AURORA borealis, or AURORA septen trionalis, in physiology, the northern dawn or light, sometimes called streamers, is an extraordinary meteor, or luminous ap pearance, skewing itselfin the night-time in the northern part of the heavens ; and most usually in frosty weather. It is usual ly of a reddish colour, inclining to yellow, and sends out frequent corruscations of pale light, which seem to rise from the horizon in a pyramidal undulating form, and shoot with great velocity up to the zenith. The aurora borealis appears fre. quently in form of an arch, chiefly in the spring and autumn after a dry year. The arch is partly bright, partly dark, but ge nerally transparent ; and the matter of which it consists is also found to have no effect on the rays of light which pass through it. Dr. IIamilton observes, that he could plainly discern the smallest speck in the Pleiades, through the densi ty of those clouds which formed the au rora borealis in 1763, without the least diminution of its splendour, or increase of twinkling.
This kind of meteor, which is more un common as we approach towards the equator, is almost constant during the long winter, and appears with the great est lustre in the polar regions. In the Shetland isles, the "merry dancers," as the northern lights are there called, are the constant attendants of clear evenings, and afford great relief amidst the gloom ' of the long winter nights. They common ly appear at twilight near .the horizon, of a dun colour, approaching to yellow ; they sometimes continue in that state for several hours, without any perceptible motion ; and afterwards they break out into streams of stronger light, spreading into columns, and altering slowly into 10,000 different shapes, and varying their colours from all the tints of yellow to the most obscure russet. They often cover the whole hemisphere, and then exhibit the most brilliant appearance. Their motions at this time are most amazingly quick; and they astonish the spectator with the rapid change of their form. They break out in places where none were seen before, skimming briskly along the heavens, are suddenly extin guished, and are succeeded by an uniform dusky tract. This again is brilliantly il luminated in the same manner, and as suddently left a dark space. In some nights they assume the appearance of large columns, on one side of the'deep est yellow, and on the other gradually changing, till it becomes undistinguished from the sky. They have generally a strong tremulous motion from one end to the other, and this continues till the whole vanishes. As for us, who see only the extremities of these northern phe nomena, we can have but a faint idea of their splendour and motions. According to the state of the atmosphere, they dif fer in colour ; and sometimes assuming the colour of blood, they make a dread ful appearance. The rustic sages, who observe them, become prophetic, and terrify the spectators with alarms of war, pestilence, and famine : nor, indeed, were these superstitious presages peculiar to the northern islands : appearances of a similar nature are of ancient date ; and they Were distinguished by the appella tions of" phasmata," " trabes," and " bo lides," according to' their forms and co lours. In old times they were either more rare, or less frequently noticed : but when they occurred, they were suppos ed to portend great events, and the timid imagination formed of them aerial conflicts.
In the northern latitudes of Sweden and Lapland, the aurora borealis are not only singularly beautiful in their appear ance, but afford travellers, by their al most constant effulgence, a very beauti ful light during the whole night. In Hud son's bay, the aurora borealis diffuses a variegated splendour, which is said to equal that of the full moon. In the north eastern parts of Siberia, according to the description of Gmelin, these northern lights are observed to "begin with sin gle bright pillars rising in 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 velo city, and, finally, almost cover the whole sky up to the zenith, and produce an ap pearance as if a vast tent was expanded in the heavens, glittering with gold, ru bies, and sapphire. A more beautiful spectacle cannot be painted ; but who ever should see such a northern light for the first time, could not behold it without terror. For, however fine the illumina tion may be, it is attended, as I have learn ed from the relation of many persons, with such a hissing, cracking, and rush ingnoise through the air, as if the largest fire-works were playing off. To describe what they then hear, they make use of the expression spolochi chodjat,' that is, 'the raging host is passing.' The hun ters, who pursue the white and blue foxes in the confines of the Icy sea, are often overtaken in their course by these northern lights. Their dogs are then so much frig': iened, that they will not move, but lie obstinately on the ground, till the noise has passed. Commonly, clear and calm weather follows this kind of northern lights. I have heard thi s account, not from one person only, but confirmed by the uniform testimony of many, who have spent part of several years in these very northern regions, and inhabited different countries, from the Yenisei to the Lena ; so that no doubt of its truth can remain.
This seems indeed to be the real birth place of the aurora borealis." This account of the noises attending the aurora borealis, allowing for some de gree of exaggeration, has been corro borated by other testimonies. A person, who resided seven years at Hudson's Bay, confirms M. Gmelin's relation of the fine appearance and brilliant colours of the northern lights, and particularly of their rushing noise, which he affirms he has frequently heard, and compares it to the sound produced by whirling round a stick swiftly at the end of a string. A similar noise has also been heard in Swe den. Mr. Nairne, also, being in North ampton at a time when the northern lights were remarkably bright, is confi dent he perceived a hissing or whizzing sound. Mr Belknap, of Dover, in New Hampshire, North America, testifies to this fact. M. Cavallo says, that the crack ling noise is distinctly audible, and that he has heard it more than once. Similar lights, called aurora australes, have been long since observed towards the south pole, and their existence has been more lately ascertained by Mr. Forster, who assures us, that, in his voyage round the world with Captain Cooke, he observed them in high southern latitudes, though attended with phenomena somewhat dif ferent from those which are seen here. On February 17, 1773, in south latitude 58°, " a beautiful phenomenon (he bays) was observed during the preceding night, which appeared this and several following nights. It consisted of long co lumns of' a clear white light, shooting up from the horizon to the eastward, almost to the zenith, and gradually spreading on the whole southern part of the sky. These columns were sometimes bent sideways at their upper extremities ; and though in most respects similar to the our lights (aurora borealis) of our hemi sphere, yet differed from them in being always of a whitish colour, whereas ours assume various those of a fiery and purple hue. The sky was ge nerally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point." The periods of the appearance of these northern lights are very inconstant. In some years they occur very frequently, and in others they are more rare ; and it has been observed, that they are more common about the time of the equinoxes than at other seasons of the year. Dr. Halley( see Philos. Trans. No. 347, p. 406,) has collected together several observa tions, which form a kind of history of this phenomenon. After having particularly described the various circumstances which attended that observed by himself and many others, in March 1716, and which was singularly brilliant, he pro ceeds with informing us, that the first ac count of similar phenomena, recorded in the English annals, is that of the appear ance which was noticed January 30, 1560, and called " burning spears," by the au thor of a book entitled " A Description of Meteors," by W. F. D. D. reprinted at London, in 1654. The next appearance of a like kind, recorded by Stow, occurred on October, 7, 1564. In 1574, as Camden and Stow inform us, an aurora borealis was seen for two successive nights, viz. 14th and 15th of November, with appear ances similar to those observed in 1716, and which are now commonly noticed. The same phenomenon was twice seen in Brabant, in 1775, viz. on the 13th of February and the 28th of September ; and the circumstances attending it were described by Cornelius Gemma, who compares them to spears, fortified cities, and armies fighting in the air. In the year 1580, M. Mastline observed these phasmata, as he calls them, at Baknang, in the county of Wirtemberg, in Germa ny, no less than seven times in the space of twelve months ; and again at several different times, in 1581. On September 2d, 1621, the same phenomenon was seen over all France ; and it was particularly described by Gassendus, in his " Physics," who gave it the name of " aurora borea lis." Another was seen all over Germany in November, 1623, and was described by Kepler. Since that time, for more than eighty years, we have no account of any such phenomenon, either at home or abroad. In 1707, Mr. Neve observed one of small continuance in Ireland ; and in the same year, a similar appearance was seen by Romer, at Copenhagen; and dur ring all interval of eighteen months, in the years 1707 and 1708, this sort of light had been seen no less than five times. Hence it should seem, says Dr. Halley, that the air or earth, or both, are not at all times disposed to produce this pheno menon, though it is possible it may hap pen in the daytime, in bright moon shine, or in cloudy weather, and so pass unob served. Dr. Halley further observes, that the aurora borealis of 1716, which he de scribed, was visible from the west of Ire land to the confines of Russia, and to the east of Poland ; extending at least near 30° of longitude, and from about the 50th degree of north latitude, over almost all the North of Europe ; and in all places, at the same time, it exhibited appearances similar to those which he observed at London. He regrets, however, that he was unable to determine its height, for want of contemporary observations at dif ferent places.