AURORA, the morning twilight, or that faint light which appears in the morning when the sun is within 18 degrees of the horizon.

Aurora, the goddess of the morning, according to the Pagan mythology. She was the daughter of Hyperion and Theia, according to Hesiod, but of Titan and Terra, according to others. It was under this name that the ancients deified the light which foreruns the rising of the sun above our hemisphere. The poets represent her as rising out of the ocean in a chariot, with rosy fingers dropping gentle dew. Virgil describes her ascending in a flame-coloured chariot with four horses.

Aurora, one of the New Hebrides Islands in the South Sea, in which Mr Forster supposes the Peak d’ Etoile mentioned by Mr Bougainville to be situated. It is about 12 leagues long, but not above five miles broad in any part; and it stretches nearly north and south. The middle lies in lat. 15. 6. S. long. 168. 24. E.

Aurora Borealis, Polar Light, Northern Lights, or Streamers, is a beautifully luminous meteor, appearing in the form of beams or rays, and sometimes in that of arches or crowns. The rays are seldom stationary, generally flitting with greater or less velocity throughout the heavens; the arches are sometimes single; sometimes several concentric ones are seen. These lights, or meteoric coruscations, are most brilliant in the arctic regions, appearing mostly in the winter season and in frosty weather. In the Shetland Islands, the merry dancers, as they are there and elsewhere called, are the constant attendants of clear evenings, and serve to diminish materially the gloom of the long winter nights. They commonly appear at twilight near the horizon, and sometimes continue in that state for several hours without any sensible motion; after which they send forth streams of stronger light, which rise from the horizon in a pyramidal undulating form, shooting with great velocity up to the zenith, assuming columnar and other shapes, and varying in colour from a reddish yellow to the darkest russet. At other times they cover the whole hemisphere with their flickering and fantastical coruscations. On these occasions their motions are amazingly quick, and they astonish the spectator with rapid changes of form. They break out in places where none were seen before, skimming briskly along the heavens; then they are suddenly extinguished, leaving behind a uniform dusky track, which, again, is brilliantly illuminated in the same manner, and as suddenly left a dull blank. Some nights they assume the appearance of vast columns; exhibiting on one side tints of the deepest yellow, and on the other melting away till they become undistinguishable from the surrounding sky. They have generally a strong tremulous motion from end to end, which continues till the whole vanishes. According to the state of the atmosphere, their colours vary. They sometimes assume the hue of blood, on which occasion their appearance is considered portentous. I hen the rustic sages become prophetic, and terrify the gazing spectators with the dread of war, pestilence, and famine. This superstition is not peculiar to the northern islands; nor are these appearances of recent date. The [7:4:195]ancients called them chasmata, and trahes, and bolides, according to their forms or colours.

In early times, it is said, these meteoric lights were extremely rare, and on that account were the more taken notice of. From the days of Plutarch to those of our sage historian Sir Richard Baker, they were supposed to portend great events, and lively imaginations shaped them into aerial conflicts:
Fierce fiery warriors fight upon the clouds,
In ranks, and squadrons, and right form of war.

Dr Halley tells us, that when he observed a great aurora borealis in 1716, he had begun to despair of ever seeing one at all; none having appeared, at least of any considerable extent, from the time he was born until then. But notwithstanding this long interval, it seems that, in some periods, the aurora borealis had been seen much more frequently; and perhaps this, as well as many other natural phenomena, may be subject to periodical changes and variations.

The only thing resembling a distinct history of this phenomenon is that which has been given by the learned Dr Halley, in the Philosophical Transactions, No. 347. The first account he gives is of the appearance of what is called by him burning spears, which were seen at London on the 30th January 1560. This account is taken from a book entitled A Description of Meteors, by W. E. D. D., reprinted at London in 1654. The next appearance, according to the testimony of Stow, was on the 7th October 1564. In 1574 also, according to Camden, and Stow above-mentioned, an aurora borealis was observed two nights successively, viz. on the 14th and 15th of November, having much the same appearances as that described by Dr Halley in 1716, and which we now so frequently observe. Again, an aurora was twice seen in Brabant, in the year 1575, viz. on the 13th of February and 28th of September. Its appearances at both these times were described by Cornelius Gemm, professor of medicine in the university of Louvain, who compares them to spears, fortified cities, and armies fighting in the air. After this, Michael Maestlin, tutor to the great Kepler, assures us, that atBaknang in the county of Wurtemberg in Germany, these phenomena, which he styles chasrnata, were seen by himself no less than seven times in 1580. In 1581 they again appeared, in an extraordinary manner in April and September, and in a less degree at some other periods of the same year. In September 1621, a similar phenomenon was observed all over France, and described by Gassendus, who gave it the name of aurora borealis ; yet neither this, nor any similar appearance posterior to 1574, is described by English writers till the year 1707, which, as Dr Halley observes, shows the prodigious neglect of curious matters that then prevailed. From 1621 to 1707, indeed, there is no mention made of an aurora borealis having been seen by any body; and, considering the number of astronomers who during that period were in a manner continually poring over the heavens, we may very reasonably conclude that nothing of the kind really made its appearance until after an interval of eighty-six years. A small one was seen in November 1707; and during that and the following year, the same appearances were repeated five times. The next on record is that mentioned by Dr Halley in March 1716, which from its brilliancy attracted universal attention, and was considered by the vulgar as marking the introduction of a foreign race of princes. Since that time these meteors have been much more frequent. Many of those observed in this island have been described. The following account of a splendid aurora borealis, as seen from the Gosport Observatory on the 7th January 1831, will give an idea of the brilliancy of this meteor, and of its colours, lustres, arches, and beams.

“In the afternoon of this day there was a peculiar brightness in the atmosphere near the horizon, for several degrees on each side of the true north point, which indicated the approach of an aurora; indeed we have reason to: suspect that it was a faint appearance of one; while the, sun shone in all his splendour, without the interposition of cloud or vapour. Shortly after sunset an aurora borealis gradually rose above the northern horizon, and at a quarter past five o’clock it had assumed the form of an arch of refulgent light, ten degrees in height, and seventy degrees in width. From this time till half-past five it continued to increase in the intensity of its light, expanding to the western point of the horizon, and fifty-five degrees to the eastward of north, which made the chord of the aurora a hundred and fifty-five degrees. Now, a bright flame-coloured rainbow-like arch, between three and four degrees broad, and pretty well defined at its upper edge, emanated from the curved edge of the aurora to an altitude of thirty-five degrees; and while it remained apparently stationary, a beautiful rainbow-like arch, still more brilliant, formed about ten degrees south of the zenith, by streamers suddenly springing up from the N. E. by E. and W. by S. points of the horizon, and meeting in the zenith, so that these two bows presented themselves at the same time. At thirty-five minutes past five, the latter bow, in some parts four and in others six degrees in width, divided a little to the eastward of its vertex; and the long streamers which formed it passed off gently to the southward in very bright patches, two in the south-east and one in the south-west quarters, like luminous clouds, and continued in sight nearly a quarter of an hour. One of these bright patches nearly covered Orion several minutes. At forty minutes past five another rainbow-like arch, equally wide and bright, was formed by long streamers from about the same points of the horizon, whose point of convergence was the same, and its course through the feet of Gemini, near the Pleiades, through Aries, the square of Pegasus, the head of Equuleus, and the bow of Antinous. It passed off gradually towards the south; and at a quarter before six the planet Mars, then near the meridian, and about forty-five degrees in altitude, rested, as it were, conspicuously on it. At six it had gone far towards the southern horizon, and could scarcely be perceived, leaving the sky unusually clear and bright. By this time the bow over the aurora had much increased in altitude, and was nearly effaced.

“At a few minutes past six, after a great many coloured columns of light had risen from the north-east and northwest quarters, and passed the zenith, the aurora sunk considerably towards the horizon; but its upper edge remained bright and very well defined. Some of the streamers or columns were long, others short, and the widest generally remained long enough to pass through a gradation of prismatic colours. At half-past six the aurora again increased in altitude, and vivid coruscations radiated from every part of its arch, and, on intermixing with each other, formed wide columns, which were so grand with crimson tints as to astonish every spectator. Between seven and eight the aurora had spread at least two thirds over the heavens, and as far as the shoulders of Orion on the eastern side of the meridian, when large perpendicular columns, and short pointed luminous coruscations, rising from the aurora, glittering like spears and conical points in nearly parallel rows, now mixing and then dividing, all passed through red, orange, lake, crimson, green, and purple tints; so that the appearance altogether over so great an extent of the heavens was exceedingly grand, particularly when contrasted with the cerulean sky, and its spangled constellations, in the southern portion of the hemisphere.
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“At ten minutes before eight the aurora was in its greatest splendour. At five minutes before eight another luminous rainbow-like arch· stretched across the heavens from the eastern point of the horizon, and displayed several prismatic colours while passing southward. Soon after eight a large dark space, in and near the horizon, presented itself several degrees on each side of the magnetic north, and the aurora, still far over the heavens, gradually diminished. At nine it again ascended, and wide columns rose* from every part of its arch, and passed through the same colours as before-mentioned. Between nine and ten the magnetic needle, which in the early part of the evening stood at twenty-four degrees west of the true north, was disturbed, and receded upwards of half a degree northward, either by the influence of the aurora, or by a change of wind from north-east to southwest, and of course a change in its electrical state. At a quarter before eleven there was a grand display of about twelve or fourteen glowing columns from the aurora, several of which passed beyond the zenith, when a perfect red rainbow-like arch, ten degrees above the aurora, was visible. At eleven another bow three degrees and a half wide rose from the aurora, and passed through Aries, Cassiopeia, Ursa Minor, and the square of Ursa Major, until, reaching the zenith, it gradually disappeared. At half-past eleven the aurora again began to sink slowly, and did not rise afterwards. At five minutes before twelve, a large brilliant meteor, the only one observed through the night, passed under Ursa Major. At one o’clock a. m. the highest part of the aurora, about the magnetic north, had sunk to within six or seven degrees of the horizon; yet bright coruscations occasionally emanated from it till two, when the observations were discontinued, as no more interesting meteoric appearances were likely to occur.”

The auroras observed, one at Cumberland House and another at Fort Enterprize, in North America, arc described by Lieutenant Hood, in Captain Franklin’s Narrative. The account of the former is as follows :—

“For the sake of perspicuity,” says Mr Hood, “I shall describe the several parts of the aurora observed at Cumberland House, which I term beams, flashes, and arches. The beams are little conical pencils of light, ranged in parallel lines, with their pointed extremities towards the earth, generally in the direction of the dipping needle. The flashes seem to be scattered beams, approaching nearer to the earth, because they are similarly shaped, and infinitely larger. I have called them flashes, because their appearance is sudden, and seldom continues long. When the aurora first becomes visible, it is formed like a rainbow, the light of which is faint, and the motion of the beams undistinguishable. It is then in the horizon. As it approaches the zenith, it resolves itself at intervals into beams, which, by a quick undulating motion, project themselves into wreaths, afterwards fading away, and again brightening, without any visible expansion or concentration of matter. Numerous flashes are seen in different parts of the sky. That this mass, from its short distance above the earth, would appear like an arch to a person situated at the horizon, may be demonstrated by the rules of perspective, supposing its parts to be nearly equidistant from the earth. An undeniable proof of it, however, is afforded by the observations of the 6th and 7th of April, when the aurora, which filled the sky at Cumberland House, from the northern horizon to the zenith, with wreaths and flashes, assumed the shape of arches at some distance to the southward.

“But the aurora does not always make its first appearance as an arch. It sometimes rises from a confused mass of light in the east or west, and crosses the sky towards the opposite point, exhibiting wreaths of beams, or coronae boreales, in its way. An arch, also, which is pale and uniform at the horizon, passes the zenith without displaying any irregularity or additional brilliancy; and we have seen three arches together very near the northern horizon, one of which exhibited beams, and even colours; but the other two were faint and uniform. On the 7th of April an arch was visible to the southward, exactly similar to that in the north, and it disappeared in fifteen minutes. It had probably passed the zenith before sunset. The motion of the whole body of aurora is from the northward to the southward, at angles not more than twenty degrees from the magnetic meridian. The centres of the arches were as often in the magnetic as in the true meridian.

“The colours do not seem to depend on the presence of any luminary, but to be generated by the motion of the beams, and then only when that motion is rapid, and the light brilliant. The lower extremities quiver with a flery red colour, and the upper with orange. We once saw violet in the former. The number of aurorae visible in September was two, in October three, in November three, in December five, in January five, in February seven, in March sixteen, in April fifteen, and in May eleven. Calm and clear weather was the most favourable for observation; but it is discernible in cloudy weather, and through mists. We could not perceive that it affected the weather. The magnetic needle, in the open air, was disturbed by the aurora, whenever it approached the zenith. Its motion was not vibratory, as observed by Mr Dalton; and this was, perhaps, owing to the weight of the card attached to it. It moved slowly to the east or west of the magnetic meridian, and seldom recovered its original direction in less than eight or nine hours. The greatest extent of its aberration was 45". A delicate electrometer, suspended at the height of fifty feet from the ground, was never perceptibly affected by the aurora; nor could we distinguish its rustling noise, of which, however, such strong testimony has been given to us, that no doubt can remain of the fact.”

The account of the aurora observed at Fort Enterprize is also interesting and instructive with reference to the theory of the phenomenon.

“The shapes of the aurora observed at Fort Enterprize, at its entry into the horizon, and progress through the sky, may be reduced under two general descriptions. In the first I shall class those which are formed like rainbows or arches in the earliest stage of their appearance. They rise with their centres sometimes in the magnetic meridian, and sometimes several degrees to the eastward or westward of it. The number visible at the same time seldom exceeds five, and is seldom limited to one. The altitude of the lowest, when first seen, is never less than four degrees. As they advance towards the zenith, their centres (or the parts most elevated) preserve a course nearly in the magnetic meridian, or parallel to it. But the eastern and western extremities vary their respective distances, and the arches become irregularly broad streams in the zenith, each dividing the sky into two unequal parts, but never crossing one another till they separate into parts. Those arches which were bright at the horizon increase their brilliancy in the zenith, and discover the beams of which they are composed when the interior motion is rapid. This interior motion is a sudden glow, not proceeding from any visible concentration of matter, but bursting out in several parts of the arch, as if an ignition of combustible matter had taken place, and spreading itself rapidly towards each extremity. In this motion the beams are formed, such as are described in the preceding observations upon the subject. They have two motions; one at right angles to their length or sideways; and the other a tremulous [7:4:197]ßnd short vibration, in which they do not exactly preserve their parallelism to each other. By the first they project themselves into wreaths, serpentine forms, or irregular broken curves. The wreaths, when in the zenith, present the appearance of coronae boreales. The second motion is always accompanied with colours; for it must be observed, that beams are often formed without any exhibition of colours; and I have not, in that case, perceived the vibratory motion. The beams, in different aurorae, and sometimes in the same, are of different magnitudes, arising, probably, from their various distances. These evolutions, often repeated, destroy the shape and coherence of the several arches, though they doubtless retain the arched appearance to the eye of a spectator at the southern horizon; for it would be absurd to suppose that these changes occur only in the zenith of one particular place. The observations at different places in 1820 afford satisfactory proof to the contrary; and the number of arches often increased or diminished in their advance to the zenith, by a dismemberment of which, from their distance, we could not distinguish the particulars. However, their several parts passed gradually to the southward, where they assumed the form of an arch. They are also sometimes distributed into flashes, and other detached portions, which pass to the south-eastward. The revolution of an arch from north to south occupies a space of time varying from twenty minutes to two hours. At Cumberland House, the arches were in many instances almost stationary for several hours, a proof that if their motion was not slower, their distance from the earth was greater, than at Fort Enterprize. The arches, which are faint at the horizon, very frequently pass the zenith without any increase of brilliancy or apparent internal motion.

“The second general class of aurorae are those which propagate themselves from different points of the compass, between north and west towards the opposite points; sometimes also originating in the south-east quarter, and extending themselves towards the north-west. They may be subdivided, like the former, into the distant arches which pass to the southward without much visible change in their appearance, and those which discover beams, and separate at intervals into wreaths, flashes, and irregular segments, exhibiting all the phenomena described above. In explaining the mode by which the two general classes of aurorae are conducted into the horizon, I shall call the motion of the arches, which is in a plane seldom deviating more than two points from the magnetic meridian, the direct motion; and that by which the aurorae propagate themselves nearly at right angles to the magnetic meridian, the lateral motion. Let us suppose a mass of aurorae to be modelled at its birth in a longitudinal form, crossing the meridians at various angles, the whole to be impelled with a direct motion towards the magnetic south, but the parts having different velocities, and each extremity continually removing itself by a lateral motion from the centre, so as to increase the length of the mass. If the centre enter the northern horizon, it will appear like an arch, the real extremities being invisible; and its direct motion will carry it to the southward in that form. But if one extremity first enter the horizon, it will extend itself by its lateral motion to the opposite point, passing, at the same time, by its direct motion to the southward. Of the unequal velocities of the parts, we had many instances, in the direct motion of the arches, by which the centres were often carried from the zenith sixty or seventy degrees to the southward, while the extremities did not materially alter their positions. Nor can this be accounted for by any application of the rules of perspective, because arches exactly similar sometimes changed the bearings of their extremities in proportion to the advance of their centres; and at Cumberland House, on two occasions, the extremities of arches arrived at the east and west points of the compass, while their centres remained only ten degrees above the northern horizon.”

It was for a long time matter of doubt whether this meteor made its appearance only in the northern hemisphere, or whether it was also to be observed near the south pole. But the occurrence of the aurora in the antarctic regions was clearly ascertained by Mr Forster, who, in his voyage round the world with Captain Cook, observed it in the high southern latitudes, though with phenomena somewhat different from those which are seen here. On the 17th February 1773, when the navigators were in latitude 58° south, “a beautiful phenomenon was observed,” says Mr Forster, “during the preceding 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 spreading on the whole southern part of the sky. These columns were sometimes bent sidewise at their upper extremities; and though in most respects similar to the northern lights (aurora borealis) of our hemisphere, yet differed from them in being always of a whitish colour, whereas ours assume various tints, especially those of a fiery and purple huc. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point.” The accounts of subsequent navigators, particularly 7 Weddell, though given more in detail, differ in no material respect from that just quoted, and therefore need not be cited at length. They prove that this splendid meteor is not confined to the northern, but occurs also in the southern regions of the globe, though with considerable diversity in the accompanying phenomena.

Colour. — The colours of the polar lights are of various tints. The rays or beams are steel gray, yellowish gray, pea-green, celandine green, gold yellow, violet blue, purple, sometimes rose red, crimson red, blood red, greenish red, orange red, and lake red. Some of the beams appear as if tinged with black, and resemble dense columns of smoke. The arches are sometimes nearly black, passing into violet blue, gray, gold yellow, or white bounded by an edge of yellow. The colours are also sometimes vivid and prismatic. Maupertuis describes a very remarkable red-coloured polar light which he saw at Oswer Zornea on the 18th December 1786. An extensive region of the heavens towards the south appeared tinged of so lively a red, that the whole constellation Orion seemed as if dyed in blood. The light was for some time fixed, but soon became movable; and after having successively assumed all the tints of violet and blue, it formed a dome, of which the summit approached the zenith in the south-west. Its splendour was so great as to be in no degree affected by the bright light of the moon. Maupertuis adds, that he observed only two of these northern lights in Lapland, which are of very rare occurrence in that country, although the aurora there assumes a great variety of tints; hence they are considered by the natives as of portentous omen, and as the forerunner of some great calamity. These red-coloured polar lights have of late years been observed in the Shetland Islands; in many parts of Scotland, as in the neighbourhood of Edinburgh; and in England, from its northern to its southern extremity.

Lustre of the Polar Lights.-— The lustre varies in kind as well as intensity. Sometimes it is pearly, sometimes imperfectly vitreous, sometimes also metallic. Its degree of intensity varies from a very faint radiance to a light nearly equalling that of the moon.

Progressive Motions. — The reverend James Farquharson states that “the aurora borealis follows a determinate order [7:4:198] 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, 3° or 4° 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 25° 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 3° or 4°, sometimes, however, 15° or 20°.” (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 Mairanalso assigns 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 musfhave 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 aurorae, it is highly probable that the aurorae said by him to have been seen at the same time at great distances were different aurorae, 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[7:4:199]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 Enterprize, 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 aurorae. Biot, also, in the island of Unst, observed many aurorae that could not be higher than the region of clouds. Captain Franklin in like manner observed low aurorae. “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 Enterprize), 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 aurorae 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 aurorae 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 unes not higher than a thousand or fifteen hundred feet.

Position of the Polar Lights.— The arches of the polar gits 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 dippingneedle. 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 Enterprize, 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 au-, rora will require some description. “The motion com-ļ municated to it,” says Captain Franklin, “was neither sud-ļ den 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 [7:4:200]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 301°, or about 59° 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 36° to the west of the magnetic north, but not in so great a degree as when its bearing was about 301°. 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 245° to 234°, 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. Μ., 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 10". 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 needlebox 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 ⅟₄₀th 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 10" 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 Eorealis. — Having, many years ago, both in this country and in the Shetland Islands, heard very distinctly noises proceeding from the polar lights, we [7:4:201]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. Arctic Regions, and Journal of a Voyage to the Northern Whale Fishery. ] Richardson,^[2. Remarks on the Aurora Borealis, in Franklin and Richardson’s Journey to the Shores of the Polar Sea. ] 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 Crees 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 Nairnc 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. Giesecké, 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 histnteresting 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 m the Magazin fur Naturwidenshaberne, Christiana, 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 we, since the beginning of this century, in one of the great pauses of this phenomenon; so that the present generation 'nows 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. Wargentin, in the fifteenth volume of the Transactio7is 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 ligld, usually attending this magnetical meteor may possibly be derived from electricity, which maybe 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, [7:4:202]at too great a distance to be discovered by any other test.” {Lectures, vol. i. p. 716.) The fact is, that in magnetism, the agency of electricity is now clearly made out, and they are shown to stand to each other in the relation of effect and cause, at least in as far as that all the phenomena of magnetism are producible by electricity; but no electric phenomena have ever been hitherto produced by magnetism. (Herschel’s Preliminary Discourse, p. 93.)* Hence the aurora “is certainly connected with the general cause of magnetism,” in as far as it is a joint result or effect of the cause by which it is now known that all the phenomena of magnetism are producible; but this connection is one of relation or coincidence merely; and hence it is to the agency of electricity that the phenomena of the aurora are primarily to be ascribed. “This wonderful agent,” says Mr Herschel, “which we see in intense activity in lightning, and in a feebler and more diffused form traversing the upper regions of the atmosphere in the northern lights, is present, probably in immense abundance, in every form of matter which surrounds us, but becomes sensible only when disturbed by excitements of peculiar kinds.” {Prelim. Discourse, p. 329, 330.) Mr Canton, likewise, conceives that the aurora is occasioned by the rapid transition or passage of electrical matter from positive towards negative clouds, throughout the upper region of the atmosphere, where the resistance is the least; but he supposes that the aurora which appears at the time when the magnetic needle is disturbed is not the cause of this disturbance, which he attributes to the heat of the earth, and that it is merely the electricity of the heated air above, disengaged chiefly in the northern regions, where the alteration in the temperature of the air is greatest. Be this as it may, however, the experiment contrived by Mr Canton shows clearly that the phenomena of the aurora are intimately connected with, or rather consequent on, a particular constitution of the atmosphere in regard to density and electricity. If, for example, a glass tube about three feet in length be partially exhausted of air, hermetically sealed, and then applied longitudinally to the prime conductor of an electrical machine, the whole tube will be illuminated from end to end, and this illumination will continue for a considerable time after it has been removed from the conductor. If, again, the tube be drawn through the hand either way, the light will become remarkably intense throughout its whole length; and although a great part of the electricity will be discharged by this operation, the tube will nevertheless emit flashes at intervals, if held by one extremity and kept quite steady; while, if grasped by the other hand at a different place, vivid flashes of light will dart from one end to the other, and continue to be emitted for a considerable time without any fresh excitation. It is to be observed, however, that if the density of the air included in the tube be either increased or diminished beyond a certain limit, none of these luminous appearances will be exhibited; and it is this circumstance which shows that the phenomena of the aurora depend on a certain constitution of the atmosphere in point of density and electricity, and that the meteoric appearances or coruscations can never attain the elevation ascribed to them by some philosophers, but must in every case be confined within the limits of our atmosphere, conformably to the observations of Biot and others already detailed. Beccaria, indeed, conjectures that there is a constant and regular circulation of the electric fluid from north to south; and he gives it as his opinion that the aurora borealis is this electric matter performing its transit southwards, in a state of the atmosphere which renders it visible, or approaching nearer than usual to the earth. But such a transition or circulation as that here suggested could only be produced by the diurnal revolution of the earth around its axis; and as the meteor is observed in the southern with nearly the same appearances as in the northern hemisphere, there must consequently be a circulation from south to north as well as from north to south; in other words, if there be any such tendency as that which Beccaria supposes, it must proceed equally or nearly so from both poles towards the equator. The following is a list of the most important papers, treatises, and works on this subject: Berlin Mem. 1710, i. 131; Halley, Phil. Trans. 1716, 1719, xxix. 406, xxx. 584; Hearne, Phil. Trans. XXX. 1107; Langworth, Huxham, Hallet, and Cal-lendrini, Phil. Trans, xxxiv. 132, 150; Maier, C. Petr. iv. 121; Mairan, Traite de l’ Aurore Boreale, 1733, 1754; Weidler, de Aurora Boreali, 4; Wargentin, Phil. Τrαns. 1751, p. 126, and History, 1752, p. 169, 1753, p. 85; Bergmann, Schw. Abh. 200, 251; Wiedeburg, über die Nordlichter, 8, Jena, 1771; Hüpsch, Untersuchung des Nordlichts, 8, Cologne, 1778; Van Swinden, Recueil de Mémoires, Hague, 1784; Cavallo, Phil. Trans. 1781, p. 329; E. Μ. Physique, art. Aurore Boréale; Wilke von den Neuesten Erhlarungen des Nordlichts, Schwedisches Mus. 8, Wismar, 1783; Hey, Wollaston, Hutchinson, Franklin, Pigott, and Cavendish, Phil. Trans. 1790, pp. 32, 47, 101; Dalton’s Meteorological Observations, 1793, p. 54, 153; Chiminello on a Luminous Arch, Soc. Ital. vii. 153; Ritter on the Lunar Periods of the Aurora, Gilb XV. 206.