SATURN'S MOONS AND BELTS 179 



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 tele- 

 scope, 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 



