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nucleus of different degrees of darkness in different Comets, though sometimes no nucleus can be seen. As the Comet advances towards the Sun, its faint and nebulous light becomes more brilliant, and its luminous train gradually increases in length.-When it reaches its perihelion, the intensity of its light, and the length of its tail, reach their maximum, and sometimes it shines with all the splendour of the planet Venus. During its passage from the perihelion, it is shorn from its splendour; it gradually resumes its nebulous appearance, and its tail decreases in magnitude, till it reaches such a distance from the earth that the attenuated light of the Sun, which it reflects, ceases to reach the eye. Traversing unseen by man, the remote portion of its orbit, the Comet wheels its ethereal course far beyond the time of the solar system. What region it there visits, or upon what destination it is sent, we are wholly ignorant. After a lapse of years, we perceive it again returning to our system, and tracing a portion of the same orbit round the Sun which it had formerly described.

Hevelius observed of the Comet in 1661, that its body was of a yellowish colour, bright and conspicuous, but without any glittering light. In the middle was a dense ruddy nucleus, almost equal to Jupiter's, encompassed with a much fainter, thinner matter. It changed its appearance almost every day. On the 5th of February, the nucleus was somewhat bigger and brighter, of a gold colour, but its light was more dusky than the rest of the stars; it appeared also to be divided into a number of parts. On the following day the nucleus still appeared, though less than before. One of them on the left side of the lower part of the disk, appeared to be denser and brighter than the rest; its body was round, and representing a little lucid star, the nuclei still encompassed with another kind of matter. February 10th, the nuclei were more obscure and confused, but brighter at top than at the bottom. On the 13th the head was diminished, both in magni

tude and brilliancy; and on the 28th its matter seemed much dispersed, and no distinct nucleus at all appearing.

Wigelius gives an account of another which he saw through a telescope in the year 1664. He observed th Moon and a little cloud, illuminated by the Sun at the same time, and distinctly saw that the Moon appeared of a continued luminous surface; but the Comet was very different, being perfectly like a little cloud, enlightened by the Sun's beams.

Comets also are, to appearance, surrounded with atmospheres of a prodigious size, often rising ten times higher than the nucleus. They have sometimes been seen with different phases, like the Moon.

Dr. Long describes the appearance of a Comet to the naked eye in the following manner :"The head appears sometimes like a cloudy star; sometimes it shines with a dull light like that of the planet Saturn; sometimes they have been seen to exceed stars of the first magnitude; some have surpassed Jupiter, and even Venus, in brilliancy, and to have cast a shadow, as Venus sometimes does."

There are three Comets, which have been much celebrated, viz. those which appeared in 1680, 1744, and 1759. The Comet of 1680 was remarkable for its near approach to the Sun; so near, that in its perihelion it was not above a third part of the diameter of that luminary from the surface thereof. Its great heat, in that position, was computed to be 2000 times hotter than an iron at its white heat; of course it must have been entirely dissipated, if it had been any other than a fixed and a solid body. It must also have retained its heat an immense time; for a globe of iron, of an inch in diameter, exposed to the open air, scarcely loses its heat in less than an hour; but a larger globe will retain its heat longer in proportion to its diameter, because the surface at which it grows cold varies in that proportion less than the quantity of hot matter. Therefore, a globe of red hot iron,

as big as our Earth, would searcely cool in 50,000 years.. The period of this Comet has been calculated at 575 years; and, if the computation be accurate, it will not return to the vicinity of the Earth till about the year 2225. This Comet, at its greatest distance, is about eleven thousand two hundred millions of miles from the Sun, and at its least distance it is no more than 49,000 miles. In that part of its orbit which was nearest the Sun, it was computed to move at the rate of

880,000 miles in an hour.

For

Dr. Halley, who saw the Comet which appeared in 1682, says, "That there are many things which make me believe that the Comet which Appian saw in the year 1531, was the same with that which Kepler and Longiomontanus more accurately described in the year 1607, and which I myself have seen return, and observed in 1682. All the elements agree, and nothing seems to contradict this opinion, except the inequality of the periodic revolutions; which inequality is not so great, but that it may be owing to physical causes. the motion of Saturn is so disturbed by the rest of the planets, especially Jupiter, that the periodic time of that planet is uncertain for some whole days together. How much more, therefore, will a Comet be subject to like errors, which rises, perhaps, four times higher than Saturn, and whose velocity, though increased but very little, would be sufficient to change its orbit from an ellipse to a parabola! and I am the more confirmed in my opinion of its being the same; for, in the year 1456, in the summer time, a Comet was seen passing retrograde between the Earth and the Sun, much after the same manner, which, though nobody made observations upon it, yet, from its period and manner of transit, I cannot think different from those I have just mentioned; and since looking over the history of Comets, I find, at an equal interval of time, a Comet to have been seen about Easter, in the year 1305, which is another double period of 151 years before the former.

Hence I think I may venture to foretel that it will return again in 1753."

Dr. Halley computed the effect of Jupiter upon this Comet in 1682, and found that it would increase its periodic time above a year; in consequence of which, he predicted its return at the end of the year 1758, or the beginning of 1759. M. Clairault computed the effects of both Saturn and Jupiter, and found that the former would retard its return in the last period 100 days, and the latter 511 days; and he determined the time when the Comet would come to its perihelion to be on the 15th of April, 1759, observing, however, that he might err a month, from neglecting small quantities in the computation. The Comet did pass the perihelion on the 13th of March, within thirty-three days of the time computed. Now if Dr. Halley meant the time of its passing the perihelion, and we had 100 days for the action of Saturn, which he did not take into the calculation, it will bring it very near to the time in which it passed the perihelion, and prove his computation of the effect of Jupiter to have been accurate. But if he meant the time when the Comet would first. appear, his prediction was accurate; for it was first seen on December 14th; 1758, Dr. Halley, therefore had the glory first to foretel the return of a Comet; and the event answered, in a remarkable manner, his prediction. He further observed, that the action of Jupiter, in the descent of the Comet towards its perihelion in 1682, would tend to increase the inclination of its orbit; and, accordingly, the inclination in 1682 was found to be greater by twenty-two than it was in the year 1607.

Dr. Halley suspected that the Comet in 1680, was the same that appeared in 1106, 531, and also in the year 44, before the Christian era. He also conjectured, that the Comet observed by Appian in 1532 was the same as that observed by Hevelius in 1661; if so, its period was 129 years, and it ought to have been returned in 1789, but it did not appear,

though astronomers here, and on the Continent, were watching its approach with great anxiety.

From the beginning of our era to this time, it is probable, according to the best accounts, that there have appeared 500 Comets. Before that time, above 100 others are mentioned in history; but, perhaps, half of these, had they been accurately observed, would not have proved Comets. When, however, we consider that many others may not have appeared, from being too near the Sun; from appearing in moonlight; from being in the other hemisphere; from being too small to be perceived; or which may not have been recorded; it is reasonable to suppose that the whole number is much greater. It is, on the other hand, very likely, that of the Comets that have been recorded as seen, the same may have appeared several times, and therefore the number may be less than is stated.

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Remarkable Comets appeared in the years 1807, 1808. and 1811; of these we may, in a subsequent Number, give enlarged accounts. We shall only observe, 1. That of the ninety-eight Comets, whose elements have been observed and calculated previously to the year 1808, twenty-four have passed between the Sun and orbit of Mercury; thirty-two between the orbits of Mercury and Venus; twenty-one be tween the orbits of Venus and the earth; sixteen between the orbits of the Earth and Mars; three between the orbits of Mars and Ceres; and one between those of Ceres and Jupiter.-2. That thirty-two Comets have appeared between the months of April and September, and sixty-six between September and April.-3. That the greatest part of the Comets have their perihelion nearest to their ascending nodes.—4, That fifty Comets move from west to east.-5. That the orbits of the Comets are not confined to any particular region of the heavens, like the planets that have long been known, but seem to be inclined to every possible angle of the ecliptic.

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