hang to the disk a long while before they would vanish." The seventh or innermost (Mimas) thus hung on the disc for twenty minutes, and the sixth for fourteen or fifteen. Had there been no atmosphere, both of the moons would have been at once hid behind the planet. This takes place when a star comes up to our moon, and vanishes behind it. The star is seen to go out at once; and the conclusion drawn is that this could not happen unless there were no atmosphere or very little of it in the moon to keep the star in sight for us after it had really vanished. Our atmosphere gives us twilight, morning and evening, and enables us to see the sun some minutes before he rises, and for as long after he has set. Ultimately Herschel perceived a quintuple belt, two dark and three bright, on Saturn. Sometimes also he noticed a whitish light at the poles similar to the polar spots on Mars, and due, he believed, to the same cause. But what these belts really are is a problem still unsolved. The vast body of Saturn is lighter than the same volume of water, and would float in it like cork. Our earth is about five times heavier than a globe of water of the same size, and would sink in water like lead. Whether Saturn is still a heated mass, slowly cooling down, and these clouds arise from streams of gas given off, remain problems for the future to solve.
With improved mirrors and a less powerful telescope, he watched the movements and changes of the ring. Between 1790 and 1806 he wrote seven papers for the Royal Society on Saturn and his system. Slowly he came to the conclusion, which he dismissed at first as improbable, that the ring was not single, but double, with a gulf twenty-five hundred miles [1680] in
