the earth is far from Jupiter than when it is near. For half a century Roemer's views found little credence, but we know now that he was right, and that on the average the eclipses come 8m. 18s. early when the earth is nearest to Jupiter, and 8m. 18s. late when it is on the opposite side of its orbit. This is equivalent to saying that light takes 8m. 18s. to cover the distance from the sun to the earth, so that at any moment we see the sun not as it then is, but as it was 8 minutes earlier. It has been found possible in recent years to measure by direct experiment the velocity with which light travels--186,337 miles per second--and multiplying this number by the 498s. (= 8m. 18s.) we obtain a new determination of the sun's distance from the earth. The product of the two numbers is 92,795,826, in very fair agreement with the 93,000,000 miles found in Chapter X; but, as noted there, this method, like every other, has its weak side, and the result may be a good many thousands of miles in error. It is worthy of note in this connection that both methods of obtaining the sun's distance which were given in Chapter X involve Kepler's Third Law, while the result obtained from Jupiter's satellites is entirely independent of this law, and the agreement of the several results is therefore good evidence both for the truth of Kepler's laws and for the soundness of Roemer's explanation of the eclipses. This mode of proof, by comparing the numerical results furnished by two or more different principles, and showing that they agree or disagree, is of wide application and great importance in physical science. SATURN 142. THE RING OF SATURN.--In respect of size and mass Saturn stands next to Jupiter, and although far inferior to him in these respects, it contains more material than all the remaining planets combined. But the unique feature of Saturn which distinguishes it from every other known body in the heavens is its ring, which was long a puzzle to the astronomers who first studied the planet with a telescope (one of them called Saturn a planet with ears), but, was after nearly half a century correctly understood and described by Huyghens, whose Latin text we translate into--"It is surrounded by a ring, thin, flat, nowhere touching it, and making quite an angle with the ecliptic." [Illustration: FIG. 91.--Aspects of Saturn's rings.]