Saturn revolves round the Sun in a little under 29½ years at a mean distance of 886 millions of miles. Its apparent diameter varies between 15″ and 20″; its true diameter may be put at 75,000 miles. The flattening of the poles, or “polar compression” as it is called, is greater than that of any other planet, but is usually less noticeable than in the case of Jupiter, because the ring is apt to distract the eye, except when near the edgeways phase. The compression may be taken at 1/9. CHAPTER XI. URANUS. To the Ancients Saturn was the outermost planet of the System, nothing beyond it being known. Nor indeed was it to be assumed that any more could possibly exist, because Mercury, Venus, the Earth, Mars, Jupiter, and Saturn, with the Sun, made 7 celestial bodies of prime importance; and 7 was the number of perfection; and there was thus provided one celestial body to give a name to each of the days of the week. But Science is not sentimental; and when men of Science come upon what looks like a discovery they do their best to bring their discovery to a successful issue, however much people’s prejudices may seem to stand in the way at the moment. On a certain evening in March, 1781, Sir William Herschel, then gradually coming into notice as a practical astronomer, was engaged in looking at different fields of stars in the constellation Gemini when he lighted on one which at once attracted his special attention. Altering his eyepiece, and substituting a higher magnifying power he found the apparent size of the mysterious object enlarged, which conclusively proved that it was not a star; for it is a well-known optical property of all stars that whatever be the size of telescope employed on them, and however high the magnifying power no definite disc of light can be obtained when in focus. Herschel’s new find, therefore, was plainly not a star, and no idea having in those days come into men’s minds of there being any new planets awaiting discovery, he announced as a matter of course that he had found a new comet, so soon as he ascertained that the new body was in motion. The announcement was not made to the Royal Society till April 26, more than six weeks after the date of the actual discovery, an indication, by the way, of the dilatory circulation of news a hundred years ago. The supposed comet was observed by Maskelyne, the Astronomer Royal, four days after Herschel had first seen it, and Maskelyne seems to have at once got the idea into his head that he was looking at a planet and not at a comet. As soon as possible after the discovery of a new comet the practice of astronomers is to endeavour to determine what is the shape of the orbit which it is pursuing. All attempts to carry out this in the case of Herschel’s supposed new comet proved abortive, because it was found impossible to harmonise, except for a short period of time, the movements of the new body with the form