AURORA nontAus, an extraordinary lumi nous appearance or meteor, shcwing itself in the night-time, in northern latitudes, whence it has got its name of northern lights, or northern dawn. It is also known among the vulgar by the name of stream ers, 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, au oblong luminous tract, parallel to the horizon. Sagitta, the arrow, the same form wit1•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. Bothynoe, the cave, a luminous cloud, having the appearance of a recess or hollow in the heavens, surrounded by a corona. Pithier, the ton, 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 Muschenbrock, 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.direetly 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.degrces. 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 tlre.heavens extending in length from 5 to 100 degrees, and sometimes still far ther. It is generally .white and shilling, 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 hand 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 lees bulk at place where it issues from the cloud ; while it dilates itself and grows slimmer 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. lustit. Phys. c. 41.