will return every seventy-five years; but the comet of 1763 will not again be seen by mortal eye till after the lapse of 7334 years: from all we know of the subject, the average period of comets seems to be about one thousand years. We have already been able to follow the course of some comets through their extensive tracks from the small part of their paths that were visible from the earth, and as soon as their periods shall be determined, and the variations in their motions, occasioned by the planets, accurately appreciated, the disturbances from any foreign cause will be known, and may not only lead to the discovery of planets revolving unseen beyond the boundaries now assigned to our system, but disclose the position of their orbits, and possibly their physical nature. Unseen bodies may either turn aside from their course such comets as are known to us; or they may bring others towards the sun which might otherwise for ever have remained far from our view.

One example, singular in its nature, has already occurred from the attraction of Jupiter. The comet of 1770, commonly called Lexel's comet, is remarkable for having experienced disturbances from the great bodies of our system, unprecedented in the annals of astronomy. Lexel made out that the orbit of this comet was an ellipse whose length was only three times the diameter of the earth's orbit (about 570 millions of miles), and that it would return to the sun at intervals of five years and a half. Nothing could be more surprising than this result, as the comet had never been seen before, though very brilliant; and what is perhaps more wonderful, it never has been seen again. There can be no doubt of the truth of Lexel's calculations, because the comet was visible through a very considerable part of its orbit, and they were also confirmed by the investigation of Buckhardt, who discovered the cause of this anomaly. He found that, previous to the year 1767, the comet's path had been so extensive that it required fifty years to accomplish a revolution. and as it had never before come nearer to the sun than Jupiter does, it could not be seen from the earth. In January, 1776, however, it came so near Jupiter, and his attraction diminished the size of its orbit to such a degree, that instead of fifty years it completed its revolution in five years and a half; and it would have been seen in March, 1776, had it not been so situated, with regard to the earth, as to be entirely hid by the sun's rays. The comet again met with Jupiter when coming to the sun in 1779, and remained under the influence of his attraction from June till October. While passing through the midst of the satellites, in August, it came so near Jupiter, that it was two hundred times more powerfully attracted by him than by the sun, which has increased

increased its orbit so much, that twenty years are now required to complete a revolution; and as it never comes nearer to the sun than the orbit of Ceres, it will for ever be hid from us unless compelled to approach nearer by some new disturbing force.

The comet of 1811 is by far the most splendid that has appeared within the memory of man. Its envelope was 30,000 miles thick, and the centre of the nucleus or head was separated from its interior surface by a space of 36,000 miles, so that the radius of the head must have been 66,000 miles. It had a brilliant point in the centre, 428 miles in diameter; but it must again be acknowledged that little reliance can be placed on measurements of mere specks, so ill defined and so distant. The tail of this comet, at its greatest length, extended to 123 millions of miles, but its extremity never came nearer to us than 141 millions of miles. Brilliant as the light of this comet was, when brought to a focus by a large mirror, it was not equal to a tenth part of the light of the full moon; and it had no sensible effect on the blackened ball of a thermometer, which was so delicate a test of variation of temperature that it would have indicated the hundredth part of a degree; this shows the total inefficacy of comets to influence our climate. The period of its revolution is estimated to be about 3383 years.

La Place and Herschel both ascribe the origin of comets to the vast quantity of nebulous matter that is scattered throughout space-sometimes like extensive clouds, sometimes in small patches, and not unfrequently so like comets that they are only distinguished from those bodies by remaining motionless. Sir William discovered 2000 nebulæ in the northern hemisphere alone; these his son has already increased to 2500, and in all probability he will double the number before he finishes his interesting observations in the southern half of the heavens. The nebulæ seem to be in various states of condensation—some just beginning to form, and appearing like a flimsy veil of gauze, scarcely visible even in the darkest night, with the best telescopeswhile others are so far advanced towards the solid state that they resemble bright stars seen through a haze. Comets are supposed to have been minute nebulæ within the sphere of the sun's action, which by the mutual attraction of their particles have become sufficiently dense to be attracted by the sun, and compelled to move in orbits round him. Sir William has observed, that on their approach to the sun, the subtile fluids are driven off into the envelopes and tail, while the denser parts are consolidated; that, in retreating from the sun, a great portion of the envelopes and tail is dissipated and lost in space; and he concludes that, by a repetition of this process, a solid is at length formed, or perhaps the whole dis

sipated,

sipated, according to the nature and circumstances of the primitive nebulous mass. The consolidation will be soonest effected in the comets that come nearest to the sun and return most fre quently towards him; and the rotation of a comet will hasten the process by diminishing the gravitation of the particles and facilitating evaporation. The hypothesis of the nebulous origin of comets accounts for the length of their orbits, for as they begin their motions at a prodigious distance from the sun, they consequently must return to the same point at each revolution, provided they move in oval paths. It likewise explains the cause of the great obliquity of their orbits to the plane of the ecliptic, and why their motions are sometimes direct and sometimes retrograde.

The smallness of the orbits of Encke's and Biela's comets may appear to be adverse to this theory, but as Mr. Milne, in his excellent Essay on Comets, observes, they may, like Lexel's comet, have moved formerly in longer paths, which have been subsequently reduced to their present dimensions by the action of the planets.

The various motions of comets, and the disturbances they meet with, have led to an inquiry whether the attraction of the planets may not compel a comet to quit its path round the sun, and become a satellite to the earth or some other planet. The supposition is by no means absurd, though there is no example of such an event. But however possible it may be for the earth to acquire a new satellite, La Place has shown that the moon never has been a comet, though her arid and vitrified appearance has led some to infer the contrary. Be that as it may, it is well known that thousands of comets are moving in every direction through the heavens-and that many remarkable changes have taken place, and may again occur, in their motions, from the disturbing action of the planets;-it concerns us more nearly, therefore, to inquire what effect comets have had upon the earth-whether the path of any comet may be so altered by the action of the planets as to bring it into collision with the earth-and what the result of such a shock would be.

The comet which came nearest to the earth is that of 1577, which Tycho Brahe observed to be within three times the distance of the moon, that is, 720,000 miles from the centre of the earth; but as nothing has been recorded to have happened in consequence, probably it had no sensible effect. The comet of 837 remained four days within 1,240,000 leagues of the earth's orbit, without any perceptible influence; and it has been already stated, that Lexel's comet approached to within six times the distance of the moon from us. Its own period was diminished more than two days by the earth's action, but the reaction of the comet did not

even affect our tides. Indeed, the greater number of comets move so rapidly, that even were their attraction greater than it is, there is not time for a sufficient accumulation of impetus to produce any effect on the ocean-so that we have not a deluge to dread.

come

The masses are generally so small, that there is little chance of the earth or planets being deflected from their paths by a comet, neither is there any likelihood of the earth being burnt, for the great comet of 1680, which was so intensely heated by having nearly touched the sun's surface, never can nearer to us than nine millions of miles after leaving him, and we know of no other that ever has been so close to the sun. No comet on record has ever had the smallest effect upon our climate, and M. Arago has shown that there is no connexion whatever between the number of comets that have appeared in any season and its temperature, farther than that, as most comets are telescopic objects, they can only be seen in very clear fine weather, and therefore more are discovered in a good year than in a cloudy and foggy season. Besides, as they shine by reflected light, they are probably of low temperature, and even what they have must be much reduced during the long periods in which they wander through a medium 90° below the freezing point of Fahrenheit's thermometer.

Since the proximity of comets gives no ground of alarm, the only other cause of apprehension that remains to be considered, is the chance of collision, which is by no means impossible, when we consider the multitudes of comets that are constantly traversing our system in all directions: certainly the velocity with which they move would make the concussion dreadful if the mass were of any magnitude, but the chance of collision is still less than that of appulse; and much would depend upon the direction in which the two bodies might be moving at the instant of meeting. Should the motions of both be in the same direction, each would slide off from the surface of the other without doing more than local harm. It might, indeed, cause a deflection in the path of the earth, and a change in its velocity. The most fatal effects would be produced by comets having retrograde motions, the course of which might be directly opposite to that of the earth, and the momentum might be sufficient to destroy the progressive motion of both bodies, in which case the sun's attraction would cause both to fall to his surface. Such would be the fate of the earth if it were struck by a comet, with a mass only about four times that of the moon, and moving in a contrary direction at the rate of one million seven hundred and thirty-four thousand feet in a second :both bodies would arrive at the sun in about fifty-four and a half days. If anything had destroyed the velocity of the comet of

1680, when nearest to the sun, it would have fallen to his surface in three minutes, which places the comparative distances of that comet and the earth from the sun in a strong point of view.

Were the earth to receive a violent concussion from a comet of considerable density, the position of the axis of its diurnal revolution would, in all probability, be changed. The consequence would be a sudden rush of all the waters of the ocean from their ancient bed, which would overflow the land, sweeping before them animate and inanimate beings in one undistinguishable ruin. All the countries of the globe bear testimony to the vast and destructive effects of floods of mighty waters. The debris of multitudes of plants and animals deeply buried in the ground show that, long before man became its inhabitant, the earth was tenanted by innumerable races of beings altogether different from those which share with us the present state of things, and which must have vanished from existence thousands of ages ago, because the strata under which they are found show that sometimes the waters prevailed for numberless centuries over the beds containing them, and sometimes the dry land. Enormous masses of rock torn from their native hills have been borne over extensive countries to far distant regions; and deposits of the natives of the deep on the tops of the loftiest mountain-chains declare the irresistible force and magnitude of the vast waves which, in remote times, have carried destruction over the face of nature. These effects have been attributed to the inundations produced by the shock of a comet in former ages; but the astronomer has shown that such is not the case; that the length of the day, which is the measure of the celestial motions, is immutable and exhibits no trace of change; and that if the earth had ever been struck by a comet so as to change the axis about which it performs its diurnal rotation, the effects would still be perceptible in the variations which it would have occasioned in the geographical latitudes. As nothing is known of the earth's primitive velocity, a comet may have given it a shock, and only destroyed a part of its progressive velocity, without changing the axis of rotation. In this case the effect would have been to make it go nearer the sun, and move in a smaller orbit, which, though not absolutely impossible, is very improbable. Indeed, instead of having become warmer, the tropical nature of the fossil remains in the most northern countries of Europe and America have led to the belief that the general climate of the earth is of a lower temperature now than it was in the extremely remote ages in which these plants and animals must have flourished.

The earth is in greater danger of a shock from the comets belonging to our system than from any others. comet, which revolves in an orbit lying nearly between

two small Encke's

those of Mercury