heat, and thus open up a further view of the laws by which the Creator regulates the material world.

SECOND WEEK-TUESDAY.

AURORA BOREALIS.

THE aurora borealis is a phenomenon undoubtedly electrical, connected in some way with the magnetic poles, which sometimes beautifully illuminates our northern sky during the autumnal and winter months. Its use in the system of nature has not been distinctly ascertained, though various conjectures have been formed. Dr Halley supposed that the earth was hollow, having within it a magnetical sphere, which corresponded in virtue with all the magnets on the surface; and that the aurora was the magnetic effluvia rendered by some means visible, and passing through or beyond the atmosphere from the north pole of the central magnet to that of the south. Boccaria adopts a similar idea, but attributes the phenomenon to the electric instead of the magnetic fluid, which, indeed, is now proved to be the same thing. The fallacy of this opinion has, however, been since shown by the fact, that the fluid, whatever it is, darts upward toward the zenith in the southern as well as in the northern hemisphere, whereas, were there a circulation such as has been conjectured, the course of the fluid would in the south have been reversed, descending from the zenith to the horizon. The supposition of Dr Faraday, therefore, is, that the electric equilibrium of the earth is restored by the aurora conveying the electricity from the poles to the equator.

Without attempting to settle a point with regard to which sufficient data have not been collected, I shall content myself with describing some of the remarkable appearances of this very curious and interesting pheno

menon.

One circumstance worthy of notice has already been stated, namely, that the aurora bears some reference, not

to the poles of the earth's rotation, but to what have been called the magnetic poles. It generally forms a kind of stationary luminous arch, of which the magnetic pole is the centre, and across this arch the coruscations are rapid, sudden, and frequently of various colours. Its history is curious, no very distinct account having been recorded of its appearance in the classic ages of the world, though we do hear of "trabes," and "bolides,” and "chasmata," which seem to refer to some celestial phenomena of a similiar nature. The first modern account of the aurora on record, is that mentioned as having been seen at London, on the 30th January 1560; and during the subsequent part of that century, its appearance does not seem to have been very uncommon. During the seventeenth century, however, it seems to have been scarcely observed at all, except in one instance, in 1621, when it excited the astonishment of the inhabitants of France, and first acquired the name of the aurora borealis. Since the beginning of the eighteenth century, its visits in this and other northern countries have been frequent but capricious, there being intervals of several years, during which they have been either intermitted altogether, or have been of such a nature as to attract little observation.

In the northern regions, the aurora appears with the greatest brilliancy; but it does not seem that the intenseness increases, as might be expected, in proportion to the nearness of approach to the magnetic pole. In the Shetland Islands, it cheers the winter nights almost constantly during clear weather. Its phenomena are there called the merry dancers, and are thus described; They commonly appear at twilight, near the horizon, of a dun colour, approaching to yellow; sometimes continuing in that state for several hours, without any sensible motion; after which they break out into streams of stronger light, spreading into columns, and altering slowly into ten thousand different shapes, varying their colours from all the tints of yellow to the obscurest rus

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set. They often cover the whole hemisphere, and then make the most brilliant appearance. Their motions, at these times, are amazingly quick, and they astonish the spectator with the rapid change of their form. They often put on the colour of blood, and make a most dreadful appearance."

The aurora is said, in the colder latitudes, to be attended with a peculiar hissing noise. Gmelin mentions this very distinctly and positively in the interesting account which he gives of it, as it appears in Siberia. "These northern lights," says he, "begin with single bright pillars, rising in the north, and almost at the same time in the north-east, which, gradually increasing, comprehend a large space of the heavens, rush about from place to place, with incredible velocity, and finally almost cover the whole sky, up to the zenith. The streams are then seen meeting together in the zenith, and produce an appearance as if a vast tent was expanded in the heavens, glittering with gold, rubies, and sapphires. A more beautiful spectacle cannot be painted; but, whoever should see such a northern light, for the first time, could not behold it without terror; for however fine the illumination may be, it is attended, as I have learned from the relation of many persons, with such a hissing, cracking, and rushing noise throughout the air, as if the largest fireworks were playing off. To describe what they then hear, they make use of the expression 'spolochi chodjat; that is, the raging host is passing. The hunters who pursue the white and blue foxes, in the confines of the Icy Sea, are often overtaken in their course by these northern lights. Their dogs are then so much frightened, that they will not move, but lie obstinately on the ground till the noise has passed. Commonly clear and calm weather follows this kind of northern lights. I have heard this account, not from one person only, but confirmed by the uniform testimony of many, who have spent part of several years in these very northern regions,

* Encyclopedia Britannica, article Aurora Borealis.

and inhabited different countries, from the Yenesei to the Lena; so that no doubt of its truth can remain."

In Captain Franklin's narrative of his journey to the Polar Sea, there are some scientific observations on the phenomena of the Aurora, which throw considerable light on this curious and interesting subject. The meteor is usually conceived to have its place very high above the earth; but exceedingly different elevations have been assigned to it by different philosophers. Euler supposed it to be some thousands of miles distant, others have fixed its place at a few hundred miles, and others again much lower. The diffused nature of the appearance in this country, renders it difficult to make any accurate observation on the subject; but if the Aurora should continue occasionally to assume the form of a moveable luminous arch, gliding slowly in a well defined continuous body towards the zenith, as it has lately done in several instances and in different seasons, observations taken from various stations might settle the point. Be this as it may, and however high the northern lights may actually rise in this comparatively southern latitude, it seems to be ascertained by Captain Franklin and his companions, that, in the higher latitudes of North America, and still nearer the Pole, the region of the Aurora is not many miles above the earth. They discovered, by actual observation, that, in several instances, it did not rise higher than six or seven miles; and both there and in Siberia, it would seem to be often much lower even than this. The same kind of appearances, as are described by Gmelin, above quoted, appear sometimes to occur on the other side of the Atlantic. These, however, are not frequent; and the more usual phenomena purtake much of the nature of the following, which I quote from Mr Richardson's interesting observations on this meteor:

"When the Aurora had exhibited itself in this form for a considerable space of time, the whole mass of light suddenly appeared in motion, and, sweeping round on

each side, was gathered together to the southward of the zenith. Immediately thereafter, a large portion of it was seen in the S.E., assuming an exact resemblance to a curtain suspended in a circular form in the air, and hanging perpendicularly to the earth's surface. The lower edge of this curtain was very luminous, and had a waving motion; and the illusion was farther heightened by the momentary appearance of perpendicular dark lines or breaks in the light, in rapid succession round the circle, exactly as the waving of a curtain would cause the dark shades of its folds to move along it. This beautiful curtain of light was about 40° high, and of a pale yellowish colour, and sent forth on the one side a process which approached the S.E by E. point of the horizon, and the other was connected with a long regular arch, terminating in the N.W. horizon, similarly constructed, and having the same waving motion with the curtain itself. All this time the sky was perfectly clear, except in the southern quarter, which, to the height of 4° or 5°, was occupied by dark clouds, apparently intermediate between stratus and cirro-stratus.

"Half an hour after its first appearance, this curtainformed Aurora was resolved into a number of detached irregular portions, which sometimes increased rapidly in every direction, until they met with other masses, either before existing, or appearing at the instant, and formed an uniform sheet of light, which covered the whole sky. The formation of this great sheet of light was so rapid, that the eye could only trace its progress partially, and its dissolution and reappearance were equally sudden."*

SECOND WEEK-WEDNESDAY.

METEORIC SHOWERS.

I HAVE now to mention another celestial phenomenon of a very singular nature, connected with two days in

* Franklin's Narrative, p. 621.