The light of Japetus has been found to be subject to a variation depending on the position of the satellite in its orbit. From this circumstance it has been inferred, with a strong degree of probability, that the satellite effecta a complete rotation on its axis in the same time as that in which it revolves around its primary, as in the case of the moon revolving around the earth. By far the most considerable of the satellites is Titan, which Sir John Herschel considers may be equal in magnitude to the planet Mars.
The appearance which Saturn presented to Galileo, when he first 'turned his telescope on the planet, was utterly inexplicable. The illustrious philosopher remarked, that the planet seemed to consist not of one but of three bodies, which almost touched each other and always maintained the same relative position. The three bodies were arranged in the same straight line, the largest body being in the centre, and the two others on the east and west sides of it. Pursuing his observations he found that the two lateral bodies continued gradually to diminish, until at length, after the lapse of about two years, they entirely vanished, leaving the planet quite round. He now felt much alarmed, lest those who derided his discoveries as optical illusions having no real existence, should adduce this mysterious transformation of the planet in support of their views. Writing to Weiser on Decem ber 4, 1612, he says, " What is to be said concerning so strange a metamorphosis I Are the tire lesser stars consumed after the manner of the solar spots ? Have they vanished and suddenly Lied I Has Saturn perhaps devoured his own children ? Or were the appearances indeed illusion or fraud, with which the glasses have so long deceived me, as well as many others to whom I have showed them / Now, perhaps, is the time come to revive the well nigh withered hopes of those who, guided by mere profound contemplations, have discovered the fallacy of the new observations, and demonstrated the utter im possibility of their existence. I do not know what to say in a case so
surprising, so unlooked-for, and so novel. The shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me." It was reserved for Iluyghens to ascertain the real nature of the appendage with which Saturn is furnished. lie originally announced Ins discovery in the form of an enigma, in a pamphlet entitled De Batumi Luna Obsen.atio Nova,' published in 1656. The logogriphe ran thus :— sa •a ass eeeee d cecee g h 'WM 11I1 mm nannnnnan noon pp q r:r a tttt In 1659 he published a work entitled '83-sterna Saturnium; in which he reveals the real nature of his discovery, and gives a detailed explanation of the various circumstances connected with it. Restoring the letters of the logogriphe to their original places, they now stood thus :— Stannic cingitor tenni piano, museum colnorente, ad ecliptic= inellnato; or, as thus translated : The planet is encompassed by a slender fiat ring, everywhere uncon nected with its surface, and inclined to the ecliptic.
Nothing can be more convincing or beautiful than the explanation which this theory affords of the various phenomena presentssl by tin planet. When the position of the planet in its orbit hi such that the plane of the ring passes through the sun, the edge only of the ring Is exposed to the solar rays, and the extent of the illuminated surface being very small, it is incapable of producing a sensible impression on the visual organ. In this position, then, the ring is invisible, and the planet presents a round appearance like the sun or the full moon. The ring also disappears when its plane passes through the earth ; for