has shone for some 

 time, and after it has sent forth several bright and 

 fiery rays. Then, from the superior edge of the 

 cloud, there issue rays in the form of jets, which are 

 sometimes many, sometimes few in number, some- 

 times close together, sometimes removed several de- 

 grees asunder. These jets diffuse a very brilliant 

 light, as if a luminous or fiery liquor were driven with 

 impetuosity from a syringe. The jet increases in 

 brightness, and has lets bulk at the place where it 

 issues from the cloud ; while it dilates itself and grows 

 dimmer as it goes farther and farther off. Then there 



arises from a large opening in the cloud, a luminous 

 train or column, of which the motion is at first gen- 

 tle and uniform, and which increases in size as it ad- 

 vances. The dimensions and duration of these co- 

 lumns, however, vary considerably. Their light is 

 sometimes white, sometimes reddish, or even blood 

 colour; and, as they advance, their colours change, 

 till they form a kind of arch in the heavens. When 

 several of these columns, which have issued from dif- 

 ferent places, encounter each other in the zenith, they 

 intermingle with each other, and form at their junc- 

 tion a small thick cloud, which seems as it were to 

 kindle, and sends forth a light considerably more 

 brilliant than that of any of the separate columns. 

 This light changes to green, blue, and purple ; and 

 quitting its original situation, it directs itself towards 

 the south, under the form of a small bright cloud. 

 When no more columns are seen to issue, the cloud 

 assumes the appearance of the morning dawn, and in- 

 sensibly dissipates itself. Musch. Instit. Phys. c. 41 . 

 The duration of the aurora is very various. Some- 

 times it is formed and disappears in the course of a 

 few minutes. At other times, it lasts during the whole 

 night, or even for two or three days together ; and 

 Muschenbroek observed one in 1734, that lasted teu 

 days and nights successively ; and another in 1735, 

 that lasted from the 22d to the 31st of March. The 

 lucid columns are so transparent, that stars of the first 

 and second magnitude are easily seen through them ; 

 these also frequently shine through the white border 

 of the horizontal cloud, and sometimes, though rare- 

 ly, through the opaque cloud itself. But many parts 

 of the luminous substance are so thin, that the smal- 

 lest stars which are visible to the naked eye may be 

 distinguished through them. 



In high northern latitudes, as those of Sweden, 

 Lapland, and Siberia, the auroras boreales are singu- 

 larly resplendent, and even terrific. They frequently 

 occupy the whole of the heavens ; and, according to 

 the testimony of Maupertuis, Middleton, Krafft, and 

 others, eclipse the splendour of the stars, planets, and 

 moon, and sometimes even of the sun himself. In the 

 north-eastern districts of Siberia, according to the de- 

 scription of Gmelin, cited and translated by Dr Blag- 

 den, [Phil. Trans, vol. Ixxiv. p. 228.) the aurora is 

 observed to " begin with single bright pillars, rising iu 

 the north, and almost at the same time in the north- 

 east, which, gradually increasing, comprehend a large 

 space of the heavens, rush about from place to place 

 with incredible velocity, and finally almost cover the 

 whole 6ky up to the It nith, and produce an appear- 

 ance as it a vast tent w.is expanded in the heavens, 

 glittering with j>old, rubies, and sapphire. A more 

 beautiful spectacle cannot be painted ; but whoever 

 should see such a northern light tor the first lime, 

 could not behold it without terror. For, however 

 fine the illumination may be, it is attended, as I Lave 

 learned from the relation of many persons, with such 

 a hissing, cracking, and rushing noise through the air, 

 as if the largest fireworks wi re playing oft. To de- 

 scribe what they then hear, they make use of the ex- 

 pression spolochi chodjat, that is, the raging host 

 is passing. The hunters who pursue the white and 

 blue foxes in the confines of the Icy sea, are often 

 overtaken in their course by these northern lights.. 



4 



Aurora 



Borealis. 



