CHAP. LIX. ABOUT THE SUPPORT AND MOVEMENTS OF THE HEAVENLY ABOUT WHILE I have been talking, however, of the sun, and moon, and stars; of the nebules, planets, comets, and of all this multitude of minor celestial bodies at present supposed to be contained and moving in the heaven; and even of the earth itself, as contained and moving like the rest; your little minds, my dear and eager listeners! have been busy with perplexing thoughts, as to how these bodies are supported in the heaven, how they are made to move in it, and how they are made to move with regularity, and come back, day after day, year after year, and thousands of thousands of years, to the same places with respect to each other, and to the same places in the heaven? You conclude, that there is some power, some force, which accomplishes all this; and you ask me, what is that force, that power, or by what means these wonderous things are done? It is very natural, for example, when we see or hear of even the planets only, travelling so swiftly, to inquire why they always keep moving round in the same paths? Why they always revolve in the same manner round the sun? Why they do not break away from their orbits, and fly into distant and unknown regions of space? To these questions I might answer, that God keeps them always in their places. And this answer would be true; but yet God works by means. He makes the grass grow, but he does it by means of sun and rain. He keeps the planets in their orbits, but what are the means by which he does so? This is what I am now going to tell you. All bodies possess attraction; by this I mean, that all bodies mutually draw If each other by an unseen power, called attraction. you throw a stone into the air, it comes down immediately to the earth. We say it falls, but why does it fall? Because the earth attracts it, or draws it back again. It is by this power of attraction that the inhabitants of the earth are kept on its surface. Those on the other side of the earth remain just as firmly upon it as we do, because the earth attracts all things that are in, or upon, or (as we see with its atmosphere) around its body. Well the sun has the same power of attraction as the earth, so the sun and the earth attracts the sun. attracts the earth, The consequence earth and sun, are is, that these two bodies, the inclined to come together. In other words, the earth is powerfully drawn toward the sun; and, unless it were in some way prevented, it would immediately set out, and go to the sun. But how is the earth prevented from taking that journey? I will tell you. When a body is by force of this But it is pre set in motion by being thrown forward, it is inclined to proceed in a straight line. Now the earth has such a motion; and consequently, were it not restrained, it would, motion, fly away from the sun. vented from doing so, by the attraction of the sun. The attraction of the sun, gives the earth a circular movement, and thus keeps it in its orbit. To make you understand this perfectly, I beg you to look at my picture of a boy whirling an apple round his head. There, the apple is drawn toward the boy by a string, just as the earth is drawn toward the sun by attraction. The apple is thrown forward; and, if the string were to break, it would fly off in a strait line. But the string holds it fast, and, giving it a circular movement, makes it pass round the boy's head, just as the earth in its orbit passes round the sun. This is the place, too, where I should speak of those distinguished French astronomers, La grange and Laplace, whose brilliant demonstrations have made it certain, that every variation of the planetary motions is but periodical, or regularly alternate, like that of the pendulum of a clock; and that therefore (contrary to what had been previously apprehended) the order of these motions is indefinitely permanent, being subject to no change but such as is periodical. |