TERRESTRIAL LIGHT. Aurora borealis, or northern lights, as they have hitherto been called, is now generally admitted to be a magnetic phenomenon. [TERRESTRIAL MAGNETISM.] As lightning shows a restoration to equilibrium from a disturbance in the electrical condition of the atmosphere, so these northern and southern lights denote the conclusion of a magnetic storm which has foretold its approach by its universal influeuce on a frecly.suspended magnet, even at places far distant from the place where the phenomenon itself is visible.
The appearances which ordinarily present themselves during a dis play of terrestrial light are thus graphically described by Humboldt with his accustomed accuracy :—" Deep iu the horizon, nearly in the situation where it is intersected by the magnetic meridian, the heaven, up to this moment clear, grows black. There is a kind of hazy hank or screen produced, which rises gradually and attains an altitude of from 8" to 10% Tho oolour of the dusky segment passes over into brown or violet. Stars are visible in it, but they are seen as in a portion of the sky obscured with dense smoke ; a broad bright lumi nous aro or seam, first white, than yellow, bounds the dusky segment. The highest point of the luminous arc, when it has been carefully measured, has been found to be not exactly in the magnetic meridian, but to vary between 5 and 18 degrees from it, towards the aide on which the magnetic declination of the place of observation lies. The luminous bow, in constant motion, flickering and changing its form Incessantly, sometimes remains visible for hours before anything like rays and pencils of rays shout from it and rise to the zenith. Tho more intense the discharges of the northern lights, the more vividly do the colours play from violet and bluish-whito, through every shade and gradation, to green and purplish•red. The magnetic fiery columns shoot up, at one time singly from the luminous arch, even mingled with black rays like thick smoke ; at another, many columns arise simultaneously from several and opposite points of the horizon, and unite in a flickering sea of flame, to the splendour of which no description can do justice, and whose luminous waves assume another and a different shape at every instant. The Intensity is at times so great that Loweniiru per
ceived its oscillations, in bright sunshine, on the 29th of January,1756. The motion increases the brilliancy of the phenomenon. Around the point of the vault of heaven which corresponds with the directiou of the dipping-needle, the rays at length collect together, And form the corona or crown. This surrounds the summit, as it were, of a vast canopy, the dome of heaven, with the mild radiance of its streaming but not flickering rays. It is only in rare instances that the phone menon.proceeds the length of forming the corona completely. With Re appearance, however, the whole is at an end. The rays now become rarer, shorter, less intensely coloured. The crown and the luminous arches break up. By-and-bye nothing but broad, motionless, and almost ashy-gray pale gleaming fleecy masses appear irregularly dispersed over the whole vault of heaven : these vanish in their turn, and before the last trace of the murky fuliginous segment, which still shows itself deeply on the horizon, has disappeared. Of the whole brilliant spectacle. nothing at length remains but a white delicate cloud, feathered at the edge, or broken up, as a cirrocumulus, into small rounded !names or heap., at equal distances." (` v. i.) The two illustrations given above are selected, for their dissimilarity, from the work of M. de 3Iairau. Fig. I represents en aurora as seen at Breuilkpont, in Normandy, nearly in the latitude of Paris, on September 24th, 1726. It consisted entirely of streams of light, (seen ic without any darker meteor. F aurora ig. 2 shows an auro as observed at the same and which lasted for several minutes, on Oct. 19, 1726.