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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, wbich (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 withio 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 ugh 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 cylin. drical 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 pro. perties. 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 suu'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.
• Second Voyage, I. 53.
Phil. Trans, 1774, | Id. 1781.
Account of surprising lights in the Air, March 6, 1716; with
an attempt to explain their principal Phænomena.
By Edmund Halley, J.V.D. Savilian Professor of Geometry, Oxon, and
Secretary to the Royal Society. On Tuesday, March 6, 0. S. 1716, the afternoon having been very serene and calm, and somewhat warmer than ordinary, about seven o'clock, out of what seemed a dusky cloud, in the N.E. parts of the heaven, and scarcely ten degrees high, the edges of which were tinged with a reddish yellow, as if the moon had been hid behind it, there arose very long luminous rays or streaks, per. pendicular to the horizon, some of which seemed nearly to ascend to the zenith. Presently after, that reddish cloud was swiftly propagated along the northern horizon, into the N.W. and still far. ther westerly; and immediately sent forth its rays after the same manner from all parts, now here, now there, observing no rule or order in their rising. Many of these rays seeming to concur near the zenith, formed there a corona, or image, which drew the at. tention of all spectators, who according to their several conceptions made very differing resemblances of it; but by which com. pared together, those that saw it not may well comprehend after what manner it appeared. All agree that this spectrum lasted only a few minutes, and showed itself variously tinged with co. lours, yellow, red, and a dusky green; nor did it keep in the same place; for when first it began to appear, it was seen a little to the northward of the zenith ; but gradually declining toward the south, the long striæ of light, which arose from all parts of the northern semicircle of the horizon, seemed to meet together, not much above the head of Castor, or the northern Twin, and there soon disappeared.
After the first impetus of this ascending vapour was over, the corona we have been describing appeared no more ; but still, without any order as to the time, or place, or size, luminous radii like the formar continued to arise perpendicularly. Nor did they proceed as at first, out of a cloud, but oftener would emerge at once out of the pure sky, which was at that time more than ordinary se.
rene and still. Nor were they all of the same form. Most of them seemed to end in a point upwards, like erect cones ; others like truncated cones or cylinders, so much resembled the long tails of comets, that at first sight they might well be taken for such. Again, some of these rays would continue visible for several mi. nutes ; when others, and those the much greater part, just shewed themselves, and died away. Some seemed to have little motion, and to stand as it were fixed among the stars, while others with a very perceptible translation moved from east to west under the pole, contrary to the motion of the heavens ; by which means they would sometimes seem to run together, and at other times to fly one another, affording a surprising spectacle to the beholders.
After this sight had continued about an hour and a half, the beams began to rise much fewer in number, and not near so high, and gradually that diffused light, which had illustrated the north. ern parts of the hemisphere, seemed to subside, and settling on the horizon formed the resemblance of a very bright crepusculum. On the first information of the thing, I immediately ran to the windows, which happened to look to the south and south west quarter; and soon perceived, that though the sky was very clear, yet it was tinged with a strange sort of light; so that the smaller stars were scarcely to be seen, and much as it is when the moon of four days old appears after twilight. We perceived a very thin vapour pass before us, which arose from the precise east part of the horizon, ascending obliquely, so as to leave the zenith about fifteen or twenty degrees to the northward. But the swiftness with which it proceeded was scarcely to be believed, seeming not infe. rior to that of lightning ; and exhibiting as it passed on a sort of momentaneous nubecula, which discovered itself by a very diluted and faint whiteness ; and was no sooner formed, but before the eye could well take it, it was gone, and left no signs behind it. Nor was this a single appearance; but for several minutes, about six or seven times in a minute, it was again and again repeated ; these waves of vapour, if I may so call it, regularly succeeding one another, and nearly at equal intervals ; all of them in their ascent producing a like transient nubecula.
By this particular we were first assured, that the vapour we saw, whatever it was, became conspicuous by its own proper light, without help of the sun's beams; for these nubecula did not discoVOL. IV.