CONVERSATION XXIV. Of Comets. TUTOR. Besides the seven primary planets, and the eighteen secondary ones, or satellites, which we have been describing, there are other bodies belonging to the solar system, called comets, to which Thomson in his Summer beautifully alludes: Amid the radiant orbs That more than deck, that animate the sky, The rushing comet to the sun descends, With awful train projected o'er the heavens, SUMMER, line 1702. Charles. Do comets resemble the planets in any respects? Tutor. Like them, they are supposed to revolve about the sun in elliptical orbits, and to describe equal areas in equal times; but they do not appear to be adapted for the habitation of animated beings, owing to the great degrees of heat and cold to which, in their course, they must be subjected, in consequence of the great eccentricity of most of their orbits. The comet seen by Sir Isaac Newton, in the year 1680, was observed to approach so near the sun, that its heat was estimated by that great man, to be 2000 times greater than that of red-hot iron. James. It must have been a very solid body to have endured such a heat without being entirely dissipated. Tutor. So indeed it should seem; and a body shus heated must retain its heat a long time; for a red-hot globe of iron, of a single inch in diameter, exposed to the open air, will scarcely lose all its heat in an hour; and it is said, that a globe of red-hot iron, as large as our earth, would scarcely cool in 50,000 years. Charles. Are the periodical times of the comets known? Tutor. Very few of them with any degree of certainty; it was long ago supposed that the periods of three of them had been distinctly ascertained. The first of these appeared in the years 1531, 1607, and 1682, and it was expected to return every 75th year ; and one which, as had been predicted by Dr. Halley, appeared in 1758, which was supposed to be the same.* Of late, it has been determined that some comets have periods of only a few years. Thus, The second of them appeared in 1532, and 1661, and it was expected that it would again make its appearance in 1789, but in this the astronomers were disappointed. The third was that which appeared in 1680, and its period being estimated at 575 years, it cannot, upon that supposition, return until the year 2255. This last comet at its greatest distance is eleven thousand two hundred millions of miles from the sun, and its least distance from the sun's centre was but four hundred and ninety thousand miles; in this part of its orbit it travelled at the rate of 880,000 miles in an hour. James. Do all bodies move faster or slower in proportion as they are the comet of 1772 and 1805, has a period of 6 years and 207 days. It passed its perihelion March 19th, 1826. nearer to, or more distant from, their centre of motion? Tutor. They do, for if you meditate upon the last six or seven lectures, you will recollect that the Herschel, which is the most remote planet in the solar system, travels at the rate of 16,000 miles an hour: Saturn, the next nearer in the order 21,000 miles; Jupiter, 28,000 miles; Mars 53,000 miles; the earth 65,000 miles; Venus 75,000 miles; and Mercury at the rate of 105,000 miles in an hour. But here we come to a comet, whose progressive motion, in that part of its orbit which is nearest to the sun, is more than equal to eight times the velocity of Mercury. Charles. Were not comets formerly dreaded as awful prodigies, intended to alarm the world? Tutor. Comets are frequently ac |