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to be zodiacal light; which, according to him, is no more than the sun's atmosphere: this light, happening on some occasions to meet the upper parts of our hemisphere, about the limits where uni-. versal gravity begins, which it passes. On the contrary, Euler conceived its cause to be particles of our atmosphere driven be yond their limits by the impulse of the solar light. He supposes the zodiacal light and the tails of comets to have a similar origin.

But ever since the identity of lightning and of electric matter has been determined, philosophers have been led to geek for the explication of the aurora wholly, or for the most part, in the prin. ciples of electricity. Beside the more obvious and known ap pearances which constitute a resemblance between this meteor and the electric matter by which lightning is produced, it has been observed that the aurora, like lightning, occasions a very sensible fluctuation in the magnetic needle; and that when it has extended lower than usual in the atmosphere, the corruscations have been attended with various sounds of rumblings and hissing, especially in Russia, and the other more northern parts of Europe, as no ticed by Signior Beccaria and M. Messier. Mr. Canton, soon after he obtained electricity from the clouds, offered a conjecture that the aurora is occasioned by the dashing of electric fire in positive toward negative clouds, at a great distance through the upper part of the atmosphere, where the resistance is least; and adds, that the aurora is said by the northern people to be remark. ably strong when a sudden thaw happens after severe cold. The best paper upon this subject is that which was written by Dr. Franklin, and read to the Royal Academy of Sciences at Paris, in 1779; entitled by him "Suppositions and Conjectures towards forming an hy pothesis for the explanation of the Aurora Borealis," His reasoning is as follows: "Air heated by any means becomes rarefied, and specifically lighter than other air in the same situa tion not heated; and when lighter it rises, and the neighbouring cooler and heavier air takes its place. If in the middle of a room you heat the air by a stove, or pot of burning coals, near the floor, the heated air will rise to the ceiling, spread over the cooler air till it comes to the cold walls; there being condensed and made heavier, it descends, to supply the place of that cool air which had moved towards the stove or fire, in order to supply the place of the heated air which had descended. Thus there will

be a continual circulation of air in the room, which may be rendered visible by making a little smoke, for that smoke will rise and circulate with the air.

"A similar operation is performed by nature on the air of this globe. Above the lower regions of our atmosphere the air is so rare as to be almost a vacuum. The air heated between the tropics is continually rising; its place is supplied by northerly and southerly winds, which come from the cooler regions. The light heated air floating above the cooler and denser, must spread northward and southward, and descend near the two poles, to supply the place of the cool air, which had moved towards the equator. Thus a circulation of air is kept up in our atmosphere, as in the room above mentioned. That heavier and lighter air may move in cur. rents of different and opposite directions, appears sometimes by the clouds that happen to be in those currents, as plainly as by the smoke in the experiment above mentioned; also, in opening a door between two chambers, one of which has been warmed, by holding a candle near the top, near the bottom, and near the middle, you will find a strong current of warm air passing out of the warmed room above, and another of cool air entering below, while in the middle there is little or no motion.

"The great quantity of vapour rising between the tropics forms clouds, which contain much electricity; some of them fall in rain, before they come to the polar regions. Every drop brings down some electricity with it; the same is done by snow or hail; the electricity so descending, in temperate climates, is received and imbibed by the earth. If the clouds be not sufficiently discharged by this gradual operation, they sometimes discharge themselves suddenly, by striking into the earth, where the earth is fit to receive their electricity. The earth, in temperate and warm climates, is generally fit to receive it, being a good conductor.

"The humidity contained in all the equatorial clouds that reach the polar regions, must there be condensed, and fall in snow. The great cake of ice that eternally covers those regions may be too hard frozen to permit the electricity, descending with that snow, to enter the earth. It may therefore be accumulated upon that ice. The atmosphere being heavier in the polar regions than in the equatorial, will there be lower; as well from that cause, as from the smaller effect of the centrifugal force: consequently

the distance of the vacuum above the lower part of the atmosphere will be less at the poles than elsewhere, and probably much less than the distance (upon the surface of the globe) extending from the pole to those latitudes in which the earth is so thawed as to receive and imbibe electricity. May not then the great quantity of electricity brought into the polar regions by the clouds, which are condensed there, and fall in snow, which electricity would enter the earth, but cannot penetrate the ice; may it not, as a bottle overcharged, break through that low atmosphere, and run along in the vacuum over the air towards the equator; diverging as the degrees of longitude enlarge; strongly visible where densest, and becoming less visible as it more diverges; till it finds a passage to the earth in more temperate climates, or is mingled with the upper air? If such an operation of nature were really performed, would it not give all the appearances of an aurora borealis? And would not the auroras become more frequent after the approach of winter; not only because more visible in longer nights, but also because in summer the long presence of the sun may soften the surface of the great ice-cake, and render it a con. ductor, by which the accumulation of electricity in the polar regions will be prevented?

"The atmosphere of the polar regions being made more dense by the extreme cold, and all the moisture in that air being frozen, may not any great light arising therein, and passing through it, render its density in some degree visible during the night time, to those who live in rarer air of more southern latitudes? And would it not in that case, although in itself a complete and full circle, extending perhaps ten degrees from the pole, appear to spectators so placed (who could see only a part of it) in the form of a segment, its chord resting on the horizon, and its arch elevated more or less above it, as beheld from latitudes more or less distant; darkish in colour, but yet sufficiently transparent to permit some stars to be seen through it.

"The rays of electric matter issuing out of a body, diverge by mutually repelling each other, unless there be some conducting body near to receive them and if that conducting body be at a greater distance, they will first diverge, and then converge, in order to enter it. May not this account for some of the varieties of figures, seen at times, in the motions of the luminous matter of

the auroras: since it is possible, that in passing over the atmo. sphere from the north, in all directions or meridians, towards the equator, the rays of that matter may find, in many places, por. tions of cloudy region, or moist atmosphere under them, which (being in the natural or negative state) may be fit to receive them, and towards which they may therefore converge and when one of those receiving bodies is more than saturated, they may again diverge from it, towards other surrounding masses of such humid atmosphere, and thus form the crowns, as they are called, and other figures mentioned in the histories of this meteor ?"

That similar corruscations are frequently visible in southern lati. tudes was not known until Captain Cook made his first voyage round the world. He speaks of an appearance of this kind on the 16th of September, 1770, about ten o'clock at night; it consisted of a dull, reddish light, and reached about twenty degrees above the horizon; its extent was very dif. ferent at different times, but it was never less than eight or ten points of the compass. Through and out of this, passed rays of light of a brighter colour, which vanished and were renewed nearly in the same time as those in the aurora borealis, but had little or no vibration. The body of it bore S.S.E. from the ship; and continued without any diminution of its brightness till twelve o'clock, when the observers retired. The ship was at this time within the tropic of Capricorn.

In the course of his second voyage, Captain Cook remarks, that on February 17, 1773, "a beautiful phenomenon was ob served in the heavens; it consisted of long colours of a clear white light, shooting up from the horizon to the eastward, almost to the zenith, and spreading gradually over the whole southern part of the sky. These columns even sometimes bent sideways at their upper extremity; 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. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freez

ing point; the ship being then in 58° south.*" Forster speaks of the same phænomenon, as well as six others, on different nights, as February 19, 20, 21, and 26, March 15 and 20.†

Dalton conceives, and it is the common hypothesis of the day, that the apparent beams of the aurora are the projections of cylindrical portions of a magnetic fluid, which are actually parallel to the dipping needle, and hence seem to converge to the magnetic pole; and that the light is produced by the transmission of electricity through them, which somewhat disturbs their magnetic properties. The arches are always perpendicular to the magnetic meridian; and being more permanent in their form, afford an opportunity of determining the height; which from one observa. tion, on a base of 22 miles, Dalton calculated to be 150 miles.

This calculation is almost a mean between the computations of other observers: for while Cavendish states it at from 52 to 71 miles, Cramer advances it to 160 leagues, and Eames to not less than 200 leagues. Cavendish, however, admits that the nature of the light may make the appearance different in different places, and renders distant observations fallacious.

Eames supposes the aurora to be derived from the sun's atmo. sphere, extending in some directions beyond the earth's orbit; and attributes the nebulæ of stars and the tails of comets to a similar substance.

Van Swinden observes that the variation of the magnetic needle increases when the aurora borealis is approaching.||

Winn remarks, that the aurora is generally followed the day after by a stream from the south or southwest.§

Blagden and Gmelin offer several testimonies of a rustling noise that occasionally accompanied it.

EDITOR.

• Second Voyage, 1. 53.

+ Observations in a Voyage round the World, p. 120.

Petr. Acad. 1783,

Phil. Trans, 1774.

Id. 1781.

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