AURORA

, the morning twilight; or that faint light which appears in the morning when the sun is within 18 degrees of the horizon.

AURORA BOREALIS, Northern Light, or Streamers; a kind of meteor appearing in the northern part of the heavens, mostly in the winter season, and in frosty weather. It is usually of a reddish colour, inclining to yellow, and sends out frequent coruscations of pale light, which seem to rise from the horizon in a pyramidal undulating form, and shooting with great velocity up to the zenith. It appears often in form of an arch, | which is partly bright, and partly dark, but generally transparent. And the matter of it is not found to have any effect on the rays of light, which pass freely through it. Dr. Hamilton observes, that he could plainly discern the smallest speck in the Pleiades through the density of those clouds which formed part of the Aurora borealis in 1763, without the least diminution of its splendour, or increase of twinkling. Philos. Essays, pa. 106.

Sometimes it produces an Iris. Hence M. Godin judges, that most of the extraordinary meteors and phenomena in the skies, related as prodigies by historians, as battles, and the like, may probably euough be reduced to the class of Auroræ boreales. Hist. Acad. R. Scienc. for 1762, pa. 405.

This kind of meteor never appears near the equator; but, it seems, is frequent enough towards the south pole, like as towards the north, having been observed there by voyagers. See Philos. Trans. N° 461, and vol. 54; also Forster's account of his voyage round the world with Captain Cook, where he describes their appearance as observed for several nights together, in sharp frosty weather, which was much the same as those observed in the north, excepting that they were of a lighter colour.

It seems that meteors of this kind have appeared sometimes more frequently than others. They were so rare in England, or else so little regarded, that none are recorded in our annals since that remarkable one of Nov. 14, 1574, till the surprising Aurora borealis of March 6, 1716, which appeared for three nights successively, but by far more strongly on the first: except that five small ones were observed in the year 1707 and 1708. Hence it would seem, that the air, or earth, or both, are not at all times disposed to produce this phenomenon.

The extent of these appearances is also amazingly great. That in March 1716 was visible from the west of Ireland, to the confines of Russia and the east of Poland; extending at least near 30 degrees of longitude, and from about the 50th degree in latitude, over almost all the north of Europe; and in all places, at the same time, it exhibited the like wondrous appearances. Father Boscovich has determined the height of an aurora borealis, which was observed by the Marquis of Polini the 16th of December, 1737, and found it was 825 miles high; and Mr. Bergman, from a mean of 30 computations, makes the average height of the aurora borealis amount to 70 Swedish, or 469 English miles. But Euler supposes the height to be several thousands of miles; and Mairan also assigns to them a very elevated region.

Many attempts have been made to determine the cause of this phenomenon. Dr. Halley imagines that the watery vapours, or effluvia, exceedingly rarefied by subterraneous fire, and tinged with sulphureous streams, which many naturalists have supposed to be the cause of earthquakes, may also be the cause of this appearance: or that it is produced by a kind of subtile matter, freely pervading the pores of the earth, and which, entering into it nearer the southern pole, passes out again with some force into the æther, at the same distance from the northern. This subtile matter, by becoming more dense, or having its velocity increased, may perhaps be capable of producing a small degree of light, after the manner of effluvia from electric bodies, which, by a strong and quick friction, emit light in the dark; to which sort of light this seems to have a great affinity. Philos. Trans. N° 347. See also Mr. Cotes's description of this phenomenon, and his method of explaining it, by streams emitted from the heterogeneous and fermenting vapours of the atmosphere, in Smith's Optics, pa. 69; or Philos. Trans. abr. vol. 6, part 2.

The celebrated M. de Mairan, in an express treatise on the Aurora Borealis, published in 1731, supposes its cause to be the zodiacal light, which, according to him, is no other than the sun's atmosphere: this light happening, on some occasions, to meet the upper parts or our atmosphere about the limits where universal gravity begins to act more forcibly towards the earth than towards the sun, falls into our air to a greater or less depth, as its specific gravity is greater or less, compared with the air through which it passes. See Tract. Phys. et Hist. de l'Aurore Boreale. Suite des Memoires de l'Acad. R. des Scien. 1731. Also Philos. Trans. N° 433, or Abridg. vol. 8, pa. 540.

However M. Euler thinks the cause of the aurora borealis not owing to the zodiacal light, as M. de Mairan supposes; but to particles of our atmosphere, driven beyond its limits by the impulse of the solar light. And on this supposition he endeavours to account for the phenomena observed concerning this light. He supposes the zodiacal light, and the tails of comets, to be owing to a similar cause.

But ever since the identity of lightning and the electric matter has been determined, philosophers have been naturally led to seek for the explication of aerial meteors in the principles of electricity; and there is now no doubt but most of them, and especially the aurora borealis, are electrical phenomena. Besides the more obvious and known appearances which constitute a resemblance between this meteor and the electric matter by which lightning is produced, it has been observed that the aurora occasions a very sensible fluctuation in the magnetic needle; and that when it has extended lower than usual in the atmosphere, the flashes have been attended with various sounds of rumbling and hissing, especially in Russia and the other more northern parts of Europe; as noticed by Sig. Beccaria and M. Messier. Mr. Canton, soon after he had obtained electricity from the clouds, offered a conjecture, that the aurora is occafioned by the dashing of electric fire positive towards negative clouds at a great distance, through the upper part of the atmosphere, where the resistance is least: and he supposes that the aurora which happens at the time when the magnetic needle is disturbed by the heat of the earth, is the electricity of the heated air above it. and this appears chiefly in the northern regions, as the alteration in the heat of the air in those parts is the greatest. Nor is this hypothesis improbable, when it is considered, that electricity is the cause of thunder and lightning; that it has been extracted from the air at the time of the aurora borealis; that the inhabitants of the northern countries observe it remarkably strong when a sudden thaw succeeds very cold severe weather; and that the tourmalin is known to emit and absorb the electric fluid only by the increase or diminution of its | heat. Positive and negative electricity in the air, with a p<*>oper quantity of moisture to serve as a conductor, will account for this and other meteors, sometimes seen in a serene sky. Mr. Canton has since contrived to exhibit this meteor by means of the Torricellian vacuum, in a glass tube about 3 feet long, and sealed hermetically. When one end of the tube is held in the hand, and the other applied to the conductor, the whole tube will be illuminated from end to end, and will continue luminous without interruption for a considerable time after it has been removed from the conductor. If, after this, it be drawn through the hand either way, the light will be remarkably intense through the whole length of the tube. And though a great part of the electricity be discharged by this operation, it will still flash at intervals, when held only at one extremity, and kept quite still; but if, at the same time, it be grasped by the other hand in a different place, strong flashes of light will dart from one end to the other; and these will continue 24 hours or more, without a fresh excitation. Sig. Beccaria conjectures that there is a constant and regular circulation of the electric fluid from north to south; and he thinks that the aurora borealis may be this electric matter performing its circulation in such a state of the atmosphere as renders it visible, or approaching nearer than usual to the earth. Though probably this is not the mode of its operation, as the meteor is observed in the southern hemisphere, with the same appearances as in the northern. Dr. Franklin supposes, that the electric fire discharged into the polar regions, from many leagues of vaporised air raised from the ocean between the tropics, accounts for the aurora borealis; and that it appears first, where it is first in motion, namely in the most northern part; and the appearance proceeds southward, though the fire really moves northward. Franklin's Exper. and Obs. 1769, pa. 49. Philos. Trans. vol. 48, pa. 358, 784; Ib. vol. 51, pa. 403; Lettere dell' Ellettricismo, pa. 269; or Priestley's Hist. of Electricity. See also an ingenious solution of this phenomenon, on the same principles, by Dr. Hamilton, in his Philos. Essays. Mr. Kirwan (in the Transactions of the Royal Irish Academy, ann. 1788) has some ingenious remarks on the auroræ borealis & australis. He gives his reasons for supposing the rarefaction of the atmosphere in the polar regions to proceed from them, and these from a combustion of inflammable air caused by electricity. He observes, that after an aurora borealis the barometer commonly falls, and high winds from the south generally follow.