What is the origin of these remarkable phenomena? The ancients asked the question, and the moderns reply by repeating it. Before proceeding to describe the magnificent auroral displays of August 28th and September 2d, let us examine authorities upon this subject, and see if we cannot arrive at some satisfactory solution of the phenomena. The following is the description given by Humboldt in "Cosmos":—
"An aurora borealis is always preceded by the formation in the horizon of a sort of nebulous veil, which slowly ascends to a height of 4°, 6°, 8°, and even to 10°. It is towards the magnetic meridian of the place that the sky, at first pure, begins to get brownish. Through this obscure segment, the color of which passes from brown to violet, the stars are seen, as through a thick fog. A wider arc, but one of brilliant light, at first white, then yellow, bounds the dark segment. Sometimes the luminous arc appears agitated, for hours together, by a sort of effervescence, and by a continuous change of form, before the rising of the rays and columns of light, which ascend as far as the zenith. The more intense the emission of the polar light, the more vivid are its colors, which, from violet and bluish white, pass through all the intermediate shades of green and purple-red. Sometimes the columns of light appear to come out of the brilliant arc mingled with blackish rays, resembling a thick smoke; sometimes they rise simultaneously from different points of the horizon, and unite themselves into a sea of flames, the magnificence of which no painting could express; for, at each instant, rapid undulations cause their form and brilliancy to vary. Motion appears to increase the visibility of the phenomena. Around the point in the heaven which corresponds to the direction of the dipping needle produced, the rays appear to meet and form the boreal corona. It is seldom that the appearance is so complete, and is prolonged to the formation of the corona; but when the latter appears, it always announces the end of the phenomenon. The rays then become more rare, shorter, and less vividly colored. Soon nothing further is seen on the celestial vault than wide, motionless, nebulous spots, pale, or of an ashy color; they have already disappeared, when the traces of the dark segment whence the appearance originated still remain on the horizon."
The connection that seems to exist, says De la Rive, between the polar light and the appearance of a certain species of clouds is confirmed by all observers; all have affirmed that the polar light emitted its most brilliant rays when the high regions of the air contained heaps of cirri,—strata of sufficient tenuity and lightness to cause a corona to arise around the light. Sometimes these clouds are grouped and arranged almost like the rays of an aurora borealis; they then appear to disturb the magnetized needle. Father Secchi has remarked, that magnetic disturbances are manifested at Rome whilst the sky is veiled with clouds that are slightly phosphorescent, which, at night, present the appearance of feeble aurorae boreales.
After a brilliant aurora borealis, we have been able to recognize, on the following morning, trains of clouds, which, during the night, had appeared as so many luminous rays.
The absolute height of aurorae boreales has been very variously estimated by different observers. It has long been thought that we might determine it by regarding, from two places widely distant from each other, the same part of the aurora,—the corona, for example. But we have started from a very inaccurate assumption, namely, that the two observers had their eyes directed to the same point at the same time,—whilst it is now well proved that the corona is an effect of perspective, due to the apparent convergence of the parallel rays situated in the magnetic meridian; so that each observer sees his own aurora borealis, as each sees his own rainbow. The aspect of the phenomenon depends also upon the positions of the observers. The seat of the aurora borealis is in the upper regions of the atmosphere; though sometimes it appears to be produced in the less elevated regions where the clouds are formed. This, at least, is what follows from some observations, especially from those of Captain Franklin, who saw an aurora borealis the light of which appeared to him to illuminate the lower surface of a stratum of clouds; whilst some twenty-five miles farther on, Mr. Kendal, who had watched the whole of the night without losing sight of the sky for a single moment, did not perceive any trace of light. Captain Parry saw an aurora borealis display itself against the side of a mountain; and we are assured that a luminous ring has sometimes been perceived upon the very surface of the sea, around the magnetic pole. Lieutenant Hood and Dr. Richardson, being placed at the distance of about forty-five miles from each other, in order to make simultaneous observations, whence they might deduce the parallax of the phenomenon, and consequently its height, were led to the conclusion that the aurora borealis had not a greater elevation than five miles. M. Liais, having had the opportunity of applying a method, which he had devised for measuring the height of aurorae boreales, to an aurora seen at Cherbourg Oct. 31, 1853, found that the arc of the aurora was about two and a half miles above the ground, at its lower edge.
Various observations made by Professor Olmsted, in conjunction with Professor Twining, of New Haven, led him, on the contrary, to fix the elevation on different occasions at forty-two, one hundred, and one hundred and sixty miles. He claims that it is rarely less than seventy miles from the earth, and never more than one hundred and sixty. He also claims that its origin is cosmical,—or, in other words, that the earth, in revolving in its orbit, at certain periods passes through a nebulous body, which evolves this strange light in more or less brilliancy, as the body is larger or smaller. To support this theory, he attempted to establish that there were fixed epochs for its display in the highest degree of brilliancy. The length of these periods was from sixty to seventy years, and the next appearance was to be in 1890. The remarkable displays of August 28th and September 2d show the fallacy of his conclusions in this respect.
Mairon and Dalton had also thought that the aurora borealis was a cosmical, and not an atmospheric phenomenon. But M. Biot, who had himself had an opportunity of observing the aurora in the Shetland Isles in 1817, had already been led to recognize it as an atmospheric phenomenon, by the consideration that the arcs and the coronae of the aurora in no way participate in the apparent motion of the stars from east to west,—a proof that they are drawn along by the rotation of the earth. Hence, almost all observers have arrived at the same conclusions; we will in particular cite MM. Lottin and Bravais, who have observed more than a hundred and forty aurorae boreales. It is therefore now clearly proved that the aurora borealis is not an extra-atmospheric phenomenon. To the proofs drawn from the appearance of the phenomenon itself we may add others deduced from certain effects which accompany it, such as the noise of crepitation, which the dwellers nearest to the pole affirm that they have heard when there is the appearance of an aurora, and the sulphurous odor that accompanies it. Finally, if the phenomena took place beyond our planet and its atmosphere, why should they take place at the polar regions only, as they often do?
J. S. Winn, in a letter to Dr. Franklin, dated Spithead, August 12th, 1772, says: "The observation is new, I believe, that the aurora borealis is constantly succeeded by hard southerly or southwest winds, attended with hazy weather and small rain. I think I am warranted from experience in saying constantly, for in twenty-three instances that have occurred since I first made the observation it has invariably obtained; and the knowledge has been of vast service to me, as I have got out of the Channel when other men as alert, and in faster ships, but unapprised of this circumstance, have not only been driven back, but with difficulty escaped shipwreck."
Colonel James Capper, the discoverer of the circular nature of storms, remarks: "As it appears, that, on all such occasions, the current of air comes in a direction diametrically opposite to that where the meteor appears, it seems probable that the aurora borealis is caused by the ascent of a considerable quantity of electric fluid in the superior regions of the atmosphere to the north and northeast, where, consequently, it causes a body of air near the earth to ascend, when another current of air will rush from the the opposite point to fill up the vacuum, and thus may produce the southerly gales which succeed the aurora borealis."
The bark "Northern Light," arrived at Boston from Africa, was at sea on the night of the great exhibition of the aurora borealis, the 28th of August. The vessel was struck by lightning twice, after which the red flames of the aurora burst upon the astonished vision of the crew. Most of them are confident that they smelt a sulphurous odor all night.