CHAP. XVII. ABOUT WEIGHING THE SUN. HOW MANY TIMES THE SUN IS HEAVIER THAN THE EARTH. I COPY the following sentences, about weighing the sun, and about how much the sun is heavier than the earth, and how much the earth is lighter than the sun, from a note in Mrs. Somerville's very excellent book, called the Connexion of the Physical Sciences. They will at least make my little readers "prick up their ears," and also give them a specimen of what extraordinary things are, to be learned among the sciences even now, and may yet add to the amount. "As hardly any thing," says Mrs. Somerville, appears more impossible, than that man should have been able to weigh the sun as it were in scales, and the earth in a balance, the method of doing so may have some interest. The at traction of the sun is as the quantity of matter in the sun to the quantity of matter in the earth; and as the force of this reciprocal attraction is assured by its effects, the space the earth would fall through in a second by the sun's attraction, is, to the space the sun would fall through by the earth's attraction, as the mass of the sun to the mass of the earth. Thus the weight of the sun will be known, if the lengths of these two spaces can be found in miles, or parts of miles. Nothing can be easier;"—and, here, the fair but profound philosopher whom I quote, first remarking that the distance from the centre of the earth to the surface of the earth is four thousand miles, but the distance from the surface of the earth to the surface of the sun ninety-five millions of miles, and some other things; calmly and justly assures us, that all the rest comes naturally, "by a single question in the rule of three !" After this, and after stating to us how to work the sum,-"By this simple process," she concludes, "it is found, that if the sun were placed in one scale of a balance, it would require three hundred and fifty-four thousand, nine hundred and thirty-six earths, to form a counterpoise." That is, that the sun is three hundred and fifty-four thousand, nine hundred and thirty-six times as heavy as the earth; or, that the earth is three hundred and fifty-four thousand, nine hundred and thirty-six times as light* as the sun. But all this, as you see, is only a comparative reckoning. Neither Mrs. Somerville (learned as that excellent lady is,) nor any body else, however learned or excellent, can answer us either of the previous questions, how heavy is the sun, or how light the earth? So that we are still without a * I need not here remind my little readers, that the word light is employed to signify the reverse of heavy, as well as the reverse of dark. chance of knowing, either in pounds troy, or in pounds avoirdupoise, or in tons, or in hundredweights, what either the sun or the earth absolutely weighs; which is what I am afraid you expected to hear when I began my chapter, and when you found my author talking of our being able to " weigh the sun as it were in scales, and the earth in a balance;"-assuring us that "nothing can be easier;" and proposing to set before us "the method." She teaches us, indeed, how to weigh one heavenly body against another; but not what it is that any one of those bodies absolutely weighs. By the same process as in the present case, we shall compare, by and by, the weight of the sun with the smallest of the satellites of Jupiter. |