On the 3d of March, 1821, when a shock of earthquake was felt at Dover and the neighbouring places, nothing in the atmospheric state indicated in this country that anything extraordinary was happening at so short a distance. At about 4 P.M. the barometer stood at 29° 520', the thermometer at 47° 3'. The air was dark and overcast; it blew a strong gale from the W.S.W.

OBSERVATIONS ON THE AURORA BOREALIS OF THE 7th OF JANUARY, THE 11th OF JANUARY, AND THE 7th OF MARCH, 1831.

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BY THE HON. CHARLES HARRIS.

IN consequence of the account of the Aurora Borealis of the 7th of January, given by Mr. Christie in the last Number of this Journal, we have been favoured, by the Hon. Mr. Harris, with the following extracts from his Meteorological Journal, kept at Heron Court.

Friday, January 7th, 1831.-Magnificent Aurora Borealis at night. It first appeared about 5h 30m P.M., in the shape of a white cloud in the north, 10° in depth and 55° high, extending from west to east, much denser towards its extremities, where it was edged off with prismatic tints of red and green, It seemed coming over from N., and a narrow band of it passed over as far as about 20° on the south of the zenith. In the north it soon became traversed with bright columns, with here and there a hazy patch of flame colour, especially in the N.W. It was most beautiful about nine P.M., when the cloud came nearly over head, being driven, as it were, by the wind from the north. Its eastern extremity, which was very dense and luminous, drove by as far as E.S.E. Suddenly, however, it streamed back again, in bright streaks, towards due north, look

ing as if the fiery cloud had burst. In the midst of these streaks appeared bright patches, especially N.E., of the most brilliant flame colour. At one time, these united and formed a most beautiful track of pale crimson from N.W. to N.E., the flame colour in some places occurring outside the white cloud against the clear sky. The northern edge of the cloud gradually became dense and ragged, while the sky in the north, to the height of twenty degrees, was of the most inky blackness. Excepting the bright prismatic patches of crimson, faint green, and sometimes yellow, it had all the appearance of a spent snow-squall strongly lighted by the moon. By ten P.M. the light gradually withdrew from the zenith, and settled very like a bank of cloud, extending from west to east, about 10° in depth, the sky beneath, for 20°, being perfectly black. In the brightest parts of the cloud the stars were seen, but faintly; and, indeed, Ursa Major, at one time, was scarcely visible. The thermometer at the time 24°, barometer 30.60.

Tuesday 11th.-Aurora Borealis visible at eight. About 8 P.M. it appeared in the form of two horizontal luminous bands of cloud, about 20° high, and extending from N.W. to N.E., with a distinct dark space intervening. About 8h 30m they were rendered more confused by a hazy white light streaming up through them from the north, giving the luminous strata the appearance which geologists call a fault.' The eastern edge of the light was very clearly defined: indeed it had the appearance of a strong light streaming through a half-closed aperture. Barometer 30.03, thermometer about 30°. At nine P.M. the night became cloudy, and no observation could be made.

Monday, March 7th.-Aurora Borealis very fine at night. I observed it first about 8h 40m P.M. Its lower edge then formed a very regular arch from N.W. by W. to N.E.; the greatest altitude of which was in N.N.W., about 18°. A large star (Deneb in Cygnus about N. by W. ?) was 2° above its lower edge. The light suddenly seemed to become concentrated in the N.E., where the lower edge became irregular and approached the horizon like the folds of a curtain; at the same time three or four streams of vertical white light shot up in the N.E., N.N.E., and N.: these almost immediately faded away;

the Aurora became as before, the light becoming insensibly fainter from the lower edge to about 45°, where it was no longer perceived. After the coruscations in the N.E., the main body of light seemed to move rapidly to the westward, the whole phenomenon, however, becoming much brighter. The lower edge again became ragged, and approached the horizon; and in the W.N.W. brilliant columns of light, tinged with pale flame colour, shot up to the altitude of 50°. From this time to about 9h 15m the Aurora was in its greatest beauty; a large body of light appearing constantly travelling from N.W. to N., and vice versa. Whenever coruscations were about to be thrown up, the lower edge of the cloud became like the base of a thunder-shower, the ragged points being at times brilliantly luminous: from these points the columns of light shot up with great intensity. About 9h 15m a most splendid coruscation shot up from due N. It appeared to extend down to the horizon, and shot up to the height of about 50°, the column itself being 4° wide. The upper extremity, and, indeed, the greater part of the sky from N. to W.N.W., at the height of about 55°, was of a beautiful pale flame colour. Beautiful columns of light were at the same time shooting up from the N.W. and N.N.W., and faint nebulæ of light were visible about 10° to 15° N. of the zenith; the Aurora then again faded into a large luminous track from N.W. to N.E., and its lower edge but ill defined: the flamecolour remained for some time on the upper edge. Small cirostrati in the N. and N.E. were thrown out in the most striking relief; and at 9h 15m Deneb was, for a few seconds, but faintly visible. The light against the north side of a house was as strong as that of the moon at the quarter. At 10h P.M. the Aurora was very faint; its lower edge about 7° from the horizon, from which the light gradually faded off, and was faintly perceptible at 45°. The sky beneath, however, was of that inky blackness so peculiar to this phenomenon. Barometer, 9 P.M., 29.82, thermometer 40.5. About 4 P.M., in the N.E. and N.W., a thin brown haze was visible, a very unusual circumstance, with a westerly wind and in unsettled weather; the haze split off at times in horizontal tiers, and it was impossible to discover whether it was haze or distant cirrus; indeed,

it was this that induced me to look out; and I have little doubt, from a similar circumstance mentioned by Parry in his first Tour, that this was the daylight appearance of the Aurora.

ON THE HEIGHT ABOVE THE SURFACE OF THE EARTH OF A LUMINOUS ARCH OF THE AURORA BOREALIS, ON THE 7th OF JANUARY, 1831.

By S. H. CHRISTIE, Esq., M.A., F.R.S., &c.

THE height of the Aurora Borealis above the surface of the earth has been so variously estimated, that any observavations which determine limits to the height of a particular phenomenon become interesting, although these limits may not be extremely close. The most permanent of the phenomena are the luminous arches, and these are therefore the best adapted for determining the height; but even these appear to be by no means stationary and as more than one are sometimes visible, it may, in many cases, be doubtful, whether simultaneous observations, made by distant observers, refer to the same arch. The Aurora of the 7th of January last made its first appearance in this neighbourhood, in the south-east, and in a few minutes afterwards, a single well-defined arch, and which was visible but for a short time, was formed across the southern meridian. If then the commencement of the Aurora happened to be observed, in the form of an arch, at a considerable distance to the south of the place where I observed the altitude of the arch, there could be scarcely any doubt of the identity of the luminous band forming these arches. It appears from Mr. Harris's account of the commencement of the phenomenon, that he observed an arch, at Heron Court, to be elevated 55° above the northern horizon, at the same time that I observed one, at Blackheath, to pass over the planet Mars, then not far from the meridian, and about 46° above the southern horizon. Mr. Harris observed nothing to the south of the zenith until some time after the first appearance of the arch: I observed no arch towards the north for a considerable time afterwards; and as we observed at as nearly

as possible the same time, 5h 30m P. M. in both cases, there can, I think, be no doubt that our observations were made on the same luminous band in the same position. Having, through the kindness of Mr. Faraday, been favoured with Mr. Harris's interesting observations on the Aurora, I have computed the height of this arch above the earth's surface, from these observations and my own; and although there may be some doubts with respect to the absolute height, as determined from these, in consequence of the unfavourable relative positions of the two places of observation, yet, as the limits which they determine are very different from the height most recently assigned to similar phenomena, I do not hesitate to publish the results.

Assuming that the arch was formed by a band of light, of great extent, in a line nearly at right angles to the meridian, and parallel to the earth's surface, it is evident that, although different portions of this band were actually observed, the absolute height will be determined from the observed altitudes of the highest points, and the arc between the parallels to this band on the earth's surface, in the same manner as if the observations had been made on the same portion, from places in a plane at right angles to the arch.

Let then, B be the angles of elevation of the same point in the arch, at two places A and B, in a vertical plane passing through that point; y the arc on the earth's surface between A and B; the angle contained by two lines drawn from the point in the arch, the one to A, the other to the centre of the earth then if ♪ = 7 (a+B+y), we shall have

or if ẞ is the greater of the two angles a, B, and

If r is the radius of the earth, and h the height of the arch above its surface,