AURORA BOREALIS, or NORTHERN LIGHTS, (Ger. itiordlicht), the name given to the luminous phenomenon which is seen towards the n. of the heavens by the inhabi tants of the higher latitudes. During the winter of the northern hemisphere, the inhabitants of the arctic zone are without the light of the sun for months together, and their long dreary night is relieved by the light of this beautiful meteor, which occurs with great frequency in these regions. Those who have explored the southern seas have seen the same phenomenon in the direction of the south pole, so that the term polar lights might be more appropriate than northern lights to designate the aurora. The appearance of the A. B. has been described by a great variety of observers, both in horthern and central Europe, all of whom give substantially the same account of the manner in which the phenomenon takes place. It is briefly as follows: A dingy aspect of the sky in the direction of the n. is generally the precursor of the aurora; and this gradually becomes darker in color, and assumes the form of a circular segment surrounded by a luminous arch, and resting at each end on the horizon. This dark segment, as it is called, has the appearance of a thick cloud, and is frequently seen as such in the fading twilight before the development of the auroral light. Its density must, however, be very small, as stars are sometimes seen shining brightly through it. This dark segment is bounded by a luminous arch of a bluish-white color, which varies in breadth from 1 to 0 diameters of the moon, having the lower edge sharply defined, and the tippet edge only when the breadth of the arch is small. This arch may be considered to be a part of a luminous ring elevated at a considerable distance above the earth's surface, and having its center corresponding with some point near the north pole. An observer several degrees s. of this auroral ring would see towards the IL only a small arc of it, the larger part being hid by the earth; to one situated not so far s., it would appear as a larger and higher arch; to one placed below it, it would be seen as an arch passing through the zenith; and to one situated within the ring and further n., it would be found as an arch culminating in the s. On this supposition, nearly all the various positions of the auroral arch may be accounted for. The center of the ring
corresponds probably with the magnetic n., which.is at present situated in the island of Boothia Felix. Hence it is that in Greenland, which is situated to the e. of this island, the auroral arch has been seen stretching from n. to s. with its highest point in the w. The luminous arch, once formed, remains visible for several hours, and is in a constant state of motion. It rises and falls, extends toward the e. and towards the w., and breaks sometimes iu one part, sometimes in another. These motions become all the more observable when the arch is about to shoot forth rays; then it becomes luminous at one point, cats in upon the dark segment, and a ray of similar brightness to the arch mounts with the rapidity of lightning towards the zenith. The ray seldom keeps the same form for any length of time; but undergoes continual changes, moving eastward and westward, and fluttering like a ribbon agitated by the wind, After some time, it gradually fades in brightness, and at last gives way to other rays. When the aurora attains its full brightness and activity, rays are projected from every part of the arch, and if they do not rise too high, it presents the appearance of a comb furnished with teeth. When the rays are very bright, they sometimes assmne a green, sometimes a violet, a purple, or a rose color, giving to the whole a variegated and brilliant effect. The accompanying sketch. taken from Mt1flees Kosmische Phydlc, of the A. B. in Nor way, represents a beautiful aurora of this comb-shaped character; the effect of color. however, is wanting to complete the picture. When the rays darted by the luminous arch are numerous and of great length, they culminate in a point which is situated in the prolongation of the dipping needle, somewhat s.e. of the zenith. There they form what is called the boreal erovn; and the whole heavens, towards the e., w., and n., pre sent the appearanee.of a vast cupola of fire, supported by columns of variously colored light. When the rays are darted less brilliantly. the crown first. disappears, then, here and there, the light becomes faint and intermittent, till at last the whole phenomenon fades from the sky.