diciously observes) is analogous and harmonical in universal nature.' I shall conclude this paper with the moral reflections of an elegant writer: 'I cannot forbear reflecting on the insignificance of human art, when set in comparison with the designs of Providence. In the pursuit of this thought, I considered a comet, or, in the language of the vulgar, a blazing star, as a sky-rocket discharged by a hand that is Almighty. Many of my readers saw that in the year 1680; and, if they are not mathematicians, will be amazed to hear, that it travelled with a much greater degree of swiftness than a cannonball, and drew after it a tail of fire that was fourscore millions of miles in length. What an amazing thought is it to consider this stupendous body traversing the immensity of the creation with such a rapidity, and, at the same time, wheeling about in that line which the Almighty had prescribed for it! That it should move in such inconceivable fury and combustion, and, at the same time, with such an exact regularity! How spacious must the universe be, that gives such bodies as these their full play, without suffering the least disorder or confusion by it! What a glorious show are those beings entertained with, that can look into this great theatre of nature, and see myriads of such tremendous objects wandering through those immeasureable depths of ether, and running their appointed courses! Our eyes may, hereafter, be strong enough to command this magnificent pros pect, and our understandings able to find out the several uses of these great parts of the universe. In the mean time, they are very proper objects for our imagination to contemplate, that we may form more extensive notions of Infinite Wisdom and Power, and learn to think humbly of ourselves, and of all the little works of human invention '.' There are three comets which have been much celebrated; viz. that which appeared in 1680, just mentioned, and those of 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 iron at its white heat; of course, it must have been entirely dissipated, if it had been any other than a fixed and 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 scarcely cool in 50,000 years. The period of this comet has been calculated at 575 years; and, if the computation be accurate, it will no 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 is not 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. Dr. Halley, who saw the comet which appeared in 1682, says, that there are many things which make me believe, that the comet which Apian saw in the year 1531, was the same with that which Guardian, No. 103. Kepler and Longimontanus 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. For 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 1758.' 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 T 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 33 days of the time computed. Now, if Dr. Halley meant the time of its passing the perihelion, and we add 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 14, 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 22 than it was in 1607. Dr. Halley suspected that the comet in 1680 was the same that appeared in 1106, 531, and also in 44 before the Christian æra. He also conjectured, that the comet observed by Apian in 1532, was the same as that observed by Hevelius in 1661; if so, its period was 129 years, and it ought to have 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 æra 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 moon light; 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'. Very remarkable comets appeared in the year 1807, 1808, and 1811, but as the nature and limits of this work forbid us from entering into a detailed description of them, we shall merely observe, (1) That of the 98 comets whose elements have been observed and calculated previously to the year 1808, 24 have passed between the Sun and the orbit of Mercury; 33 between the orbits of Mercury and Venus; 21 between the orbits of Venus and the Earth; 16 between the orbits of the Earth and Mars; 3 between the orbits of Mars and Ceres; and 1 between those of Ceres and Jupiter. (2) That 32 comets have appeared between the months of April and September, and 66 between September and April. (3) That the greater part of the comets have their perihelion nearest to their ascending nodes. (4) That 50 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 at every possible angle of the ecliptic2. Sce Vince's Complete Sytem of Astronomy. |