640 SATURN the apparent brightness of the planet. As the rings attain their greatest opening very near- ly at the aphelion and perihelion of Saturn's orbit, the^-atio just obtained fairly represents the relative brightness when he is in those parts of his orbit ; but he appears much less bright relatively when at his mean distance than would be the case if he had no rings, for the rings then turn their edge almost exactly toward the earth. Saturn completes the cir- cuit of his orbit in 10,759-2198 days, or 29 years 167'2 days, in an orbit inclined about 2 29' 58" to the plane of the ecliptic. Saturn comes next to Jupiter in volume and mass. In fact, he surpasses all the remaining planets taken together nearly three times. His mean diameter is about 70,150 m., his polar diameter about 3,500 m. less, his equatorial diameter about 3,500 m. greater. Ills compression is about -fa. lie exceeds the earth 697 times in volume ; but his density is only 0*13 of that of the earth, so that his mass only exceeds hers about 89*7 times. He rotates on his axis in about lOJ^ hours, and his equator is inclined about 27 D to the plane of his orbit. Saturn when first observed with tbe telescope by Galileo presented a triple appearance, as if two smaller orbs were symmetrically placed on either side of a larger one. Afterward Ga- lileo supposed the planet had two appendages resembling handles (ansce) in shape and position. Sometimes the arnce appeared so large as to form a continuous ring; at other times they disappeared altogether. After they had been for some time invisible, they reappeared, and gradually increased in magnitude. Galileo and Hevelius were able to detect a dark space ap- parently enclosed within the rounded curve of the nnnic. Huygens explained these appear- ances as caused by an opaque, flat, thin, and circular ring surrounding the equator of Sat- urn, but nowhere touching the planet's globe. This ring being inclined to the ecliptic and moving always parallel to itself, it follows that for half the Saturnian year one side of the ring is illuminated, and for the other half the other side. When the ring is turned edge- wise either to the sun or to the earth, or when the earth is on the darkened side of the ring, it is invisible except in telescopes of great pow- er. As the distance of the earth from the sun is very small compared with that of Saturn, it follows that the invisibility caused either by the ring's plane passing through the earth, or by the earth and sun being on different sides of the plane, must always occur when the planet is near one or other of the two points on its orbit where the plane passes through the sun. Later observations by Cassini, Bell, Sir W. Herschel, and others, but more particularly the recent observations of the Bonds at Harvard college and Dawes and Lassell in England, have added greatly to our knowledge of Saturn's ring system. The ring is found to consist of two chief bright rings separated by a circular gap ; but each of the bright rings is probably divided into several subordinate rings. But perhaps the most remarkable part of this remarkable system is a dark ring within the bright system of rings. This dark ring is so obvious with very moderate telescopic power that the idea is naturally suggested that it is a recent forma- tion ; and therefore the history of its discovery and of observations prior thereto deserves to be carefully noted. It appears from a paper by Galle of Berlin in the Nachrichten, No. 756, that a dark ring was seen with the large Ber- lin refractor in 1838. An account of Galle's observations, accompanied by drawings exhib- iting the part of the ring seen across the body of the planet, was read by Encke before the Berlin academy in the same year. But little notice seems to have been attracted by this re- markable announcement ; so that the actual dis- covery of the ring (made in such a way as to secure general recognition) must be attributed to Prof. G. P. Bond of Harvard and the Rev. W. R. Dawes in England, in November, 1850 ; Bond, who discovered the ring on Nov. 15, having the priority by a few days. Dawes at this time, and Bond somewhat later, called at- tention to the darkening of the inner bright ring toward the inside, where it adjoins on the dark ring. Dawes's account, when he announced the discovery of the dark ring, was as follows: "The exterior portion of the inner bright ring to about one fourth of its whole breadth was very bright ; but interior to this the shading off did not appear, as under ordi- nary circumstances, to become deeper toward the inner edge without any distinct or sudden gradations of shade ; on the contrary, it was clearly seen to be arranged in a series of nar- row concentric bands, each of which was dark- er than the next exterior one. Four such were distinctly made out; they looked like steps, leading down to the black chasm between the ring and the ball. The impression I received was that they were separate rings, but too close together for the divisions to be seen in black lines." Later Capt. Jacobs at Madras recog- nized the fact that the dnsky ring is semi-trans- parent, the outline of the globe of the planet being distinctly visible through the ring. On this account, and because of the peculiar aspect of the dark ring when it crosses the body of the planet, the name "crape ring" has been assigned to it. Otto Struve has shown that the inner or dusky ring is not a modern appen- dage to the planet, as might bo suggested by the fact that it remained so long undiscovered ; but that at the beginning of the 18th century the dark band thrown across the planet was distinguished from the shadow of the bright rings, and was called by observers the equato- rial belt. Another curious result of Struve's researches was to show, by comparisons of measures made by Huygens, Cassini, Bradley, Sir W. Herschel, W. Struve, Encke, and himself, that " the inner edge of the inferior bright ring is gradually approaching the body of the plan- et, while at the same time the total breadth of
