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become clear, be poured it out carefully, without disturbing such of the iron sediment as still remained, which now appeared reduced almost to impalpable powder. This powder was afterwards removed into another vessel, in order to dry it; but as he had not obtained a proper quantity of it by this first step, he was obliged to repeat the process many times.
Having at last procured enough of this very fine powder, the next thing to be done was to make a paste of it, and that with some vehicle which would contain a considerable quantity of the phlogistic principle ; for this purpose he had recourse to linseed oil in preference to all other fluids With these two ingredients only, he made a stiff paste, taking a particular care to knead it well before he moulded it into convenient shapes. Sometimes, while the paste continued in its soft state, he would put the impression of a seal on the several pieces; one of which is in the British Museum. This paste was then put upon wood, and sometimes on tiles, in order to bake or dry it before a moderate fire, at about a foot distance. The doctor found, that a moderate fire was most proper, because a greater degree of heat made the composition frequently crack ia many places.
The time required for the baking or drying of this paste was generally five or six hours, before it attained a sufficient degree of hardness. When that was done, and the several baked pieces were become cold, he gave them their magnetic virtue in any direc. tion he pleased, by placing them between the extreme ends of bis large magazine of artificial magnets for a few seconds or more, as he saw occasion. By this method the virtue they acquired was such, that when any one of those pieces was held between two of his best ten guinea bars, with its poles purposely inverted, it im. mediately of itself turned about to recover its natural direction, which the force of those very powerful bars was not sufficient to counteract.
[Phil. Trans, 1779.
AURORA BOREALIS AND AUSTRALIS.
General History and Remarks. The reader who carefully attends to the different sections of the present chapter, will see the propriety of our having preceded it by a chapter explanatory of the general laws and phænomena of magnetism : since he will tind sufficient reason, if we mistake not, for concurring in the general opinion of the day, that this splendid meteor is the result of a combinative of the two powers of magnetism and electricity.
When the light or aurora appears chiefly in the north part of the heavens, it is AURORA BOREALIS, or NORTHERN LIGHTs; and when chiefly in the south part, AURORA AUSTRALIS, or SOUTHERN Lights. Where the corruscation is more than ordinarily bright and streaming, which, however, seldom occurs except in the north, it is denominated LUMEN BOREALE; and where these streams have assumed a decided curvature, like that of the rain-bow, they are distinguished by the name of LUMINOUS ARCHES.
The Aurora is chiefly visible in the winter season and in frosty weather. It is usually of a reddish colour, inclining to yellow, and sends out frequent corruscations of pale light which seem to rise from the horizon in a pyramidical, undulating form, shooting with great velocity up to the zenith. This meteor never appears near the equator, but of late years has frequently been seen to. ward the south pole, and when in that situation is, as above, called Aurora Australis, or southern lights; though this is to use the same term in two different senses.
It seems that the aurora borealis has appeared at some periods more frequently than at others. They were so rare in England, or else were so little regarded, that none are recorded in our annals between a remarkable one observed on the 14th November 1754, and a very brilliant one on the 6th of March, 1716, and the two succeeding nights, but much the strongest on the first night, ex. cept that five small ones were noticed in the years 1707 and 1708. Hence it may be inferred, that either the air or earth, or perhaps both, are not at all times in such a state as tends to produce this phenomenon.
The extent of these appearances are also amazingly great; that in March, 1716, was visible from the west of Ireland to the confines of Russia, and the east of Poland, extending over, at least, thirty degrees of longitude, and from abont the fiftieth degree of latitude over almost all the northern part of Europe ; and in all places, at the same time, it exhibited the like wonderful features. Father Boscovich determined the height of an aurora borealis ob. served by the Marquis of Polini, 16th December, 1737, and found it 825 mises high; and M. Bergman, from a mean of thirty com. putations, makes the average height to be 70 Swedish, or 469 English miles. Euler supposes the aurora to be more than double that height; but in this opinion he stands alone ; for M. Mairan, in a treatise which he wrote expressly upon this phenomenon, entitled, “ Traité Physique et Historique de l'Aurore Boreale," fixes the height, on an average, at 175 leagues from the earth, which is equal to 464 English miles.
Many conjectural opinions have been formed concerning the cause of this phenomenon; Dr. Halley imagined that the watery vapours, or effluvia, exceedingly rarefied by subterraneous fire, and singed with sulphurous streams, which many naturalists have supposed to be the cause of earthquakes, might also be the cause of this appearance; or that it is produced by a kind of subtile matter freely pervading the pores of the earth, and which entering into it near the southern pole, passes out again, with some force, into the ether, at the same distance from the northern. The sub. tile matter, by becoming more dense, or having its velocity in. creased, may perhaps be capable of producing a small degree of light, after the manner of eflluvia from electric bodies, which by a strong and quick friction emits light in the dark, to which sort of light the aurora seems to bear a great atfinity. Phil. Trans. No. 347.
M. de Mairan, in the treatise above quoted, supposed its cause to be zodiacal light; wbich, 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 appearances 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, tbat 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 hypothesis 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 ren. dered 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 betweeu the tropics is continually rising ; its place is supplied by northerly and south. erly 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 cor. 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 re. ceive 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