CONVERSATION XLIV. Of the Herschel Planet, Tutor. We have but one more planet to describe, that is the Herschel. James. Was it discovered by Dr. Herschel? Tutor. It was, on the 13th of March, 1781, and therefore by astronomers in general it is denominated the Herschel planet; though by the Doctor himself it was named the Georgium Sidus, or Georgian star, in honour of his present majesty George the Third, who has for many, years been a liberal patron to this great and most indefatigable astronomer. Charles. I do not think that I have ever seen this planet. Tutor. Its apparent diameter is too small to be discerned readily by the naked eye, but it may be easily discovered in a clear night, when it is above the horizon, by means of a good telescope, its situation being previously known from the Ephemeris. James. Is it owing to the smallness of this planet, or to its great distance from the sun, that we cannot see it with the naked eye? Tutor. Both these causes are combined: in comparison of Jupiter and Saturn it is small, his diameter being less than 35,000 miles in length; and his distance from the sun is estimated at more than one thousand eight hundred millions of miles from that luminary, around which, however, he performs his journey in eighty-two of our years, consequently he must travel at the rate of 16,000 miles an hour. Charles. But if this planet has been discovered only twenty-two years, how is it known that it will complete its revolution in eighty-two years? Tutor. By a long series of observations it was found to move with such a velocity, as would carry it round the heavens in that period. Moreover, when it was first discovered, it was in Gemini, and in August, 1803, it had advanced in the 11° of Libra, more than a fourth part of its journey; and now in June, 1809, it is in the eighth of Scorpio. James. How many moons has the Herschel ? Tutor. He is attended by six satellites or moons, of which, the one nearest to the planet performs his revolution round the primary in five days and twenty-three hours, but that which is the most remote from him takes 107 days and 16 hours for his journey. Charles. Is there any idea formed as to the light and heat enjoyed by this planet? Tutor. His distance from the sun is nineteen times greater than that of the earth, consequently since the square of 19 is 361, the light and heat experienced by the inhabitants of that planet must be 361 times less than we derive from the rays of the sun. The proportion of light enjoyed by the Herschel has been estimated at about equal to the effect of two hundred and forty-nine of our full moons. ” CONVERSATION XLV. 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, 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. 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 redhot 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 thus 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. See Enfield's Institutes of Natural Philosophy, p. 296, second edition. Charles. Are the periodical times of the comets known? Tutor. Not with any degree of certainty; it was 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. 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 of the present day have been disappointed. The third was that which appeared in 1680, and its period being estimated at 575 years 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 eight hundred and eighty thousand miles in an hour. James. Do all bodies move faster or slower in proportion as they are nearer to, or more distant from their centre of motion? Tutor. They do, for if you look back upon the last six or seven lectures, you will see 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 VOL. I.-X |