The aurora is by no means confined to the northern hemisphere. In the high southern latitudes, it was long ago observed, that there is a similar phenome non. (See Phil. Trans. No. 461. and vol. liv. No. 53.) And, if the existence of the aurora australis before in some measure doubtful, it has been com pletely ascertained by the second voyage round the world performed by Captain Cooke. " On February 17. 1773," says Mr Forster, who accompanied Cooke in the capacity of naturalist, ",in south lat. 58°, a beautiful phenomenon was observed during the pre eeding night, which appeared again this and several following nights. It consisted of long columns of a clear white light, shooting up from the horizon to the eastward, almost to the zenith, and gradually spread ing over the whole southern part of the sky. These columns were sometimes bent. sideways at their upper extremities ; arid though in most respects similar to the northern lights of our hemisphere, yet differed from them in being always of a whitish colour ; where as ours assume various tints, especially those of a fiery and purple hue. The sky was generally clear when they appeared, and the air sharp and cold, the ther mometer standing at the freezing point." Various attempts have been made to determine the height of the aurora borealis, but with very little suc cess. Bergman, from a mean of thirty computations, makes the height of this phenomenon to be 72 Swe dish, or about 468 English miles. Father Boscovich calculated the height of an aurora borealis, observed on the 16th December 1737, by the Marquis of Po leni, to have been 825 miles : Mairan supposed the far greater number of aurora to be at least 600 miles above the surface of the earth ; and Euler assigned them an elevation of several thousands of miles. Dr Blagden, however, limits their height to about 100 miles, which he supposes to be the region of fire-balls; remarking that instances are upon record, in which the northern lights have been seen to join, and form luminous balls, darting about with great velocity, and even leaving a train behind them like common me teors.—Phil. Trans. vol. lxxiv. p. 227.
Respecting the cause of this beautiful phenomenon, a great variety of theories have been proposed. When the science of meteorology an imperfect state, it was natural to ascribe the aurora borealis to fiery or sulphureous vapours exhaled from the bowels of the earth, and rising into the region of the air ; and Muschenbroek is at pains to point out certain chemical mixtures which send forth a phosphorescent steam or vapour, in many respects resembling the northern lights. Dr Halley, also, at first proposed a similar theory : conceiving that the .watery vapours which are rarefied and sublimed by subterraneous fire, might carry along with them sulphureous vapours suffi cient to produce this luminous appearance in the atmosphere. He soon, however, abandoned this hypothesis, which is evidently very insufficient to account for the phenomena ; and supposed that the aurora borealis might be occasioned by the circu lation of the magnetic effluvia of the earth from one pole to the other. It %vas an hypothesis of this phi losopher, that the earth is a hollow sphere, inclosing within it another sphere, which has a strong magne tic virtue, to two poles which are nearly but not per fectly coincident with the poles of the world. The
inner sphere he supposed to have a slow revolution on its axis, independent of the diurnal rotation of the earth, by which he accounted for the variation of th e magnetic needle ; and he supposed that there is a con stant circulation of the magnetic fluid from the north to the south pole through the air ; which is counter balanced by a circulation from the south to the north pole, through the pores of the earth. The magnetic effluvia, darting upwards from the north pole into the higher regions of the atmosphere, acquire such an impetus as to render the circumambient ether lumi nous ; and give rise to all the phenomena of the au rora borealis. It has never, however, been shewn, that magnetic effluvia can in any case produce light ; and according to this theory, the aurora ought at the south pole to direct itself towards the earth ; where as Mr Forster found it moving rapidly towards the zenith, just as it does in the northern hemisphere.
The celebrated academician M. de Mairan, in 1731, published a treatise upon the aurora borealis, in which he ascribes this phenomenon to the impulse of the zodiacal light upon the atmosphere of the earth. The zodiacal light is a luminous train, which is visible at certain seasons, a little before sun-rise, or after sun set, in the shape of a pyramid or lens, stretching along the zodiac. It was first discovered by Cassini, who conceived it to be the atmosphere of the sun, formed by a very rare fluid luminous in itself, or il luminated by the sun's rays, but not equally through out ; being much more luminous, and more extended around the equator of the sun ; in which direction it forms a very oblate spheroid, or rather lens, of which the transverse section coincides with the plane of the sun's equator. See ASTRONOMY Index".
" It is proved by observation," says M. de Mairan, " that this solar atmosphere extends sometimes as far as the earth's orbit, and even farther. When, there fore, it is at its greatest extent, the earth will be im mersed in it ; in which case a quantity of the luminous matter, influenced by gravitation, falls upon the earth's atmosphere, and descends more or less within it ac cording to its weight ; each luminous particle descend ing till it meet with a stratum of air, with which it will he in equilibrium. But as the equatorial regions have a greater centrifugal' force than the polar, on account of their greater velocity during the earth's diurnal rotation, the luminous particles of the zodia cal light must be driven by this centrifugal force, from the equator towards the poles ; and it is then that they form those luminous arches which we call the aurora borealis. (See Tract. Phys. and Hist. del 'furore Bor.) Besides this express treatise, there are several papers on the subject of the aurora borea lis, by M. de Mairan, in the Memoirs of the French Academy ; one in particular for the year 1733, in which he records a variety of his observations on the zodiacal light, made with a view to confirm his theory ; and another for the year 174.7, in which he defends his theory against the attacks of Euler, who wrote a treatise in order to refute it, and establish a new doc trine of his own.