The following ingenious theory has been proposed, with a view to resolve this difficulty. There is a very remarkable analogy between the phenomena of elec tricity and those of magnetism, and, apparently,.an•iii timate dependence of the one upon the other. There are two species of electricity, a positive and a ne gative, and two species of magnetic polarity, a north and a south. A body positively electrified repels another body positively electrified, and attracts one that is electrified negatively; while the north pole of one magnet repels the north pole, and attracts tht south pole of another. The electric shock will de prive a magnetic needle of its power, or communicate it to it again, according to the direction in which it is laid ; and, during a thunder storm, the magnetic needle is observed to be powerfully agitated. Thus the intimate connection between electricity and mag netism seems to be satisfactorily established. Again, that imaginary line, or circle, which traverses the earth irregularly from the north towards the, south, and is called the line of no variation, because the magnetic needle, when placed upon it, points truly to the poles, is observed to have a gradual, and pretty regular, revolution around the earth, performed in about 1000 years ; so that, when 1000 years have elapsed, the line of no variation will have reached the same situation which it occup'ed at the beginning of that period. This line seems to have a sort of eon troul over the coruscations of the aurora, which are observed to follow its direction in the heavens, and, as it were, to be attracted towards it, and regulated by its influence in their course.
But it appears, by calculation, that during the scientific age of Rome, the only period at which ac curate observations of the phenomena of the heavens could be made and recorded, the line of no variation run across the continent of Europe ; and consequent ly the aurora borealis, or stream of electrical matter that passes from the north pole towards the equator, would find a read) vent from the earth into the sky, through the mountains, and other pointed bodies which are so plentifully scattered over the land. By thus flowing constantly, and so greatly subdivided, it could not give rise to any striking atmospheric phenomena. At present, the direction of this line is through the Atlantic ocean ; and of consequence, the electricity of the polar regions is not imperceptibly transmitted to the atmosphere, but proceeds in such masses, and at such irregular intervals, as to exhibit itself in the beautiful phenomena of the northern lights. A thousand years ago the line of no varia tion, no doubt, occupied the same situation as.it does at present; and the aurora then shone forth in all its splendour; but, at that period, the nations of Eu were sunk in ignorance and barbarism; and tever phenomena the heavens presented were lost to posterity, from the rudeness and want of know ledge of the people of that age. Thus, if this theory be true, when the line of no variation shall again pass over the land, the aurora borealis will become invi sible for a time ; and when this line reverts to its pre sent situation over the ocean, the aurora will once more shine forth with its wonted lustre. (m)
A new theory of the aurora borealis has lately been proposed by M. Monge. He imagines that this phenomenon is merely clouds illuminated by the sun's light, which falls upon them after numerous reflec tions from other clouds placed at different distances in the heavens. If we suppose that clouds placed in the atmosphere are enlightened by the direct rays of the sun, and reflect the light which they receive to other clouds, situated in a part of the heavens de prived of the direct light of the son, and if we sup pose this light to be necessarily reflected to other clouds, we shall have some idea of the possibility of a mass of thin clouds being illuminated by the sun when this luminary is considerably depressed below the horizon of the spectator. The intensity of these radiations will depend on the dispersion and absorp tion of the light in its successive reflections, and it will be more distinctly.perceived when the rest of the atmosphere is involved in darkness. Upon this hy pothesis, Monge has explained why the phenomenon is perceived near the poles, and why it is most fre quently seen between the vernal and autumnal equi noxes. See Lecons de Physique, par Pujciulz, 1801. p. 237.
Another theory of this singular phenomenon has been recently proposed by M. Libes. It is founded on the following principles. 1. If the electric spark is passed through a mixture of azotic and oxygen gas, nitric acid, nitrous acid, or nitrous gas, will be produced, according to the proportion which exists between the azot and oxygen. 2. The nitric acid exposed to the sun, becomes more coloured and vola tile. If a receiver is placed over a vessel containing this acid, exposed to the rays of the sun, the acid will in a few minutes become coloured, and the re ceiver will be filled with red and volatile vapours, which continue for a long time, and exhibit a bril liancy resembling the aurora borealis. 3. In the flasks which contain the nitrous acid, there is always above the acid, a red and volatile vapour, which is never condensed. 4. Nitrous gas, in contact with atmospheric air, exhales red vapours, which fly away in the atmosphere. 5. The hydrogen gas which dis-• engages itself from the surface of the earth, rises to a height in the atmosphere corresponding with its i specific gravity. 6. The heat of the sun is extreme ly feeble in the solar regions. From these principles, M. Libes concludes, that there is very little hydrogen produced in the polar regions, and that therefore there is almost none of this substance in the higher regions'of the atmosphere. The electric fluid, there fore, passing through a mixture of azot and oxygen, will produce, nitric acid, nitrous acid, or nitrous gas, and these substances, acted upon by the solar rays, will exhibit those red and volatile vapours, which form the aurora borealis. ,A more complete account of thit ingenious theory may be found in the de Physique, par Libes, or in the Dictionizaire de Physique, of the same author ; and in Rozier's Jour nal, June 1790, Feb. 1791, and vol xxxviii. p. 191.