during the clear evenings of a certain season of the year; and the inhabitants generally affirmed, that sometimes, when its streamers are very vivid, it is accompanied by the most terrific noises.

As we approach the North Pole, the more striking and wonderful are its appearances. The sailors employed in the Greenland whale-fishery give ample testimony in regard to the intensity of light and the frightful sounds which accompany it. Gruelin, in his account of Siberia, speaks of the crackling noises of the Aurora Borealis, and says that the hunters, who go in search of the blue fox to the confines of the frozen ocean, are frequently surprised by the unexpected appearance of this meteor. Their dogs are frightened by it to such a degree, that they cannot be kept from stopping, and lying on the ground, until the noise has ceased. There is a phrase peculiar to the language of that country to express the terror occasioned by these sounds. M. Edmonston, who has frequently heard these noises, in an account of the Aurora seen by him in 1818, says, he was informed by two creditable persons, that while they were on a voyage to the Shetland Islands they observed a display of the Aurora, the noise of which was so great that the sailors were afraid to remain on deck, and the light was so strong that they could observe the compass by it.

In regard to the intensity of the light of the Aurora, we have the recent testimony of the celebrated navigator, Capt. Parry, who, on his voyages of discovery, witnessed numerous beautiful displays of this phenomenon. The light was for some time equal to that of the full moon, and was very great sometimes, in cloudy weather, when the sky was completely overcast.

The same phenomenon which we call the Aurora Borealis, is observed near the South Pole, and hence called Aurora Australis. It probably appears in the same latitudes as the Aurora of the Northern regions. Mr. Forster, in his voyage round the world with Capt. Cook, observed it in latitude fifty-eight degrees South, during the month of February, 1773. It appeared several evenings in succession, and although in most respects similar to the Northern lights of our hemisphere, yet it differed from them in being always of a whitish color, whereas ours assume various tints. The sky was generally clear when they appeared, and a "nipping and an eager air" prevailed-the thermometer standing at the freezing point.

The light of the Aurora was, for a long time, the only circumstance that engaged the attention of the world; but in 1740, two Swedish observers, Celsius and Hiorter, discovered its magnetic properties. They observed, that during the appearance of the meteor, magnetic needles undergo very irregular agitations. These variations are frequently of short duration, the needle soon recovering its wonted motions; but sometimes the deflection is permanent. Their violence depends on the brightness and extent of the Aurora. A faint glimmering on the northern horizon produces only a slight, and perhaps insensible, disturbance of the needle. The motion may be very slight also, when the meteor is situated in the magnetic meridian; so that the meteor may be brilliant, without producing any sensible motion of the needle. It has also been as

certained, by repeated observations, that the columns, the summits of the arcs, and the centres of the crowns of the Aurora, are situated in the magnetic meridian of the place where they are observed. From these facts it may be concluded that the magnetic property of the Aurora Borealis is well ascertained.

We have now described the various appearances of the Aurora Borealis, and given a brief history of some of the most remarkable instances of its display. We have seen that it appears at irregular periods, and of unequal brilliancy-that it is visible in the high latitudes, but never near the equator-that it usually appears soon after twilight, and continues till about midnight, sometimes, however, being visible a few hours after that in our latitude it commonly appears in the Northern sky, while nearer the Pole it is seen in the East and West. We have observed, also, that its prominent features are the expanded arch, and the shooting column-that the centre of the arch, and the direction of the column appear to be in the magnetic meridian-that the magnetic needle is frequently affected by the brilliant meteor, and that it is sometimes accompanied by hissing, crackling noises.

It is now time to inquire, What is the cause and constitution of the meteor, and whence emanates the light which renders it visible? Various theories have been proposed, all of them founded on its uniform appearances. But, as in the present state of Meteorological science, no one of them is susceptible of actual demonstration, our proper course is, to review the best of them, and adopt that which seems most fully supported by facts and logical reasoning.

The earliest conjecture in regard to the origin of the Aurora, was, that it was caused by sulphureous vapors, which took fire as they rose from the earth. The celebrated Dr. Halley thought this cause inadequate, and supposed the Poles of the earth to be in some way connected with it. In his opinion, the earth consisted of two great magnets, one within the other and hence the cause of the variation of the needle-and that magnetical effluvia, or vapors, passed out at the Poles. Supposing the earth to be hollow, with a smaller globe included, in order that the inner one might be habitable, there might be some luminous medium between the globes, so as to make perpetual day below-some of this matter might pass through the earth, near the Poles, where its density is least, and occasion the Aurora. Had the famous Capt. Symmes of our own time and country, lived to confirm his theory of the world, which was similar to this, by actual discovery, we might perhaps have learned more of this subtle medium in the bowels of our globe. At present, we must let the theory pass, as neither precise nor probable.

In later times, since the identity of lightning with electricity has been established, the Aurora Borealis has been generally considered an electrical phenomenon, although many even now attribute it to the refraction of the sun's rays in the higher regions of the atmosphere. M. Canton, a distinguished French electrician, supposed that it was occasioned by the rushing of the two kinds of electricity, through the upper part of the atmosphere where the resistance is least. Dr. Franklin supposed it to be caused by the electric fluid discharged into the Polar re

gions from the vaporized air, raised from the ocean between the tropics. Mr. Kirwan ascribes it to the combustion of inflammable air, caused by electricity, which is quite a probable theory.

The theory of M. Libes, adopted by many philosophers, supposes the meteor to be caused by electricity, which combines the gases that constitute the atmosphere. If the electric spark come in contact with a compound of nitrogen and oxygen, they will combine and produce a gas which exhales ruddy vapors; but if hydrogen be exposed to the spark, it will be inflamed and will explode. He supposes that, in consequence of the feeble power of the sun in the Polar regions, the quantity of hydrogen raised from the earth will be very small, and, being lighter than the other gases, will rise to the higher regions of the atmosphere. Hence the electric fluid would find in its passage only a mixture of oxygen and nitrogen, which would combine, and form red, volatile vapors, which would constitute the Aurora Borealis. If the electric equilibrium takes place in the higher regions, where there is some hydrogen, an explosion would take place, which occasions the noises heard during the displays of the meteor.

This is a very ingenious theory, but it fails in several points. For the prevailing color of the Aurora is white, or a yellowish white, and the noises which are attributed to explosions in the higher regions, are, in fact, only heard when the meteor appears to be nearest to the earth, and are considered generally as a proof of its approach to it.

The best theory that has ever been proposed to account for the Aurora Borealis, is that of M. Biot, the distinguished French mathematician and naturalist. It is supported by the most rigorous deductions from careful observations, and susceptible of almost actual demonstration. He considers the Aurora as consisting of real clouds, proceeding usually from the North, and composed of some very light substances, probably metallic, capable of becoming luminous and magnetic, arranging themselves in columns, which turn towards the earth as real magnetic needles would do. This is supported by optical considerations, and the application of the known laws of electricity and magnetism. Having shown that the phenomenon takes place in our atmosphere, and that the columns always point towards the magnetic pole, he concludes that they are cylindrical, and parallel to the direction of the needle. Although, instead of being erect, they seem to incline, and converge towards the North Pole, he reconciles this appearance with the supposition that they are parallel, by applying to them the laws of perspective. It is well known that elevated objects, when seen at a distance through the atmosphere, produce optical illusions. For instance, the stars, which are at immense distances from each other, appear to be set in the same vault or concave surface; and the tails of comets seem to spread along the sky, although they extend into space, in right lines. So, when the sun is concealed behind a mass of clouds, and shines through their openings, the rays, though parallel, appear to converge towards that point of the heavens where the sun is. These laws of perspective will affect, in like manner, the appearance of the columns of the Aurora. M. Biot supposes the Aurora to consist of a forest of

these columns, all parallel to the direction of the magnet, and of course parallel to each other. Being at different distances, they will, by the laws of perspective, appear at different heights, and seem to project over each other, especially when in the horizon. If a number of the columns pass over the head of an observer, they will form a luminous erown, the rays converging to a point, as has been described.

This constitution of the meteor is rendered probable by many curious facts, which agree with the rules of perspective. For instance, when the columns are in the Northern horizon, if they move towards the South, without any change of arrangement, they ought to produce the same appearance as the trees of a forest when we approach them; that is, the columns Eastward will separate towards the East, and those Westward to the West of the meridian; while those directly in the meridian will appear to be stationary, or to rise directly to the zenith. Such an appearance was accurately observed by M. Mayer, the astronomer, at St. Petersburgh, during a large Aurora in 1726. Sometimes only a few of the columns are illuminated, and then, if they are at sufficient distance, we may examine them singly, as was observed by Dr. Halley in 1716, when small columns of equal length were inclosed between two beltsso, on another occasion, when the columns had the appearance of the pipes of an organ.

In regard to the height of the meteor, it is very probable that it occupies the region of the higher clouds, and sometimes descends quite near the earth. The evidence of this is the motion of the arcs, like that which a light breeze would give them ;-the passage of the fleecy portions of cloud-like substance, which are seen floating in the air; and particularly from the hissing, crackling noise which sometimes accompanies them.

But what is the nature of these floating substances which we suppose to be the basis of the meteor? As the metals are the only substances, with which we are acquainted, that are capable of becoming magnetic, it is most probable that the substance of the Aurora is of a metallic nature. Supposing this to be the case, the hypothesis is confirmed by the fact, that the metals are the best conductors of electricity. Now the atmosphere, in different strata, contains very unequal quantities of electricity; so that, if metallic columns are suspended in nearly a vertical position in the atmosphere, like those of the Aurora, when they float over the Polar regions they will be conductors of electricity. If the tendency of the electricity to an equilibrium be sufficient to overcome the imperfect conducting power of the columns, it will flow along them, illuminating its path, as is often observed in conductors that are not continuous. When the passage of the fluid takes place in the higher regions of the atmosphere, where its rarity offers little resistance, it will flow on silently, with all those variations of light which we observe in exhausted tubes. But if it extends to the lower strata of the atmosphere, where the resistance is great, it will occasion such hissing, crackling noises as attend the Aurora, when it approaches very near the earth.

These phenomena must be less striking as the meteor advances towards the South, not only because it is more widely spread, but especially, because the conducting columns, always conforming to the

direction of the magnetic needle, become more horizontal, and have their extremities in atmospheric strata less distant, and therefore less unequal in quantity of electricity. Moreover, a greater moisture prevails in the atmosphere of the lower latitudes, which occasions a more frequent discharge of electricity.

Supposing the substances which compose the meteor to be of a metallic nature, the passage of electricity from one particle to another would render the columns luminous, and thus afford the light of the AuBut it has been ascertained by repeated experiments, that a bar of iron may be magnetized, by suffering a current of electricity to pass over it. Consequently, the passage of electricity over the columns of the Aurora would not only render them luminous, but would, at the same time, give them a magnetic property. This celebrated author further supposes that there may be an actual combustion in the phosphoric clouds, which, detaching themselves in some cases from the meteor, carry with them the principle of their phosphorescence, and throw out jets of light, like rockets, which leave after them a whitish train. The opinion that the Aurora possessed magnetic properties was also maintained by the English philosopher, Dalton, who made similar deductions from the laws of perspective to those made by Biot.

We consider the theory of M. Biot as satisfactory, for the following

reasons:

1. The Aurora evidently consists of some cloud-like substance, for it has been remarked that it uniformly proceeds from a dark cloud in the North-we have noticed it particularly in every brilliant display of the Aurora which we have seen-and luminous fleecy clouds are sometimes seen floating separately in the atmosphere, which have been recognized as fragments of the Aurora.

2. The obvious appearances of the meteor are such as would be produced by electricity. The vivid shootings, rapid as the lightning, make it almost certain that the phenomenon is electrical. And if it be electrical, the cloud-like substances are sufficient to retain the light. But the hissing, crackling noises, which sometimes accompany the Aurora, can be accounted for in no other way, and perhaps, more than all else, confirm the theory of its electrical origin.

3. The meteor evidently possesses magnetic properties, for needles, freely suspended, undergo very irregular variations, according to the observations of philosophers in Europe and also in our own country. Moreover, the columns of the Aurora uniformly follow the direction of the magnetic needle. The magnificent arch is but a series of these columns, which, by an optical illusion, seem to converge to a point. But if the substance possess magnetic properties, it must be of a metallic nature, for we know of no other susceptible of magnetism. Supposing it to be metallic, it would, as we have seen, when electrified, become luminous and magnetic, and consequently directed towards the magnetic Pole. And this is all that the theory requires.

Now, that there may be metallic particles in the clouds which constitute the Aurora, is, we think, rendered quite probable by the fact that the meteoric stones, which sometimes fall to the earth, contain various