Mr. Todd of Adelaide has recently obtained for his observatory a fine 8-inch telescope by Mr. Cooke. With this instrument, mounted in December, 1874, he has made many valuable observations of the motions of Jupiter's satellites. Ordinarily, of course, the entry of each satellite on the planet's face and the egress therefrom, the disappearance of each satellite behind the planet or in the planet's shadow (not necessarily the same thing) and the reappearance, are effected in what may be called the normal way; and Mr. Todd's experience in this respect has been like that of other observers. But on two occasions he and his assistant, Mr. Ringwood, observed that a satellite, when passing behind the planet's edge, did not disappear at once, but remained visible as if seen through the edge, for about two minutes. The same satellite behaved thus on each occasion,--viz. the satellite nearest the planet. As this satellite travels at the rate of about 645 miles per minute, it would follow that the satellite was seen through a depth of nearly 1300 miles, or, after making all possible allowance for optical illusions, some 900 or 1000 miles. The effect of refraction cannot then be great in the air of Jupiter, to this depth below the usual limit of the upper clouds,--for otherwise the satellite would have been altogether distorted. And this very fact, that for 1000 miles or so below the highest clouds the change of atmospheric density is not sufficient to produce any noticeable refractive effects, implies that the true base of the atmosphere of Jupiter lies very far lower yet--perhaps many hundreds of miles lower. If the reader now look again at the picture at page 201, he will understand, I think, how those great round white clouds in the chief belt,--clouds thousands of miles long and broad,--are probably hundreds of miles deep also, and float in an atmosphere still deeper. All that we know about Jupiter, in fine, from direct observation, as well as all that we can infer respecting the past history of the solar system, tends to show that he is still an extremely young planet. He is the giant of the solar family in bulk, and probably he is far older than our earth in years; but in development he is, in all probability, the youngest of the sun's family of planets, and certainly far younger than the earth on which we live. [Illustration] XI _THE RINGED PLANET SATURN._ Very different from the ruddy planet which approached so closely to him in November, 1877, is Saturn, the ringed world, the most wonderful of all the planets if the complexity of the system attending on him is considered, and in size inferior only to the giant Jupiter.