2250. Many are supposed to have periods of thousands of years; and some have their orbits so modified by the attraction of the planets, as to pass off in parabolic curves, to return to our system no more. Prof. Nichol is of opinion that the greater number visit our system but once, and then fly off in nearly straight lines till they pass the center of attraction between the solar system and the fixed stars, and go to revolve around other suns in the far distant heavSir John Herschel expresses the same sentiment. ens. 303. The distances to which those comets that return must go, to be so long absent, must be very great. Still their bounds are set by the great law of gravitation, for were they to pass the point "where gravitation turns the other way," they would never return. But some, at least, do return, after their "long travel of a thousand years." What a sublime conception this affords us of the almost infinite space between the solar system and the fixed stars. 304. The perihelion distances of the various comets that have appeared, and whose elements have been estimated by astronomers, are also exceedingly variable. While some pass very near the sun, others are at an immense distance from him, even at their perihelion. Of 137 that have been particularly noticed, 30 passed between the sun and the orbit of Mercury. 44 between the orbits of Mercury and Venus. 34 66 66 66 66 Venus and the earth. Mars and Jupiter. 303. Distances to which they go? Remark respecting the law of gravi tation? What specimen of orbit given? 304. What said of perihelion distances? How many noticed? Where did ORBITS OF SEVERAL COMETS. HALLEY'S 76 YRS ENCKE'S 35YRS The orbit of Encke's comet is wholly within the orbit of Jupiter, while that of Biela's extends but a short distance beyond it. The aphelion distance of Halley's comet is 3,400 millions of miles, or 550 millions of miles beyond the orbit of Neptune. But these are all comets of short periods. 305. The number of comets belonging to, or that visit the solar system, is very great. Some have estimated them at several millions. When we consider that most comets are seen only through telescopes an instrument of comparatively modern date and that, notwithstanding this, some 450 are mentioned in ancient BIELA'S 6 YRS. annals and chronicles, as having been seen with the naked eye, it is probable that the above opinion is by no means extravagant. It is supposed that not less than 700 have been seen at different times since the birth of Christ. The paths of only about 140 have been determined. The extreme difficulty of observing comets whose nearest point is beyond the orbit of Mars, is supposed to account for the comparatively small number that have been seen without that limit; and the proximate uniformity of the distribution of their orbits over the space included within the orbit of Mars, seems to justify the conclusion, that though seldom detected beyond his path, they are nevertheless equally distributed through all the spaces of the solar heavens. Reasoning upon this hypothesis, Professor Arago concludes that there are probably seven millions of comets that belong to or visit the solar system. 306. The directions of comets are as variable as their forms or magnitudes. They enter the solar system from all points of the heavens. Some seem to come up from the immeasurable depths below the ecliptic, and, having doubled "heaven's mighty cape," again plunge downward with their fiery trains, and are lost for ages in the ethereal võid. Others appear to come down from the zenith of the universe, and, having passed their peri they pass? (What samples given in cut? Where does the orbit of Encke's comet lie? Of Biela's? Of Halley's?) 305. The number of comets? What estimate? Why probably correct? How many supposed to have been seen since the birth of Christ? (Why so few seen? How supposed to be distributed? What conclusion of Arago?) 806. Direction of comets? (Remark of late writer?) helion, reascend far above all human vision. Others again are dashing through the solar system, in all possible directions, apparently without any prescribed path, or any guide to direct them in their eccentric wanderings. Instead of revolving uniformly from east to west, like the planets, their motions are direct, retrograde, and in every conceivable direction. It is remarked by a late writer, that the average inclinations of all the planes in which the comets now on record have been found to move, is about 90°. This he regards as a wonderful instance of the goodness of Providence, in causing their motions to be performed in a manner least likely to come in contact with the earth and the other planets. 307. Of the physical nature of comets, little is known. That they are, in general, very light and vapory bodies, is evident from the fact that stars have sometimes been seen even through their densest portions, and are generally visible through their tails, and from the little attractive influence they exert upon the planets in causing perturbations. While Jupiter and Saturn often retard and delay comets for months in their periodic revolutions, comets have not power, in turn, to hasten the time of the planets for a single hour; showing conclusively that the relative masses of the comets and planets are almost infinitely disproportionate. Such is the extreme lightness or tenuity of cometary bodies, that in all probability the entire mass of the largest of them, if condensed to a solid substance, would not amount to more than a few hundred pounds. Sir Isaac Newton was of opinion, that if the tail of the largest comet was compressed within the space of a cubic inch, it would not then be as dense as atmospheric air! The comet of 1770 got entangled, by attraction, among the moons of Jupiter, on its way to the sun, and remained near them for four months; yet it did not sensibly affect Jupiter or his moons. In this way the orbits of comets are often entirely changed. 308. Comets were formerly regarded as harbingers of famine, pestilence, war, and other dire calamities. In one or two instances, they have excited serious apprehension that the day of judgment was at hand, and that they were the appointed messengers of Divine wrath, hasting apace to burn up the world. A little reflection, however, will show that all such fears are groundless. The same unerring hand that guides the ponderous planet 807. Physical nature of comets? What proofs of their light and vapory character? (What said of their probable mass? Opinion of Newton? What said of the comet of 1770? What effect on orbits?) 308. How comets formerly regarded? Why no fears of collision? (What estimate of "chances?") in its way, directs also the majestic comet; and where infinite wisdom and almighty power direct, it is almost profane to talk of collision or accident. Even those who have calculated the "chances" of collision-as if chance had any thing to do among the solar bodies-have concluded the chances of collision are about as one to 281,000,000-i. e., like the chance one would have in a lottery, where there were 281,000,000 black balls, and but one white one; and where the white ball must be produced at the first drawing to secure a prize. 309. Were a collision actually to take place between a comet and the earth, it is not probable that the former would even penetrate our atmosphere, much less dash the world to pieces. Prof. Olmsted is of opinion that in such an event, not a particle of the comet would reach the earth-that the portions encountered by her would be arrested by the atmosphere, and probably inflamed; and that they would perhaps exhibit, on a more magnificent scale than was ever before observed, the phenomena of shooting stars or meteoric showers. The idea, therefore, that comets are dangerous visitants to our system, has more support from superstition than from reason or science. The air is to us what the waters are to fish. Some fish swim around in the deep, while others, like lobsters and oysters, keep on the bottom. So birds wing the air, while men and beasts are the "lobsters" that crawl around on the bottom. Now there is no more probability that a comet would pass through the atmosphere, and injure us upon the earth, than there is that a handful of fog or vapor thrown down upon the sur face of the ocean, would pass through and kill the shell-fish at the bottom. 310. After all that is supposed to be known respecting comets, it must be admitted that they are less understood than any other bodies belonging to our system. "What regions these bodies visit, when they pass beyond the limits of our view; upon what errands they come, when they again revisit the central parts of our system; what is the difference between their physical constitution and that of the sun and planets; and what important ends they are destined to accomplish in the economy of the universe, are inquiries which naturally arise in the mind, but which surpass the limited powers of the human understanding at present to determine." 309. What probable effect in case of collision? Prof. Olmsted's opinion? (Remark respecting the air, fish, lobsters, &c. ?) 310. Are we as well acquainted with comets as with other bodies of our system? What inquiries suggested? How answered? CHAPTER X. OF THE SUN. 311. Or all the celestial objects with which we ar acquainted, none make so strong and universal an impression upon our globe as does the sun. He is the great center of the solar system-a vast and fiery orb, kindled by the Almighty on the morn of creation, to cheer the dark abyss, and to pour his radiance upon surrounding worlds. Compared with him, all the solar bodies are of inconsiderable dimensions; and without him, they would be wrapped in the gloom of interminable night. 312. The form of the sun is that of an oblate spheroid, his equatorial being somewhat greater than his polar diameter. The mean of the two is 886,000 miles. He is 1,400,000 times as large as the mighty globe we inhabit, and 500 times as large as all the planets put together. Were he placed where the earth is, he would fill all the orbit of the moon, and extend 200,000 miles beyond it in every direction. It would take 112 such worlds as ours, if laid side by side, to reach across his vast diameter. 1. The vast magnitude of the sun may be Inferred from the fact, that when rising or setting, he often appears larger than the largest building, or the tops of the largest trees. Now if the angle filled by him at the distance of two miles is over 100 feet across, what must it be at the distance of 95 millions of miles? THE SUN AND THE MOON'S ORBIT. 203.000 240.000 240.000 203 000 480.000 2. Were a railroad passed through the sun's center, and should a train of cars start from one side, and proceed on at the rate of 30 miles an hour, it would require 34 years 811. Describe the sun. How compare with the rest of the system? 812. What is his form? Diameter? Mass, as compared with our globe? With all other bodies of the system? With moon's orbit? (What sensible evidence of the vast magnitude of the sun? Illustration from railroad? Demonstration as to its comparison with moon's orbit?) |