From the time when Galilei’s inadequate glass revealed the “threefold” aspect of Saturn, and led up to Huygens’s solution of the mystery in 1654, this planet has been successively interrogated with the improved telescopes which every generation has produced. Cassini, W. Herschel, Encke, Bond, Lassell, Dawes, and Hall are names familiar to us as having materially advanced our knowledge of this unique orb, both as to his surface-configuration and as to his numerous retinue of satellites.
_Belts and Spots on the Planet._—Parallel belts are seen on the surface of Saturn, but they are much fainter than those on Jupiter, and they seldom display the spots and other irregularities interspersed with the belts of the latter planet. Well-bounded spots have rarely been distinguished on the disk of Saturn; the belts normally appear equal in tone, without breaks, condensations, abrupt curves, or branches, so that the rotation-period has only been accurately determined by Herschel and Hall. And in these cases the markings were certainly atmospheric, and probably affected by proper motions similar to those operating on Jupiter.
Cassini and Fatio remarked two bright streaks on the planet as early as 1683. Sir W. Hershel, in 1790, observed a very dark spot near the margin of the limb, and a few modern observers have been successful in distinguishing either bright or dark spots or patches, though no continuous and useful observations appear to have been secured. In the winter of 1793 Herschel noticed a very distinct quintuple belt, which consisted of three dusky and two intervening light zones. The dark belts presented a dusky yellow hue, while the spaces separating them were white. He recognized the evidences of rotation in the quintuple belt; for on the same nights, after a few hours’ interval, it exhibited considerable variation. Though seen with great precision at first, it became indistinct at a later hour, and the individual belts were placed at unequal distances.
_Rotation-Period._—Prof. A. Hall, at Washington, discovered on Dec. 7, 1876, a well-defined white spot, 2″ or 3″ in diameter, and situated just below the ring of Saturn. He watched this object till Jan. 2 following, when it had become faint and indistinct, and the planet being low and the weather unfavourable no further observations were made. The spot had fortunately been seen at four other observatories in the United States, Prof. Hall having notified its existence to them; and on discussing the results, a rotation-period was found not differing largely from Herschel’s value derived from the quintuple belt in 1793. These are, in fact, the only two determinations on which we may place confidence. They are as below:—