in its appearance and progress; that the streamers or beams generally appear first in the north, forming an arch from east to west, having its vertex at the line of the magnetic meridian; that when this arch is yet only of low elevation, it is of considerable breadth from north to south, having the streamers of which it is composed placed crossways in relation to its own line, and all directed towards a point a little south of the zenith; that the arch moves forward towards the south, contracting its lateral dimensions as it approaches the zenith, and increasing in intensity of light by the shortening of the streamers near the magnetic meridian and the gradual shifting of the angles which the streamers near the east and west extremities of the arch make with its own line, till at length these streamers become parallel to that line, and then the arch is seen as a narrow belt, or only in breadth, extending across the zenith at right angles to the magnetic meridian; that it still makes progress southwards; and, after it has reached several degrees south of the zenith, again enlarges in breadth, by exhibiting an order of appearances the reverse of that which had attended its progress towards the zenith from the north; and that the only conditions which can explain these appearances are, that the streamers of the aurora are vertical, or nearly so, and form a deep fringe or arch, which stretches a great way from east to west at right angles to the magnetic meridian, but which is of no great thickness from north to south; and that the arch moves southward, preserving its direction at right angles to the magnetic meridian. The same gentleman elsewhere remarks, that "the whole lights in the north part of the sky made a rapid progress southward; and the manner of this progress was repeatedly finely exhibited in the fringes and fragments that had reached or passed the zenith, by the extinction of streamers at their northern faces, and the formation of new ones at their southern faces. The advanced southern fringe expired when it had reached about south of the zenith; and all did so, either when they attained a similar angle south, or before they had gone so far. The confused mass of streamers in the north, as they came forward in succession to the zenith, and passed that point, unfolded themselves into narrow zones of light at right angles to the magnetic meridian, or very nearly so; for there was occasionally a small deviation from parallelism among themselves. The zones were more numerous than usual, and were separated from each other by less intervals, sometimes not exceeding or , sometimes, however, or ." (Edinb. Phil. Mag.)
Height of the Polar Lights.—Opinions differ as to the elevation of the aurora borealis above the surface of the earth, which is a point that can be determined only by a series of accurate observations. Dr Halley observed that the aurora described by him rose to a prodigious height, it being seen from the west of Ireland to the confines of Russia and Poland on the east, nor did he know how much farther it might have been visible; so that it extended at least 30 degrees in longitude, and from latitude 50 degrees N. it was seen over all the northern part of Europe; and, what was very surprising, in all those places where it was visible, the same general appearances were exhibited which Dr Halley noticed at London. But he observes, with seeming regret, that he could by no means determine its height for want of observations made at different places; otherwise he might easily have calculated the height of the aurora, as he did that of the luminous ball of 1719. Father Boscovich, says he, determined the height of an aurora borealis, observed on the 16th of December 1737 by the Marquis of Poleni, to have been 825 miles; Bergmann, from a mean of thirty computations, makes the average height of the aurora borealis to be 70 Swedish or
upwards of 460 English miles; Euler supposes it to be several thousands of miles in height; and Mairan also as signs it a very elevated region. In the 74th volume of the Philosophical Transactions Dr Blagden, speaking of the height of some fiery meteors, remarks that the "aurora borealis appears to occupy as high if not a higher region above the surface of the earth, as may be judged from the very distant countries to which it has been visible at the same time." On the 22d October 1804 it is said that the same polar light was seen at the same time in Saxony and in Liefland. Mr Dalton of Manchester, in a paper read to the Royal Society of London on the 17th April 1828, describes a polar light which he observed on the 29th March 1826, assuming the form of a regular arch at right angles to the magnetic meridian, and continuing for above an hour in the same position; thus affording a favourable opportunity for obtaining the data requisite for determining its height. He took great pains to collect as many authentic accounts as possible of the apparent position of this luminous arch with reference to the stars, when seen from various places where it had been observed in England and in Scotland. And according to Mr Dalton's view of the distribution of the meteor, it appears to have been seen in places 170 miles distant from one another in a north and south direction, and 45 miles distant from east to west, thus comprising an area of 7000 or 8000 square miles; but it must have been visible over a much greater extent. Accounts were received of its having been seen as far north as Edinburgh, and as far south as Manchester and Doncaster, and at most of the intermediate towns. From the exact correspondence of the descriptions at all these places, it was inferred that they referred to the same individual luminous appearance. In proceeding from north to south, the apparent altitude of the arch continually increased, still keeping the south of the zenith to the distance of Kendal, at which place it very nearly crossed the zenith. At Warrington, which is farther south, the culminating point of the arch was north of the zenith; and, wherever seen, the arch always seemed to terminate nearly in the magnetic east and west, at two opposite points of the horizon. The observations in which the author placed the greatest confidence for determining the height of the aurora were those made at Whitehaven and at Warrington, places which are distant 83 miles from each other, and situated nearly on the same magnetic meridian. Calculating from the data they afford, he found the height of the arch very nearly 100 miles above the surface of the earth, and immediately over the towns of Kendal and Kirkby-Stephen. This conclusion is countenanced by observations made at Jedburgh; yet if the former be compared with those at Edinburgh, the height will come out to be 150 or 160 miles, and the position vertical above Carlisle; but he thinks the former result more entitled to confidence. Assuming the height to be 100 miles, it follows that the breadth of the arch would be eight or nine miles; and its visible length, in an easterly and westerly direction from any one place, would be about 550 miles.
But we are disposed to reject these calculations of Dalton, because from the particular details in his memoirs, and the known distribution of the aurora, it is highly probable that the aurora said by him to have been seen at the same time at great distances were different aurora, and then not much elevated above the clouds. Indeed Mr Farquharson, in the Philosophical Transactions, part i. 1829, when discussing the value of Mr Dalton's observations, thinks that the observations made from Edinburgh to Warrington might be explained on the supposition that there were several nearly vertical arches of the aurora, almost contemporaneously hanging over many lines from Edin-
burgh to Warrington, at a few thousand feet above the surface. The observations of Dr Richardson, Captain Franklin, Lieutenant Hood, and others, render it highly probable that many polar lights occur at heights not higher than the region of clouds. Dr Richardson's observations seem to show that the aurora is occasionally seated in a region of the atmosphere, below a kind of cloud which is known to possess no great altitude, namely, that modification of cirro-stratus which, descending low in the atmosphere, produces a hazy sheet of cloud over-head, or a fog-bank in the horizon. Indeed Dr Richardson is inclined to infer that the aurora borealis is constantly accompanied by, or immediately precedes, the formation of one or other of the forms of cirro-stratus. On the 13th of November and 18th December 1826, at Fort Enterprise, its connection with a cloud intermediate between cirrus and cirro-stratus is mentioned; but the most vivid coruscations of the aurora were observed when there were only a few thin attenuated shoots of cirro-stratus floating in the air, or when that cloud was so rare that its existence was only known by the production of a halo round the moon. The natives of the arctic regions of North America pretend to foretell wind by the rapidity of the motions of the aurora; and they say, that when it spreads over the sky in a uniform sheet of light, it is followed by fine weather, and that the changes thus indicated are more or less speedy, according as the appearance of the meteor is early or late in the evening; an opinion not improbable, when it is recollected that certain kinds of cirro-stratus are also regarded by meteorologists as sure indications of rain and wind. Dr Richardson frequently observed the lower surface of nebulous masses illuminated by polar lights; a fact illustrative of the comparatively low situation of these auroræ. Biot, also, in the island of Unst, observed many auroræ that could not be higher than the region of clouds. Captain Franklin in like manner observed low auroræ. "The important fact," says he, "of the existence of the aurora at a less elevation than that of dense clouds was evinced on two or three occasions this night (13th February 1821, at Fort Enterprise), and particularly at 11 hours 50 min., when a brilliant mass of light, variegated with the prismatic colours, passed between an uniform steady dense cloud and the earth, and in its progress completely concealed that portion of the cloud which the stream of light covered, until the coruscation had passed over it, when the cloud appeared as before." Captain Parry, as stated in his third voyage, observed auroræ near to the earth's surface. It is said that while Lieutenants Scherer and Ross and Captain Parry were admiring the extreme beauty of a polar light, they all simultaneously uttered an exclamation of surprise at seeing a bright ray of the aurora shoot suddenly downward from the general mass of light, and between them and the land, which was only three thousand yards distant. The ray or beam of the polar light thus passed within a distance of three thousand yards, or less than two miles, of them. Further, Mr Farquharson observed in Aberdeenshire an aurora borealis not more than four thousand feet above the level of the sea; and he agrees with Richardson, Franklin, &c., in believing that the auroræ occur in a region immediately above that of the clouds, and of course vary in height with the different states of the atmosphere. But although this region was very low on the 20th December 1829, in the polar light seen from Alford, we know that at times it is several miles high, agreeing with the observations of these intelligent travellers. We have frequently seen the aurora when the height of the clouds could not be estimated at less than two or three miles, and at other times not higher than a thousand or fifteen hundred feet.
Position of the Polar Lights.—The arches of the polar lights generally cross the magnetic meridian at right
angles; when two or more appear at once they are concentric, and tend to the magnetic east and west. The beams or streamers in the direction of their length coincide with the plane of the dip of the needle, or nearly so; and each individual streamer is, in fact, parallel to the dipping needle. Dr Richardson thinks he has observed a polarity in the masses of cloud belonging to a certain kind of cirro-stratus approaching to cirrus, by which their long diameters, having all the same direction, were made to cross the magnetic meridian nearly at right angles. But the apparent convergence of such masses of cloud towards the opposite points of the horizon, which have been so frequently noticed by meteorologists, is an optical deception, produced when they are situated in a plane parallel to that on which the observer stands. These circumstances, says Dr Richardson, are here noticed, because, if it shall hereafter be proved that the aurora depends upon the existence of certain clouds, its apparent polarity may perhaps, with more propriety, be ascribed to the clouds themselves which emit the light; or, in other words, the clouds may assume their peculiar arrangement through the operation of one cause (magnetism, for example), while the emission of light may be produced by another, namely, a change in their internal constitution, perhaps connected with a motion of the electrical fluid.
Magnetic Property of the Polar Lights.—Many years ago philosophers remarked that the magnetic needle was agitated during polar lights; and hence it was inferred that these lights were somehow connected with magnetism. Other observers, again, maintained that these observations must be erroneous, as they could not in any instance perceive the compass affected by the presence of this meteor. But the late observations, of Biot, Hansteen, Gay-Lussac, Kupfer, and particularly of Richardson, Franklin, and Farquharson, have demonstrated that the magnetic needle is affected by the polar lights. As the fact is one of the most curious in meteorology, we shall now state some particulars illustrative of its nature. In Captain Franklin's observations, the horizontal compass was placed in a firm sheltered stand, fixed to the back wall of the house at Fort Enterprise, three feet above the ground, on a northern exposure; and the dipping needle was similarly fixed to the end of the storehouse, at the distance of forty feet. There was no iron near either of them. The house stood on a sand-hill, and there were no large stones in its immediate neighbourhood. The horizontal compass belonged to a small variation transit made by Dollond; and its graduated scale, of one and a half inch radius, was divided into degrees, the degrees counting from north towards west to 360. Each degree was subdivided into 20, and, by the assistance of a magnifying glass, he could read it off accurately to within three minutes. The horizontal position was preserved by means of a spirit-level attached to the instrument.
The manner in which the needle is affected by the aurora will require some description. "The motion communicated to it," says Captain Franklin, "was neither sudden nor vibratory. Sometimes it was simultaneous with the formation of arches, prolongation of beams, or certain other changes of form or action of the aurora. But generally the effect of these phenomena upon the needle was not visible immediately, but in about half an hour or an hour the needle had attained its maximum of deviation. From this its return to its former position was very gradual, seldom regaining it before the following morning, and frequently not until the afternoon, unless it was expedited by another arch of the aurora operating in a direction different from the former one.
"The arches of the aurora," he adds, "most commonly traverse the sky nearly at right angles to the magnetic
meridian, but the deviations from this direction, as has been already stated, were not rare; and I am inclined to consider that these different positions of the aurora have considerable influence upon the direction of the needle. When an arch was nearly at right angles to the magnetic meridian, the motion of the needle was towards the west; this westward motion was still greater when one extremity of an arch bore , or about to the west of the magnetic north, that is, when the extremity of the arch approached from the west towards the magnetic north. A westerly motion also took place when the extremity of an arch was in the true north, or about to the west of the magnetic north, but not in so great a degree as when its bearing was about . A contrary effect was produced when the same end of an arch originated to the southward of the magnetic west, viz. when it bore from about to , and, of course, when its opposite extremity approached nearer to the magnetic north. In these cases the motion of the needle was towards the east. In one instance only a complete arch was formed in the magnetic meridian; in another, the beam shot up from the magnetic north to the zenith; and in both these cases the needle moved towards the west.
"The needle was most disturbed on February 13, P. M., at a time when the aurora was most distinctly seen passing between a stratum of clouds and the earth, or at least illuminating the face of the clouds opposed to the observer. This and several other appearances induced me to infer that the distance of the aurora from the earth varied on different nights, and produced a proportionate effect on the needle. When the light shone through a dense hazy atmosphere, when there was a halo round the moon, or when a small snow was falling, the disturbance was generally considerable; and on certain hazy cloudy nights the needle frequently deviated in a considerable degree, although the aurora was not visible at the time. Our observations do not enable us to decide whether this ought to be attributed to an aurora concealed by a cloud or haze, or entirely to the state of the atmosphere. Similar deviations have been observed in the day-time, both in a clear and cloudy state of the sky, but more frequently in the latter case. Upon one occasion the aurora was seen immediately after sunset, whilst bright day-light was remaining. A circumstance to which I attach some importance must not be omitted. Clouds have been sometimes observed during the day to assume the forms of the aurora, and I am inclined to connect with the appearance of these clouds the deviation of the needle, which was occasionally remarked at such times. An aurora sometimes approached the zenith without producing any change in the position of the needle, as was more generally the case; whilst at other times a considerable alteration took place although the beams or arches did not come near the zenith. The aurora was frequently seen without producing any perceptible effect on the needle. At such times its appearance was that of an arch, or an horizontal stream of dense yellowish light, with little or no internal motion. The disturbance in the needle was not always proportionate to the agitation of the aurora, but it was always greater when the quick motion and vivid light were observed to take place in a hazy atmosphere. In a few instances, the motion of the needle was observed to commence at the instant a beam darted upwards from the horizon; and its former position was more quickly or slowly regained according to circumstances. If an arch was formed immediately afterwards, having its extremities placed on opposite sides of the magnetic north and south to the former one, the return of the needle was more speedy, and it generally went beyond the point from whence it first started."
A series of interesting observations on this subject were
also made in December 1829, by the reverend James Farquharson, F. R. S., of Aberdeenshire, with an apparatus transmitted to him by the Royal Society of London. This apparatus consisted of a horizontal brass circle, about one foot in diameter, graduated to divisions of ten minutes, and capable of adjustment to a perfect level by means of spirit levels and screwed feet. Concentrically within this divided circle moved a circular horizontal brass plate, its edge touching the divisions, and having at opposite points two verniers, which, by means of attached microscopes, indicated the movements which it made to 60th parts of ten minutes, or . The movement of the plate within the circle was effected by means of a screw. A circular brass needle-box was attached to the surface of the inner plate, and a vertical pointed steel wire for supporting the needle formed the centre. At opposite points in the needle-box were fixed two micrometers, with cross wires in the foci, for adjusting the needle to a level, and observing any change in its direction. The top of the needle-box was a circular plate of ground glass in a brass ring, made to slip easily off and on, and having screwed into its centre a vertical brass tube about eight inches long, for the purpose of suspending the needle with fibres of silk, for measuring the time of its oscillations. A horizontal brass pin, with a minute perforation for the silk near its middle, passed through the vertical tube near its top, and being contrived with several motions, served to adjust the suspended needle, and bring it correctly over the steel point, where its levelling could be completely ascertained. The magnetic needle itself was a rectangular plate about five inches long, half an inch broad, and th of an inch thick. An agate cup set in brass admitted of being screwed in either at the narrow or flat side of the needle; and a little fixed ring of brass, with a minute perforation in its top, rising over the cup, admitted of the ready attachment of the silk; so that the needle could be placed on the steel point, or suspended with the silk, with its flat face either vertical or horizontal. This apparatus measured, with great accuracy, very minute changes in the declination of the needle, one so small as being quite sensible by it.
The observations made with this apparatus, and published in the Transactions of the Royal Society of London for 1830, also show, in a most satisfactory manner, that the magnetic needle is actually affected by the presence of the polar lights. They equally prove that the needle is not in every instance agitated by the polar lights, even when they are very brilliant; the oscillations taking place only when the beams or fringes of the meteor are in the same plane with the dip of the needle. But as the needle is affected in those planes only where the fringes or arches are in the plane just mentioned, it is evident that observers in different latitudes may obtain very discordant results in the same evening. The observations collected by Dalton, of the appearance of the aurora of the 29th March 1826, prove that many fringes or arches may be parallel to each other at remote distances; and the observations of the President of the Royal Society, on a luminous arch in Cornwall, 29th September 1828, which appeared simultaneously with a remarkable aurora of many arches extending over the whole of Aberdeenshire, show that the meteor is sometimes active over a space nearly coincident with the extent of the kingdom; and there is reason for believing that it often extends much farther. There might, therefore, be an extensive succession of observations of disturbance and non-disturbance of the needle at the same instant from north to south over many degrees of latitude.
Noises from the Aurora Borealis.—Having, many years ago, both in this country and in the Shetland Islands, heard very distinctly noises proceeding from the polar lights, we
have always given full credit to the statements of those observers who have published accounts of this fact. It is true that late observers, as Scoresby,1 Richardson,2 Franklin, Parry, and Hood, never heard such noises, although they do not deny that they may have been heard. Thus Richardson says, "I have never heard any sound that could be unequivocally considered as originating in the aurora; but the uniform testimony of the natives, both Cree and Copper Indians, and Esquimaux, and of all the older residents in the country, induces me to believe that its motions are sometimes audible." Captain Franklin says, "I have not heard the noise ascribed to the aurora, but the uniform testimony of the natives and of the residents in this country induces me to believe that it is occasionally audible." Parry frequently listened for sounds from the polar lights, but never heard any. Lieutenant Hood says (Franklin's Narrative, p. 535), "We repeatedly heard a hissing noise, like that of a musket bullet passing through the air, which seemed to proceed from the aurora; but Mr Wentzel assured us that this noise was occasioned by severe cold succeeding mild weather, and acting upon the surface of the snow previously melted in the sun's rays. The temperature of the air was then — 35°, and on the two preceding days it had been above zero. The next morning it was — 42°, and we frequently heard a similar noise. Mr Hearn's description of the noise of the aurora agrees exactly with Mr Wentzel's, and with that of every other person who heard it. It would be an absurd scepticism to doubt the fact any longer, for our observations have rather increased than diminished the probability of it." Muschenbroeck says that the Greenland fishers in his time assured him that they had frequently heard noises proceeding from the aurora borealis. Mr Nairne is confident that he has heard a hissing and whizzing noise when the polar lights were very bright; and Mr Cavallo affirms that he more than once heard a crackling noise from polar lights. Giesecke, who resided so long in West or Old Greenland, says, "the polar lights sometimes appear very low, and then they are much agitated, and a crashing and crackling sound is heard, like that of an electric spark, or the falling of hail." Professor Parrot of Dorpat describes a magnificent polar light he witnessed on the 22d October 1804, from which a crackling and rustling noise proceeded. "We learn from the inhabitants," says Captain Brooke, in his interesting travels through Norway, "with respect to the polar or northern lights, that they had frequently heard the noise that sometimes attends them, which they describe like that of a rushing wind. At Hammerfest they said they were violent, and descended so low that it would appear almost possible to touch them." In a letter from Mr Ramm of Tonset in Norway, addressed to Professor Hansteen, and published in the Magazin für Naturwidenskaberne, Christiania, 1825, we are told that he several times heard a quick whispering noise simultaneously with the motion of the beams of the polar lights. In the same journal, Professor Hansteen says, "The polar regions being in reality the native country of the polar light, we ought to be particularly interested in obtaining any additional information on the natural history of this remarkable phenomenon; and we have so many certain accounts of the noise attending it, that the negative experience of southern nations cannot be brought in opposition to our positive knowledge. Unfortunately, we live, since the beginning of this century, in one of the great pauses of this phenomenon; so that the present generation knows but little of it from personal observation. It would
therefore be very agreeable to receive, from older people, observations of this kind, made in their youth, when the aurora borealis showed itself in full splendour. It can be proved mathematically that the rays of the northern lights ascend from the surface of the earth, in a direction inclining towards the south (an inclination which, with us, forms an angle of about 73°). If, then, this light occupies the whole northern sky, rising more than 17° above the zenith, the rays must proceed from under the feet of the observer, although they do not receive their reflecting power till they have reached a considerable elevation, perhaps beyond our atmosphere. It is therefore conceivable why we should frequently hear a noise attending the northern lights, when the inhabitants of southern countries, who see the phenomenon at a distance of many hundred miles, hear no report whatever. Wargent, in the fifteenth volume of the Transactions of the Swedish Academy, says, that Dr Gisler and Mr Hellant, who had resided for some time in the north of Sweden, made, at the request of the academy, a report of their observations on the aurora borealis." To these observations Professor Hansteen adds, that Captain Abrahamson, in the Transactions of the Scandinavian Literary Society, has given an account of several observations of noises that were heard proceeding from the northern lights. The professor concludes with stating that he himself knows several persons who have distinctly heard the same sounds; he expresses his surprise that a fact so well established should be called in question; and he relates, with some sharpness, a conversation he once had on this subject with an Englishman, who remarked that the Norwegian tales of noises proceeding from polar lights were akin to the ghost stories of this country.
Theory of the Aurora Borealis.—The theory of this phenomenon is involved in the copious details which have been given under the different foregoing heads; and, indeed, from all that is known, it appears reasonable to infer that it is intimately connected with electricity and magnetism. "The aurora borealis," says Dr Young, "is certainly in some measure a magnetical phenomenon; and if iron were the only substance capable of exhibiting magnetic effects, it would follow that some ferruginous particles must exist in the upper regions of the atmosphere. The light usually attending this magnetical meteor may possibly be derived from electricity, which may be the immediate cause of a change in the distribution of the magnetic fluid contained in the ferruginous vapours which are imagined to float in the air." (Lectures, vol. i. p. 687.) This assumption of ferruginous particles or vapours, however, seems purely gratuitous and imaginary; and as iron is not the only substance or matter capable of exhibiting magnetic effects, light itself being susceptible of polarization, the above hypothesis is therefore untenable even on the ground on which it has been rested by its author. But it is nevertheless certain that the cause of this luminous meteor is intimately connected with magnetism and electricity; or rather, as the magnetic is variously modified and affected by the electric power, with the phenomena of electro-magnetism. "It is doubtful," says Dr Young in another place, "whether the light of the aurora borealis may not be of an electric nature; the phenomenon is certainly connected with the general cause of magnetism;" and he refers in support of this opinion to the ascertained fact that the primitive beams or rays of light are constantly observed in a direction parallel to that of the dipping needle; adding, that "although the substance be magnetical, the illumination which renders it visible may still be derived from the passage of electricity,
1 Arctic Regions, and Journal of a Voyage to the Northern Whale Fishery.
2 Remarks on the Aurora Borealis, in Franklin and Richardson's Journey to the Shores of the Polar Sea.