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 defined 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-colored chariot with four horses.

**Aurora Borealis, Northern Twilight**, or **Streamers**, a kind of meteor appearing in the northern part of the heavens, mostly in the winter-time, and in frosty weather. It is now so generally known, that no description is requisite; and indeed the variety of its appearances is so great, as to render a particular description almost impossible.

The aurora borealis, though now so common, seems formerly to have been very rare: and indeed we can scarce avoid attributing to this meteor many of the prodigies that are mentioned by ancient historians, as the flame that appeared over the city of Constantinople, those before the siege of Jerusalem, &c., and its appearances being accounted ominous, shews that it hath in those ages been seen very seldom, in comparison to what it is now. This we are assured of by Dr Halley, who tells us, that when he saw a great aurora borealis in 1716, he had begun to despair of ever seeing one at all; none having appeared, at least in any considerable degree, from the time he was born till then. Notwithstanding this long interval, however, it seems that in some periods the aurora borealis had been seen much more frequently; and perhaps this, as well as other natural phenomena, may have some stated times of returning.

The only thing that resembles a distinct history of this phenomenon, is what we have from the learned Dr Halley, Phil. Trans. n° 347. The first account he gives, is of the appearance of what is called by the author, burning spears, and was seen at London on January 30th, 1560. This account is taken from a book entitled *A Description of Meteors* by W. F. D. D. and reprinted at London in 1654. The next appearance, on the testimony of Stow, was on October 7th, 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, with much the same appearances as described by Dr Halley in 1716, and which we now so frequently observe. Again, the same 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 Gemma, professor of medicine in the university of Lovain, who seems to have been struck with the utmost horror at the sight; and compares them to spears, fortified cities, and armies fighting in the air. Concerning these four appearances, Dr Halley remarks that they all happened at the same age of the moon, viz. two days after the change. After this, Michael Maellin, tutor to the great Kepler, affirms us, that at Baknang in the county of Wurtemberg in Germany, these phenomena, which he styles *chaosmata*, 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 times of the same year. In 1621, September 24, this phenomenon was observed all over France, and described by Gassendus, who gave it the name of *aurora borealis*: yet neither this, nor any similar appearances posterior to 1574, are described by English writers, till the year 1707; which, as Dr Halley observes, shows the prodigious neglect of curious matters which at that time prevailed. From 1621 to 1707, indeed, there is no mention made of an aurora borealis being seen by any body; and conferring the number of astronomers who during that period were in a manner continually poring on the heavens, we may very reasonably conclude that no such thing did make its appearance till after an interval of 86 years. In 1707, a small one was seen in November; and during that year and the next, the same appearances were repeated five times. The next on record, is that mentioned by Dr Halley in March 1716; since which time they have been so common, that no accounts have been kept of them.

It was for a long time a matter of doubt whether Mr Foster's meteor made its appearance only in the northern hemisphere, or whether it was also to be observed near the south pole. This is now ascertained by Mr Foster; who, in his late voyage round the world along with captain Cook, affirms us, that he observed them in the high southern latitudes, though with phenomena somewhat different from those which are seen here. On Feb. 17, 1775, as they were in Lat. 58° south, "A beautiful phenomenon (says he) was observed 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 hue. The stars were sometimes hid by, and sometimes faintly to be seen through, the substance of these southern lights (*aurora australis*), which have hitherto, as far as I can find, escaped the notice of voyagers. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point." On the 16th and 19th of March the same year, the aurora australis was again observed, and the last time rather brighter than before, and the columns formed an arch across the sky.

Dr Halley observed that the aurora borealis described by him arose to a prodigious height, it being seen high from the west of Ireland to the confines of Russia and Poland on the east; nor did he know how much further it might have been visible: so that it extended at least 30 degrees in longitude; and, from lat. 50 north, 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 appearances were exhibited which Dr Halley observed at London. 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 as easily have calculated the height of this aurora borealis, as he did of the fiery globe. globe in 1719. To other philosophers, however, he gives the following exhortation. "When therefore for the future any such thing shall happen, all those that are curious in astronomical matters are hereby admonished and intrusted to set their clocks to the apparent time at London, for example, by allowing so many minutes as is the difference of meridians; and then to note, at the end of every half hour precisely, the exact situation of what at that time appears remarkable in the sky; and particularly the azimuths of those very tall pyramids so eminent above the rest, and therefore likely to be seen forthwith: to the intent that, by comparing these observations taken at the same moment in distant places, the difference of their azimuths may serve to determine how far these pyramids are distant from us."—With this pathetic advice no two philosophers have yet thought proper to comply, notwithstanding the multitude of opportunities that have since occurred; a shameful instance of philosphic indolence! The only faint attempt towards ascertaining this matter we find in the Philosophical Transactions, Vol. LXIV., where a correspondent in North America acquaints Dr Franklin then at London, that an aurora borealis had been seen in America, and asks whether on the same night there had been any such appearance at London. The Doctor replies, that there had indeed been an aurora borealis that night; and that, if it had likewise been seen in America, it must have been very high.

With regard to the cause of the aurora borealis, many conjectures have been formed. The first which naturally occurred, was, that it was occasioned by the ascent of inflammable sulphureous vapors from the earth, which taking fire in the rarer regions of the atmosphere, exhibited, by its flashing in different directions, the beautiful appearances we often observe in the aurora borealis. To this supposition Dr Halley objects the immense extent of such phenomena, and that they are constantly observed to proceed from north to south, but never from south to north. This made him very reasonably conclude, that there was some connection between the poles of the earth and the aurora borealis. The Doctor was in a great measure unacquainted with the electric power, though he well knew that of magnetism: he supposed therefore, that this earth was hollow, having within it a magnetic sphere, which corresponded in virtue with all the natural and artificial magnets on the surface; the poles of the central magnet not corresponding exactly with those of the outer shell of earth, he thought might be the reason why the magnetic needle did not always point due north; and the magnetic effluvia passing through the earth, from one pole of the central magnet to another, might sometimes become visible, in their course, which he thought was from north to south, and thus exhibit the beautiful curvatures of the aurora borealis. This conjecture, for it could be no more, has commonly been looked upon as an extravagant flight of fancy; but if we consider the matter thoroughly, we shall find it an undoubted proof of a most profound and solid genius. Dr Halley, as already observed, was in a manner totally ignorant of the powers of electricity, and therefore was obliged to work with such materials as he had. Had he known that a stroke of electricity would give polarity to a needle that had it not, or reverse the poles of one that had it before, he would undoubtedly have concluded the electric and magnetic effluvia to be the same, and that the aurora borealis was this fluid performing its circulation from one pole of the earth to the other. In fact, this very hypothesis is adopted by S. Beccaria; and by the supposed circulation of the electric fluid he accounts for the phenomena of magnetism and the aurora borealis in a manner perfectly similar to that of Dr Halley, only changing the phrase magnetic effluvia for electric fluid. The following is the account given us by Dr Priestley, of Beccaria's sentiments on this matter.

"Since a sudden stroke of lightning gives polarity to magnets, he conjectures, that a regular and constant circulation of the whole mass of the fluid from north to south may be the original cause of magnetism in general. This is truly a great thought; and, if just, will introduce greater simplicity into our conceptions of the laws of nature.

"That this ethereal current is insensible to us is no proof of its non-existence, since we ourselves are involved in it. He had seen birds fly so near a thunder-cloud, as he was sure they would not have done had they been affected by its atmosphere.

"This current he would not suppose to arise from one source, but from several, in the northern hemisphere of the earth. The aberration of the common centre of all these currents from the north point may be the period of the variation, and the obliquity with which the currents strike into the earth may be the cause of the dipping of the needle, and also why bars of iron more easily receive the magnetic virtue in one particular direction.

"He thinks that the aurora borealis may be this electric matter performing its circulation in such a state of the atmosphere as renders it visible, or approaching the earth nearer than usual. Accordingly very vivid appearances of this kind have been observed to occasion a fluctuation in the magnetic needle."

Here we must first remark, that, if this thought, as Dr Priestley says, is truly great, its greatness is justly due to Dr Halley: we cannot suppose S. Beccaria to have been ignorant of Dr Halley's magnetic hypotheses; and if he was not ignorant of it, it is impossible to avoid believing him to be tacitly obliged to it. On the thought itself, however, we must observe, that a circulation of an immense quantity of electric fluid round the earth bears a striking similitude to one of Descartes' vortices; and as such a circulation hath by no means been proved, this very circumstance ought to make us cautious of receiving it. But besides this, Mr Forster, in the passage above quoted, furnishes us with a direct disproof of this circulation; with which, though neither Dr Halley nor S. Beccaria could be acquainted, they might have thought of it as a final proof either of the truth or falsehood of their hypotheses.—If the aurora borealis is no other than the electric fluid performing the abovementioned circulation, it ought to dart from the horizon towards the zenith in the northern hemisphere, and from the zenith to the horizon in the southern one: but Mr Forster plainly tells us, that the columns shot up from the horizon towards the zenith as well in the southern hemisphere as in the northern; so that if the aurora borealis is to be reckoned the flashings of electric matter, its course is plainly directed from both poles towards the equator, Concerning the cause of this phenomenon Mr Canton has the following query: "Is not the aurora borealis the flashing of electrical fire from positive towards negative clouds at a great distance, through the upper part of the atmosphere where the resistance is least?" But to this we must reply in the negative; for in this case it would flash in every direction according to the position of the clouds, as well as from north to south. Besides this query, he conjectures, that when the needle is disturbed by the aurora borealis, that phenomenon proceeds from the electricity of the heated air; and supposes the air to have the property of becoming electric by heat, like the tourmalin. But neither does this hypothesis appear at all probable; because, in such a case, the aurora borealis ought to be most frequent in summer when the air is most heated, whereas it is found to be the reverse.—Lastly, with these electrical hypotheses we shall contrast that of Mr Mairan, who imagined this phenomenon to proceed from the atmosphere of the sun, particles of which were thrown off by its centrifugal force acquired by his rotation on his axis; and that these particles falling upon the atmosphere of the earth near its equatorial parts, were from thence propelled by the diurnal motion of the earth towards the polar regions, where they formed the aurora borealis. This hypothesis, besides its being a mere supposition unsupported by one single appearance in nature, is liable to the objection already mentioned; for in this case the light should dart from the equator to the poles, and not from the poles to the equator: or if we should suppose this matter to be gradually accumulated at each of the poles, we must then make other suppositions, equally vague and ill founded, concerning its getting back with such surprising rapidity in direct opposition to the power which once brought it thither.

The first person that seems to have endeavoured to find any positive proof for the electrical quality of the aurora borealis, is Dr Hamilton of Dublin. He observes, that though this phenomenon is commonly supposed to be electrical, yet he had not seen any attempt to prove that it is so: but the only proof he himself brings is an experiment of Mr Hawkbee, by which the electric fluid is shown to put on appearances somewhat like the aurora borealis, when it passes through a vacuum. He observed, that when the air was most perfectly exhausted, the streams of electric matter were then quite white; but when a small quantity of air was let in, the light assumed more of a purple colour. The flashing of this light therefore from the dense regions of the atmosphere into such as are more rare, and the transitions through mediums of different density, he reckons the cause of the aurora borealis, and of the different colours it assumes.

Dr Hamilton's proof, then, of the electricity of the aurora borealis, consists entirely in the resemblance the two lights bear to one another; and if to this we add, that, during the time of an aurora borealis, the magnetic needle hath been disturbed, electric fire obtained from the atmosphere in plenty, and at some times different kinds of rumbling and hissing sounds heard, we have the sum of all the positive evidence in favour of the electric hypothesis.

Was the aurora borealis the first natural phenomenon the solution of which had been attempted by electricity, no doubt the proofs just now adduced would be very insufficient: but when it is considered, that we have indisputable evidence of the identity of the phenomena of thunder and of electricity; when we also consider, that the higher parts of our atmosphere are continually in a strongly electrified state; the analogy becomes so strong, that we can scarce doubt of the aurora borealis arising from the same cause. The only difficulty is, to give a good reason why the electricity of the atmosphere should be constantly found to direct its course from the poles towards the equator, and not from the equator to the poles; and this we think may be done in the following manner.

1. It is found that all electric bodies, when considerably heated, become conductors of electricity; thus highly pithy hot air, hot glass, melted rosin, sealing wax, &c. are fine all conductors, till their heat is dissipated, and then they again become electrics.

2. As the converse of every true proposition ought also to be true, it follows from the above one, that if electrics when heated become conductors, then non-electrics when subjected to violent degrees of cold ought to become electric. In one instance this has been verified by experience: water, which is a conductor when warm or not violently cooled, is found to become electric when cooled to 20° below 0 of Fahrenheit's thermometer. With regard to metallic substances, indeed, no experiments have as yet been made to determine whether their conducting power is affected by cold or not. Very probably we might not be able to produce such a degree of cold as sensibly to lessen their conducting power; but still the analogy will hold; and, as we are by no means able to produce the greatest degree of cold possible, reason will always suggest to us, that if a certain degree of cold changes one conductor into an electric, a sufficient degree of it will also change all others into electrics.

3. If cold is sufficient to change conducting substances into electrics, it must also increase the electric power of such substances as are already electric; that is to say, very cold air, glass, rosin, &c. provided they are dry, will be more electric than when they are warmer. With regard to air, which is most to our present purpose, this is rendered extremely probable, by considering that clear frosty weather is of all others the most favourable for electric experiments. They may be made indeed to equal advantage almost in any state of the atmosphere, provided sufficient pains is used, but in dry hard frosts they will succeed much more easily than at any other time.

These three axioms being allowed, the cause of the aurora borealis is easily deduced from them. The air, all round the globe, at a certain height above its surface, is found to be exceedingly cold, and, as far as experiments have yet determined, exceedingly electrical also. The inferior parts of the atmosphere between the tropics, are violently heated during the day-time by the reflection of the sun's rays from the earth. Such air will therefore be a kind of conductor, and much more readily part with its electricity to the clouds and vapours floating in it, than the colder air towards the north and south poles. Hence the prodigious appearances of electricity in these regions, shewing itself in thunder and other tempests of the most terrible kind. Immense quantities of the electric fluid are thus communicated. nicated to the earth, and the inferior warm atmosphere having once exhausted itself must necessarily be recruited from the upper and colder region. This becomes very probable from what the French mathematicians observed when on the top of one of the Andes. They were often involved in clouds, which, sinking down into the warmer air, appeared there to be highly electrified, and discharged themselves in violent tempests of thunder and lightning; while in the mean time, on the top of the mountain, they enjoyed a calm and serene sky.

In the temperate and frigid zones, the inferior parts of the atmosphere never being so strongly heated, do not part with their electricity so easily as in the torrid zone, and consequently do not require such recruits from the upper regions; but notwithstanding the difference of heat observed in different parts of the earth near the surface, it is very probable that at considerable heights the degree of cold is nearly equal all round it. Were there a like equality in the heat of the under part, there could never be any considerable loss of equilibrium in the electricity of the atmosphere; but as the hot air of the torrid zone is perpetually bringing down vast quantities of electric matter from the cold air that lies directly above it; and as the inferior parts of the atmosphere lying towards the north and south poles do not conduct in any great degree; it thence follows, that the upper parts of the atmosphere lying over the torrid zone will continually require a supply from the northern and southern regions. This, easily shows the necessity of an electric current in the upper parts of the atmosphere from each pole towards the equator; and thus we are also furnished with a reason why the aurora borealis appears more frequently in winter than in summer; namely, because at that time the electric power of the inferior atmosphere is greater on account of the cold than in summer; and consequently the abundant electricity of the upper regions must go almost wholly off to the equatorial parts, it being impossible for it to get down to the earth; hence also the aurora borealis appears very frequent and bright in the frigid zones, the degree of cold in the upper and under regions of the atmosphere being much more nearly equal in these parts than in any other. In some parts of Siberia, particularly, this meteor appears constantly from October to Christmas, and its coruscations are said to be very terrifying. Travellers agree, that here the aurora borealis appears in greatest perfection; and it is to be remarked, that Siberia is the coldest country on earth. In confirmation of this, it may also be observed, that, from the experiments hitherto made with the electrical kite, the air appears considerably more electrical in winter than in summer, though the clouds are known to be often most violently electrified in the summer time; a proof, that the electricity naturally belonging to the air is in summer much more powerfully drawn off by the clouds than in the winter, owing to the excess of heat in summer, as already observed.

A considerable difficulty, however, still remains, from the upright position which the streams of the aurora borealis are generally observed to have; whereas, according to the hypothesis above mentioned, they ought rather to run directly from north to south. This difficulty occurred to Dr Halley: but he answers it by supposing his magnetic effluvia to pass from one pole to another in arches of great circles, arising to a vast height above the earth, and consequently darting from the places whence they arose almost like the radii of a circle; in which case, being sent off in a direction nearly perpendicular to the surface of the earth, they must necessarily appear erect to those who see them from any part of the surface, as is demonstrated by mathematicians. It is also reasonable to think that they will take this direction rather than any other, on account of their meeting with less resistance in the very high regions of the air than in such as are lower.

But the greatest difficulty still remains: for we have supposed the equilibrium of the atmosphere to be broken in the day-time, and restored only in the night; whereas, considering the immense velocity with which the electric fluid moves, the equilibrium ought to be restored in all parts almost instantaneously; yet the aurora borealis never appears except in the night, although its brightness is such as must sometimes make it visible to us did it really exist in the day-time.

In answer to this it must be observed, that though the passage of electricity through a good conductor is instantaneous, yet through a bad conductor it is observed to take some time in passing. As our atmosphere therefore, unless very violently heated, is but a bad conductor of electricity; though the equilibrium in it is broken, it can by no means be instantaneously restored. Add to this, that as it is the action of the sun which breaks the equilibrium, so the same action, extending over half the globe, prevents almost any attempt to restore it till night; when flashes arise from various parts of the atmosphere, gradually extending themselves with a variety of undulations towards the equator.

It now remains to explain only one particularity of the aurora borealis, namely, that its streams do not always move with rapidity, sometimes appearing quite stationary for a considerable time, and sometimes being carried in different directions with a flow motion. To this indeed we can give no other reply, than that weak electric lights have been sometimes observed to put on the same appearance at the surface of the earth; and much more may we suppose them capable of doing so at great heights above it, where the conductors are both fewer in number and much more imperfect. When M. de Romas was making experiments with an electric kite in Italy, a cylinder of blue light about four or five inches diameter was observed surrounding the string. This was in the day-time; but had it been night, he imagined it must have been four or five feet in diameter; and as the string was 780 feet long, it would probably have seemed pyramidal, pointing upwards like one of the streams of the aurora borealis. A still more remarkable appearance, Dr Priestley tells us, was observed by Mr Hartman. He had been making electrical experiments for four or five hours together in a very small room; and upon going out of it, and returning with a light in his hand, walking pretty quick, he perceived a small flame following him at about three feet distance. Being alarmed at this appearance, he stopped to examine it, upon which it vanished. This last instance is very remarkable, and singular in its kind: from both, however, we are sufficiently warranted to conclude, that small portions of our atmosphere may by various causes be so much electrified as to shine, and likewise be moved from one place to another without parting with the electricity they have. have received, for a considerable time.

The corona, or circle, which is often formed near the zenith by the aurora borealis, is easily accounted for in the same manner. As this corona is commonly stationary for some time, we imagine it would be a very proper mark whereby to determine the distance of the meteor itself. If an aurora borealis, for instance, was observed by two persons, one at London, and the other at Edinburgh; by noting the stars among which the corona was observed at each place, its true altitude from the surface of the earth could easily be determined by trigonometry.

Under the article Atmosphere it was suggested, that no good proof had been as yet brought for the extreme rarity of the air usually supposed to take place at no very great heights above the earth. The brightness of the meteor there mentioned at 70 miles perpendicular from the surface, as also its figure, seemed to prove the air considerably denser at that distance from the earth. Though the height of the aurora borealis has never been determined, we can scarcely imagine it to be greater than that of this meteor, or indeed so great; but although its streams resemble the passage of electric light through a vacuum, it cannot be from thence inferred, that the air is at all in a state similar to the vacuum of an air-pump in those places where the aurora borealis is produced; seeing we have instances of similar appearances being produced in very dense air. The plate of an electrophorus is often so highly electrified, as to throw out flashes from different parts as soon as it is lifted up, and by proper management it may be always made to emit long and broad flashes which shall scarcely be felt by the finger, instead of small, dense, and pungent sparks; so that, though long flashes may be produced in rarefied air, it by no means follows that the same may not also be produced in denser air. As little can we infer any thing from the colours; for we observe the electric spark sometimes white, sometimes blue, and sometimes purple, in the very same state of the atmosphere, and from the same substance.

We shall conclude this article with an account of a paper presented to the Royal Society by Mr. Winn, in 1772, wherein he says that the appearance of an aurora borealis is a certain sign of an hard gale of wind from the south or south-west. This he never found to fail in 23 instances; and even thinks, that from the splendor of the meteor, some judgment may be formed concerning the ensuing tempest. If the aurora is very bright, the gale will come on within twenty-four hours, but will be of no long duration; if the light is faint and dull, the gale will be less violent, and longer in coming on, but will also last longer. His observations were made in the English channel, where such winds are very dangerous; and by attending to the aurora, he says he often got easily out of it, when others narrowly escaped being wrecked. This is an exceeding useful observation for sailors; but it cannot be expected that the winds succeeding these meteors should in all places blow from the south-west; though no doubt a careful observation of what winds succeed the aurora borealis, and other meteors, in different parts of the world, might contribute in some measure to lessen the dangers of navigation.

That the aurora borealis ought to be succeeded by winds, may be easily deduced from the hypothesis last mentioned. If this phenomenon is occasioned by the vast quantity of electric matter conveyed to the equatorial parts of the earth, it is certain that the earth cannot receive any great quantity of this matter at one place without emitting it at another. The electricity, therefore, which is constantly received at the equator, must be emitted nearer the poles, in order to perform its course; otherwise there could not be a constant supply of it for the common operations of nature. It is observed, that electrified bodies are always surrounded by a blast of air, which is sent forth from them in all directions; hence, if the electric matter find a more ready passage through one part of the earth than another, a wind will be found to blow from that quarter. If therefore one of these places happens to be in the Atlantic ocean near the coast of France, or in the bay of Biscay, the electric matter which has been received at the equator during an aurora borealis will be discharged there some time after, and consequently a wind will blow from that quarter, which will be from the south-west to those ships which are in the English channel. It cannot be imagined, however, that all the matter can be discharged from one place; and therefore, according to the different situations of those electrical vents, winds may blow in different directions; and thus the same aurora borealis may produce a south-west wind in the English channel, and a north-west one in Scotland.