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the comet whose period is 63 years was ten times nearer the earth in 1805 than in 1832, when it caused so much alarm.

Comets, when in or near their perihelion, move with prodigious velocity. That of 1680 appears to have gone half round the sun in ten hours and a half, moving at the rate of 880000 miles an hour. If its enormous centrifugal force had ceased when passing its perihelion, it would have fallen to the sun in about three minutes, as it was then only 147000 miles from his surface. So near the sun, it would be exposed to a heat 27500 times greater than that received by the earth; and as the sun's heat is supposed to be in proportion to the intensity of his light, it is probable that a degree of heat so very intense would be sufficient to convert into vapour every terrestrial substance with which we are acquainted. At the perihelion distance the sun's diameter would be seen from the comet under an angle of 73°, so that the sun, viewed from the comet, would nearly cover the whole extent of the heavens from the horizon to the zenith; and as this comet is presumed to have a period of 575 years, the major axis of its orbit must be so great, that at the aphelion the sun's diameter would only subtend an angle of about fourteen seconds, which is not so great as half the diameter of Mars appears to us when in

opposition. The sun would consequently impart no heat, so that the comet would then be exposed to the temperature of the etherial regions, which is 58° below the zero point of Fahrenheit. A body so rare as the comet, and moving with such velocity, must have met with great resistance from the dense atmosphere of the sun, while passing so near his surface at its perihelion. The centrifugal force must consequently have been diminished, and the sun's attraction proportionally augmented, so that it must have come nearer to the sun in 1680 than in its preceding revolution, and would subsequently describe a smaller orbit. As this diminution of its orbit will be repeated at each revolution, the comet will infallibly end by falling on the surface of the sun, unless its course be changed by the disturbing influence of some large body in the unknown expanse of creation. Our ignorance of the actual density of the sun's atmosphere, of the density of the comet, and of the period of its revolution, renders it impossible to form any idea of the number of centuries which must elapse before this singular event takes place.

But this is not the only comet threatened with such a catastrophe; Encke's, and that discovered by M. Biela, are both slowly tending to the same fate. By the resistance of the ether, they will

perform each revolution nearer and nearer to the sun, till at last they will be precipitated on his surface. The same cause may affect the motions of the planets, and be ultimately the means of destroying the solar system; but, as Sir John Herschel observes, they could hardly all revolve in the same direction round the sun for so many ages without impressing a corresponding motion on the etherial fluid, which may preserve them from the accumulated effects of its resistance. Should this material fluid revolve about the sun like a vortex, it will accelerate the revolutions of such comets as have direct motions, but it will retard those that have retrograde motions.

Though already so well acquainted with the motions of comets, we know nothing of their physical constitution. A vast number, especially of telescopic comets, are only like clouds or masses of vapour often without tails. Such were the comets which appeared in the years 1795, 1797, and 1798; but the head commonly consists of a mass of light, like a planet surrounded by a very transparent atmosphere, the whole, viewed with a telescope, being so diaphanous, that the smallest star may be seen even through the densest part of the nucleus; and in general their masses, when they have any, are so minute that they have no sensible diameter, like that of the comet of 1811, which

appeared to Sir Wm. Herschel like a luminous point in the middle of the nebulous matter. The nuclei, which seem to be formed of the denser strata of that nebulous matter in successive coatings, are often of great magnitude; those of the comets which came to the sun in the years 1799 and 1807 had nuclei whose diameters measured 180 and 275 leagues respectively, and the second comet of 1811 had a nucleus 1350 leagues in diameter.

The nebulosity immediately round the nucleus is so diaphanous that it gives little light; but at a small distance the nebulous matter becomes suddenly brilliant, so as to look like a bright ring round the body. Sometimes there are as

many as two or three of these luminous concentric rings separated by dark intervals, but they are generally incomplete on the side next the tail. In the comet of 1811, the luminous ring was 12400 leagues thick, and the distance between its interior surface and the centre of the nucleus was as much as 14880 leagues. The thickness of these bright diaphanous coatings in the comets of 1807 and 1799 were 14880 and 9920 leagues respectively. The transit of a comet over the sun would afford the best information with regard to the nature of the nuclei. It was computed that such an event was to take

place in the year 1827; unfortunately the sun was hid by clouds from the British astronomers, but it was examined at Viviers and at Marseilles, at the time the comet must have been projected on its disc, but no spot or cloud was to be seen.

The tails of comets proceed from the head in two streams of light somewhat like that of the aurora; these in most cases unite at a greater or less distance from the nucleus, and are generally situate in the planes of their orbits; they follow the comets in their descent towards the sun, but precede them in their return with a small degree of curvature, probably owing to the resistance of the ether, but their extent and form must vary in appearance according to the positions of their orbits with regard to the ecliptic. In some cases, the tail has been at right angles to the line joining the sun and comet. They are generally of enormous lengths,-the comet of 1811 had a tail no less than 34 millions of leagues in length, and those which appeared in the years 1618, 1680, and 1769, had tails which extended respectively over 104, 90, and 97 degrees of space; consequently, when the heads of these comets were set, a portion of the extremity of their tails was still in the zenith. Sometimes the tail is divided into several branches, like the comet of 1744, which had six, separated by dark intervals, each of them

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