The aurora borealis, or rather, the polar aurora, — for there are auroræ australes as well as auroræ boreales, — has been an object of wonder and admiration from time immemorial.
Pliny and Aristotle record phenomena identical with those which later times have witnessed. The ancients ranked this with other celestial phenomena, as portending great events.
In a Bible imprinted at London in the year 1699, the 22d verse of the 37th chapter of Job reads thus: “The brightness commeth out of the Northe, the praise to God which is terrible.” The writer of the Book of Job was very conversant with natural objects, and may have referred to the aurora borealis and the phenomena immediately connected therewith.
In 1560, we are told, it was seen at London in the shape of burning spears, a similitude which would be no less appropriate now than then. Frequent displays are recorded during the fifteen years following that date. During the latter half of the seventeenth century, the phenomena were frequently visible, oftentimes being characterized by remarkable brilliancy. After 1746, the displays suddenly diminished, and were but rarely seen for the next nine years. The present century has been favored to a remarkable degree. The displays during the years 1835, ’36, ’37, ’46, ’48, ’51, ’52, and ’59, have been especially grand.
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 auroræ 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 auroræ 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 auroræ 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 coronæ 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 auroræ 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 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.
M. de Tessan, who, in the voyage of the “Venus” around the world, had the opportunity of seeing a very beautiful aurora australis, (southern aurora,) which he describes with much care, also considers that this phenomenon takes place in the atmosphere. The summit of the aurora being in the magnetic meridian, it was elevated 14° above the horizon, and the centre of the arc was on the prolongation of the dipping needle, the dip being about 68° at the place of the observation. M. de Tessan did not hear the noise arising from the aurora, which he attributes to the circumstance that he was too far distant from the place of the phenomenon; but he reports the observation of a distinguished officer of the French navy, M. Verdier, who, on the night of October 13th, 1819, being in the latitude of Newfoundland, had heard very distinctly a sort of crackling or crepitation, when the vessel he was on board was in the midst of an aurora borealis. This was also observed in many localities during the aurora of August 28th, 1859. A New York paper, alluding to the subject, remarks: “Many imagined that they heard rushing sounds, as if Æolus had let loose the winds; others were confident that a sweeping, as if of flames, was distinctly audible.” Burns, a good observer, if ever there was one, and not likely to be aware of any theories on the subject, alludes in his “Vision” to a noise accompanying the aurora, as if it were of ordinary occurrence: —
The cauld blue North was flashing forth
Her lights wi’ hissing eerie din.
It finds confirmation also in the fact, generally admitted by the inhabitants of the northern regions, that, when the auroræ appear low, a crackling is heard similar to that of the electric spark. The Greenlanders think that the souls of the dead are then striking against each other in the air. M. Ramm, Inspector of Forests in Norway, wrote to M. Hansteen, in 1825, that he had heard the noise, which always coincided with the appearance of the luminous jets, when, being only ten years old, he was crossing a meadow covered with snow and hoar-frost, near which no forests were in existence. Dr. Gisler, who for a long time dwelt in the North of Sweden, remarks that the matter of the auroræ bo-reales sometimes descends so low that it touches the ground; at the summit of high mountains it produces upon the faces of travellers an effect analogous to that of the wind. Dr. Gisler adds, that he has frequently heard the noise of the aurora, and that it resembles that of a strong wind, or the hissing that certain chemical substances produce in the act of decomposition.
M. Necker, who has described a great number of auroræ which he observed at the end of 1839 and at the commencement of 1840, in the Isle of Skye, never himself heard the noise in question; but he remarks that this noise had been very frequently heard by persons charged with meteorological observations at the light-house of Swenburgh Head, at the southern extremity of Shetland. M. Necker is not the only observer who has not heard the noise; neither have MM. Lottier and Bravais, who have observed so great a number of auroræ ever heard it; and a great many others are in this case. This may be due to the fact that it is necessary to be very near to the aurora in order to hear the crepitation in question, and also to the fact that it is possible that it does not always take place, at least in a manner sufficiently powerful to be beard.
We have just been pointing out, as concomitant effects of the aurora borealis, a noise of crepitation analogous to that of distant discharges, and a sulphurous odor similar to that which accompanies the fall of lightning. M. Matteucci also observed at Pisa, during the appearance of a brilliant aurora borealis, decided signs of positive electricity in the air; but of all phenomena, those which invariably take place at the same time as the appearance of the aurora borealis are the magnetic effects. Magnetized needles suffer disturbances in their normal direction which cause them to deviate generally to the west first, afterwards to the east. These disturbances vary in intensity, but they never fail to take place, and are manifested even in places in which the aurora borealis is not visible. This coincidence, proved by M. Arago without any exception, during several years of observation, is such that the learned Frenchman was able, without ever having been mistaken, to detect from the bottom of the cellars of the observatory of Paris the appearance of an aurora borealis. M. Matteucci had the opportunity of observing this magnetic influence under a new and remarkable form. He saw, during the appearance of the aurora borealis of November 17, 1848, the soft iron armatures employed in the electric telegraph between Florence and Pisa remain attached to their electro-magnets, as if the latter were powerfully magnetized, without, however, the apparatus being in action, and without the currents in the battery being set in action. This singular effect ceases with the aurora, and the telegraph, as well as the batteries, could operate anew, without having suffered any alteration. Mr. Highton also observed in England a very decided action of the aurora borealis, November 17, 1848. The magnetized needle was always driven toward the same side, even with much force. But it is in our own country that the action of the aurora upon the telegraph-wires has been the most remarkable.
My attention was first called in 1847 to the probability of the aurora’s producing an effect upon the wires; but, although having an excellent opportunity to observe such an effect, I was not fortunate enough to do so until the winter of 1850, and then, owing to the feeble displays of the aurora, only to a limited extent. In September, 1851, however, there was a remarkable aurora, which took complete possession of all the telegraph-lines in New England and prevented any business from being transacted during its continuance. The following winter there was another remarkable display, which occurred on the 19th of February, 1852. It was exceedingly brilliant throughout the northern portion of our continent. I extract the following account of its effects upon the wires from my journal of that date. I should premise, that the system of telegraphing used upon the wires, during the observation of February, 1852, was Bain’s chemical. No batteries were kept constantly upon the line, as in the Morse and other magnetic systems. The main wire was connected directly with the chemically prepared paper on the disc, so that any atmospheric currents were recorded upon the disc with the greatest accuracy. Our usual battery current, decomposing the salts in the paper, and uniting with the iron point of the pen wire, left a light blue mark on the white paper, or, if the current were strong, a dark one, — the color of the mark depending upon the quantity of the current upon the wire.
Thursday, February 19, 1852.
Towards evening a heavy blue line appeared upon the paper, which gradually increased in size for the space of half a minute, when a flame of fire succeeded to the blue line, of sufficient intensity to burn through a dozen thicknesses of the moistened paper. The current then subsided as gradually as it had come on, until it entirely ceased, and was then succeeded by a negative current (which bleaches, instead of coloring, the paper). This gradually increased, in the same manner as the positive current, until it also, in turn, produced its flame of fire, and burned through many thicknesses of the prepared paper; it then subsided, again to be followed by the positive current. This state of things continued during the entire evening, and effectually prevented any business being done over the wires.
Never, however, since the establishment of the telegraphic system in this country, have the wires been so greatly affected by the aurora as upon Sunday night, the 28th of August, 1859. Throughout the entire northern portion of the United States and Canada, the lines were rendered useless for all business purposes through its action. So strongly was the atmosphere charged with the electric fluid, that lines or circuits of only twelve miles in length were so seriously affected by it as to render operation difficult, and, at times, impossible.
The effects of this magnetic storm were apparent upon the wires during a considerable portion of Saturday evening, and during the whole of the next day. At 6 P.M., the line between Boston and New Bedford (sixty miles in length) could be worked only at intervals, although, of course, no signs of the aurora were apparent to the eye at that hour. The same was true of the wires running eastward through the State of Maine, as well as those to the north.
The wire between Boston and Fall River had no battery upon it Sunday, and yet there was an artificial current upon it, which increased and decreased in intensity, producing upon the electro-magnets in the offices the same effect as would be produced by constantly opening and closing the circuit at intervals of half a minute. This current, which came from the aurora, was strong enough to have worked the line, although not sufficiently steady for regular use.
The current from the aurora borealis comes in waves, — light at first, then stronger, until we have, frequently, a strength of current equal to that produced by a battery of two hundred Grove cups. The waves occupy about fifteen seconds each, ordinarily, but I have known them to last a full minute; though this is rare. As soon as one wave passes, another, of the reverse polarity, always succeeds. I have never known this to fail, and it may be set down as an invariable rule. When the poles of the aurora are in unison with the poles of the current upon the line, its effect is to increase the current; but when they are opposed, the current from the battery is neutralized, — null. These effects were observed at times during Saturday, Saturday evening, and Sunday, but were very marked during Sunday evening.
It is hardly necessary to add here, that the effect of the aurora borealis, or magnetic storm, is totally unlike that of common or free electricity, with which the atmosphere is charged during a thunder-storm. The electricity evolved during a thunder-storm, as soon as it reaches a conductor, explodes with a spark, and becomes at once dissipated. The other, on the contrary, is of very low tension, remains upon the wires sometimes half a minute, produces magnetism, decomposes chemicals, deflects the needle, and is capable of being used for telegraphic purposes, although, of course, imperfectly.
Mr. O. S. Wood, Superintendent of the Canadian telegraph-lines, says: — “I never, in my experience of fifteen years in the working of telegraph-lines, witnessed anything like the extraordinary effect of the aurora borealis, between Quebec and Father Point, last night. The line was in most perfect order, and well-skilled operators worked incessantly from eight o’clock last evening till one o’clock this morning, to get over, in even a tolerably intelligible form, about four hundred words of the steamer ‘Indian’s’ report for the press; but at the latter hour, so completely were the wires under the influence of the aurora borealis, that it was found utterly impossible to communicate between the telegraph-stations, and the line was closed for the night.”
We have seen from the foregoing examples that the aurora borealis produces remarkable effects upon the telegraph-lines during its entire manifestation. We have, however, to record yet more wonderful effects of the aurora upon the wires, namely, the use of the auroral current for transmitting and receiving telegraphic dispatches. This almost incredible feat was accomplished in the forenoon of September 2, between the hours of half past eight and eleven o’clock, on the wires of the American Telegraph Company between Boston and Portland, and upon the wires of the Old Colony and Fall River Railroad Company between South Braintree and Fall River.
The auroral influence was observed upon all the lines running out of the office in Boston, at the hour of commencing business, (eight o’clock, A. M.,) and it continued so strong up to half past eight as to prevent any business being done the ordinary current upon the wires being at times neutralized by the magnetism of the aurora, and at other times so greatly augmented as to render operations impracticable; At this juncture it was suggested that the batteries should be cut off, and the wires simply connected with the earth.
It is proper to remark here, that the current from the aurora coming in waves of greater or less intensity, there are times, both while the wave is approaching and while it is receding, when the instruments are enabled to work; but the time, varying according to the rapidity of the vibrations of the auroral bands, is only from one quarter of a minute to one minute in duration. Therefore, whatever business is done upon the wires during these displays has to be accomplished in brief intervals of from quarter to half a minute in duration.
During one of these intervals, the Boston operator said to the one at Portland, —
“Please cut off your battery, and let us see if we cannot work with the auroral current alone.”
The Portland operator replied, —
“I will do so. Will you do the same?”
“I have already done so,” was the answer. “We are working with the aid of the aurora alone. How do you receive my writing?”
“Very well indeed,” responds the operator at Portland; “much better than when the batteries were on the current is steadier and more reliable. Suppose we continue to work so until the aurora subsides?”
“Agreed,” replied the Boston operator. “Are you ready for business?”
“Yes; go ahead,” was the answer.
The Boston operator then commenced sending private dispatches, which he was able to do much more satisfactorily than when the batteries were on, although, of course, not so well as he could have done with his own batteries without celestial assistance.
The line was worked in this manner more than two hours, when, the aurora having subsided, the batteries were resumed. While this remarkable phenomenon was taking place upon the wires between Boston and Portland, the operator at South Braintree informed me that he was working the wire between that station and Fall River—a distance of about forty miles—with the current from the aurora alone, He continued to do so for some time, the line working comparatively well. Since then I have visited Fall River, and have the following account from the intelligent operator in the railroad office at that place. The office at the station is about half a mile from the regular office in the village. The battery is kept at the latter place, but the operator at the station is provided with a switch by which he can throw the battery off the line and put the wire in connection with the earth at pleasure. The battery at the other terminus of the line is at Boston; but the operator at South Braintree is furnished with a similar switch, which enables him to dispense with its use at pleasure. There are no intermediate batteries; consequently, if the Fall River operator put his end of the wire in connection with the earth, and the South Braintree operator do the same, the line is without battery, and of course without an electrical current. Such was the state of the line on the 2d of September last, when for more than an hour they held communication over the wire with the aid of the celestial batteries alone.
This seems almost too wonderful for belief, and yet the proof is incontestable. However, the fact being established that the currents from the aurora borealis do have a direct effect upon the telegraph-wires, and that the currents are of both kinds, positive and negative, — as I have shown in my remarks upon the aurora of 1852, which sometimes left a dark line upon the prepared paper, and at other times bleached it, it is a natural consequence that the wires should work better without batteries than with them, whenever a current from the aurora has sufficient intensity to neutralize the current from the batteries.
I will try to make myself clear upon this point. It makes no difference, in working the Morse, or any other system of magnetic telegraph, whether we have the positive or the negative pole to the line; but, whichever way we point, the same direction must be continued with all additional batteries we put upon the line. Now if we put a battery upon the line at Boston, of, say, twenty-five cells, and point the positive pole eastward, and the same number of cells at Portland, pointing the positive pole westward, the current will be null, that is to say, each will neutralize the other. Now the aurora, in presenting its positive pole, we will say, increases the current upon the line beyond the power of the magnet-keeper-spring to control it, and thus prevents the line from working, by surfeiting it with the electric current; until, presently, the wave recedes and is followed by a negative current which neutralizes the battery current, and prevents the line from working for want of power. It is plain, therefore, that, if the batteries be taken off, the positive current of the aurora cannot increase nor the negative decrease the working state of the line to the same extent as when the batteries are connected; but that, whichever pole is presented, the magnetism can be made use of by the operator for the ordinary duties of the line.
At Springfield, a gentleman who observed the needle of the compass, during the auroral display of August 28th, noticed that it was deflected first to the west, and then to the east, while the waves of the aurora were in motion. The electrotype plates at the office of the “Republican” at that place were so seriously affected by the aurora, that they could not be printed from during the continuance of the phenomenon.
The aurora borealis of August 28th was surpassingly brilliant not only in the northern portion of this continent, but also as far south as the equator, — as well as in Cuba, Jamaica, California, and the greater portion of Europe. The London newspapers of the 29th contain glowing descriptions of it. A California journal says: — “During the last ten years the aurora borealis was never seen in California except on very rare occasions, and then the light was very faint or barely visible; but on the 28th ult., it appeared in wonderful splendor, — the whole northern part of the sky being of a bright crimson; and the same phenomenon, with equal magnificence, was repeated on the night of the first instant.”
In Jamaica the aurora borealis was witnessed for the first time, perhaps, since the discovery of this island by Columbus. So rare is the phenomenon in those latitudes, that it was taken for the glare of a fire, and was associated with the recent riots.
Mr. E. B. Elliot of Boston, in an interesting article upon the recent aurora, points out the simultaneous occurrence of the auroral display of February 19th, 1852, with the eruption of Mauna Loa, — the largest volcano in the world, situated on Hawaii, (one of the Sandwich Island group,) — on the 20th of February on which occasion, the side of the mountain gave way about two-thirds of the distance from the base, giving passage to a magnificent stream of lava, five hundred feet deep and seven hundred broad.
Again, on the 17th of December, 1857, between the hours of one and four in the morning, there occurred an aurora of unwonted magnificence. The first steamer arriving from Europe after that date brought the following intelligence, which is taken from one of the journals of the day: — “An earthquake took place on the night of the 17th, throughout the whole kingdom of Naples, but its effects were most severe in the towns of Salerno, Potenza, and Nola. At Salerno, the walls of the houses were rent from top to bottom. Numerous villages were half destroyed.”
Were these coincidences of extraordinary auroras with extraordinary commotions in the physical condition of our globe merely accidental? or are these phenomena due to a common cause? The latter supposition is not improbable, but the question can be fully settled only by further observations.
Mr. Meriam, “the sage of Brooklyn,” as the daily journals denominate him, considers the aurora as the result of earthquakes or volcanic eruptions. He also says: — “The auroral light sometimes is composed of threads, like the silken warp of a web these sometimes become broken and fall to the earth, and possess exquisite softness and a silvery lustre, and I denominate them the products of the silkery of the skies. I once obtained a small piece, which I preserved.”
It is due to Mr. Meriam, as well as to the scientific world, to say, that he stands alone in his convictions with regard to the aurora, both in respect of the cause and the effect of the phenomenon.
* * *
Having thus illustrated the effects of the aurora, let us now return to the discussion of its causes.
The intimate and constant connection between the phenomena of the aurora borealis and terrestrial magnetism led Humboldt to class under the head of Magnetic Storms all disturbances in the equilibrium of the earth’s magnetic forces. The presence of such storms is indicated by the oscillations of the magnetized needle, the disturbance of the currents upon the telegraph-wires, and the appearance of the aurora, of which these oscillations and disturbances are, as it were, the forerunners, and which itself puts an end to the storm, — as in electric storms the phenomenon of lightning announces that the electrical equilibrium, temporarily disturbed, is now restored.
The atmosphere is constantly charged with positive electricity, — electricity furnished by the vapors that rise from the sea, especially in tropical regions, and, on the other hand, the earth is negatively electrized. The recomposition or neutralization of the two opposite electricities of the atmosphere and of the terrestrial globe is brought about by means of the moisture with which the lower strata of the air are more or less charged. But it is especially in the polar regions, where the eternal ice that reigns there constantly condenses the aqueous vapors under the form of haze, that this recomposition must be brought about the more so, as the positive vapors are carried thither and accumulated by the tropical current, which, setting out from the equatorial regions, where it occupies the most elevated regions of the atmosphere, descends as it advances towards the higher latitudes, until it comes in contact with the earth in the neighborhood of the poles. It is there, then, chiefly, that the equilibrium between the positive electricity of the vapors and the negative electricity of the earth must be accomplished by means of a discharge, which, when of sufficient intensity, will be accompanied with light, if, as is almost always the case near the poles, and sometimes in the higher parts of the atmosphere, it take place among those extremely small icy particles which constitute the hazes and the very elevated clouds.
There can be no doubt that the occurrence of the phenomenon is materially dependent on the presence in the atmosphere of these particles of ice, forming a kind of thin haze, which, becoming luminous by the transmission of electricity, must appear simply as an illuminated surface of greater or less extent, and more or less cut up. The phenomenon actually takes place in this manner in the parts of the atmosphere that are the most distant from the earth. We perceive what are termed auroral plates of a purple or reddish-violet color, more or less extended, according as this species of veil, formed by icy particles, extends to a greater or less distance from the poles. The tenuity of this veil is such that it admits of our seeing the stars through the auroral plates. Of its existence, independently of indirect proofs, we have a direct demonstration in the observation of MM. Bixio and Baral, who, being raised in a balloon to a great height, found themselves, on a sudden, although the sky was entirely serene and the atmosphere cloudless, in the midst of a perfectly transparent veil, formed by a multitude of little icy needles, so fine that they were scarcely visible.
If we place the pole of an electro-magnet over the jets of electric light that are made to converge in extremely rarefied air, we shall see that the electric light, instead of coming out indifferently from all points of the upper surface, as had taken place before the magnetization, comes out from the points of the circumference only of this surface, so as to form around it a continuous luminous ring. This ring possesses a movement of rotation around the magnetized cylinder, sometimes in one direction, sometimes in another, according to the direction of the discharge and of the magnetization. Finally, some more brilliant jets seem to come out from this luminous circumference without being confounded with the rest of the group. Now the magnetic pole exercises over the luminous haze which we have mentioned as always present during an aurora precisely the same action which the pole of the electro-magnet exercises in the experiment just described; and what takes place on the small scale of the experiment is precisely what takes place on the large scale of the phenomenon of the aurora borealis.
The arc of the aurora borealis is a portion of a luminous ring, the different points of which are sensibly at equal distances from the earth, and which centres upon the boreal magnetic pole, so as to cut at right angles all the magnetic meridians that converge towards this pole. Such a ring, seen by an observer placed at the surface of the earth, evidently presents to him the known arc of the aurora; and its apparent summit is always necessarily situated in the magnetic meridian of the place.
The diameter of the luminous ring is greater in proportion as the magnetic pole is more distant from the surface of the earth, since this pole must be situated upon the intersection of the plane of the ring with the axis of the terrestrial globe; if we could determine rigorously the position of the aurora borealis, we should then have the means of knowing exactly that of the pole itself.
Each observer sees the summit of the auroral arc at his magnetic meridian; it is, therefore, only those who are on the same magnetic meridian who see the same summit, and who are able by simultaneous observations to take its height.
If the summit of the arc pass beyond the zenith of the observer, the latter is surrounded by the matter of the aurora borealis. This matter is nothing else than aqueous vapors traversed by the discharges, and which are in general luminous only at a certain height from the ground, either because the air is there more rarefied, or because they are themselves congealed, and more capable, consequently, of liberating their electric light. Then it is, that, from being nearer to the spot where the phenomenon is taking place, the observer hears the crepitation, or whizzing, of which we have spoken, especially if he be in an open country and in a quiet place. But if the arc do not attain to his zenith, he is situated beyond the region in which the meeting of the electric currents takes place; he sees only an arc a little more elevated to the north or the south, according as he is situated in one hemisphere or the other; and he hears no noise, on account of his too great distance. The crepitation is the result of the action of a powerful magnetic pole upon luminous electric jets in its immediate neighborhood. With regard to the sulphurous odor which some observers have perceived, it arises, as does that which accompanies the fall of lightning, from the conversion into ozone of the oxygen of the air, by the passage of electric discharges.
Gisler says, that on the high mountains of Sweden the traveller is sometimes suddenly enveloped in a very transparent fog, of a whitish-gray color inclining a little to green, which rises from the ground, and is transformed into an aurora borealis. The cirro-cumulus and the hazes become luminous when they are traversed by sufficiently energetic discharges of electricity, and when the light of day is no longer present to overcome their more feeble light. Dr. Usher describes an aurora borealis seen in the open day, at noon, May 24, 1778.
MM. Cornulier and Verdier are convinced, after carefully studying the subject, that there are almost always auroræ boreales in the high polar latitudes, and that their brilliancy alone is van able. This conviction is in accordance with the very careful observations which have now been made for four years in the northern hemisphere. It appears, as the result of these, that the aurora borealis is visible almost every clear night, but it does not show itself at all the stations at the same time. From October to March there is scarcely a night in which it may not be seen; but it is in February that it is most brilliant. In 1850 it was observed two hundred and sixty-one nights, and during 1851 two hundred and seven. The proportion of nights in which the aurora is seen is much greater the nearer we are to the magnetic pole.
De la Rive, from whose admirable treatise upon Electricity we have borrowed our general views, and whose theory we have attempted to illustrate in this paper, concludes that the aurora borealis is a phenomenon which has its seat in the atmosphere, and consists in the production of a luminous ring of greater or less diameter, having for its centre the magnetic pole. Experiment shows, as we have seen, that, on bringing about in rarefied air the reunion of the two electricities, near the pole of a powerful artificial magnet, a small luminous ring is produced, similar to that which constitutes the aurora borealis, and animated by a similar movement of rotation. The aurora borealis would be due, consequently, to electric discharges taking place in the polar regions between the positive electricity of the atmosphere and the negative electricity of the earth. These electric discharges taking place constantly, but with intensities varying according to the state of the atmosphere, the aurora borealis should be a daily phenomenon, more or less intense, consequently visible at greater or less distances, but only when the nights arc clear, — which is perfectly in accordance with observation.
The aurora australis presents precisely the same phenomenal as the aurora borealis, and is explained, consequently, in the same manner.
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