Is eight miles each second slow progress? Compared with the wild whirl of little Mercury, or even compared with the rate of our own earth’s advance, we may count it so; but certainly not, compared with our notions of speed upon earth. Eight miles each second is five hundred times as fast as the swiftest express-train ever made by man. No mean pace that for so enormous a body. For Jupiter is the very largest of all the members of the Solar System except the sun himself--quite the eldest brother of the family. His diameter is about eighty-five thousand miles, his axis being nearly eleven times as long as that of earth. Though in proportion to his great bulk not nearly so heavy as our earth, yet his bulk is so vast that more than twelve hundred earths would be needed to make one Jupiter. It must not, however, be forgotten that there is a certain amount of uncertainty about these measurements of Jupiter. He seems to be so covered with a dense atmosphere and heavy clouds that it is quite impossible for us to learn the exact size of the solid body within. Jupiter does not travel alone. Borne onwards with him, and circling steadily around him, are five moons; the smallest with a probable diameter of about one hundred miles; one about the same size as our own moon, and the others all larger. The nearest of the five, though distant from the center of the planet over one hundred and twelve thousand miles, revolves about him in twelve hours; the second, though farther from Jupiter than our moon from the earth, speeds round him in less than two of our days. The most distant, though over a million miles away, takes scarcely seventeen days to accomplish its long journey. Jupiter and his moons make a little system by themselves--a family circle within a family circle. Like the smaller planets, Jupiter spins upon his axis; and he does this so rapidly that, notwithstanding his great size, his day lasts only ten hours, instead of twenty-four hours like ours. But if Jupiter’s day is short, his year is not. Nearly twelve of our years pass by before Jupiter has traveled once completely round the sun. So a native of earth who had just reached his thirty-seventh year, would, on Jupiter, be only three years old. Passing onward from Jupiter, ever farther and farther from the sun, we leave behind us another vast and empty space--empty as we count emptiness, though it may be that there is in reality no such thing as emptiness throughout the length and breadth of the universe. The width of the gap which divides the pathway of Jupiter from the pathway of his giant brother-planet Saturn is nearly five times as much as the width of the gap separating the earth from the sun. The distance of Saturn from the sun is not much less than double the distance of Jupiter.