Page:In the high heavens.djvu/47

 him. Careful measurements having been made it has been shown that Jupiter is 1,200 times bigger than our earth: in other words, that 1,200 globes, each as large as this earth, rolled together into one, would only form a ball as big as this mighty planet. Astronomers also have the means of weighing a great planet as well as of measuring it. How this weighing is to be effected I shall not here pause to describe; suffice it to say that the little moons by which Jupiter is attended afford by their movements the means of answering the question; and the answer is a significant one, for we find that Jupiter is about 300 times as heavy as the earth. This gives us, indeed, an impressive idea of the magnificence of the mightiest of the planets. Were a pair of gigantic weighing scales constructed, and Jupiter placed in one of these scales, then it would require 300 globes each as heavy as the earth to be placed in the other before the mighty balance could turn. Yet when we remember that Jupiter is 1,200 times as large as the earth we may well feel surprised at learning that he is only 300 times as heavy. Were the constitution of the planet at all like that of our earth, then the weights and the sizes should observe the same proportions, just as one solid iron ball, when ten times as big as another, will be ten times as heavy. The lightness of Jupiter in comparison with his size is really the point that merits our astonishment. He is, indeed, not so very much heavier than a globe of water the same size would be, while our earth is five times as heavy as a globe of water equally large. The true explanation is that Jupiter is so swollen by these enormous masses of cloud which surround him as to give him a bigness altogether out of proportion to his mass. Therefore, as Mars gives