142 PEOGEESS IN ASTRONOMY. 



already appeared to indicate that the results of the inquiry when 

 completed will be of the most important kind. The authors have 

 already come to the conclusion that the lunar and terrestrial sea bot- 

 toms much resemble each other, inasmuch as both have convex sur- 

 faces. The lunar seas began 1)}^ sinking of vast regions; the formidal)le 

 volcanic eruptions of which the moon has been the scene have taken 

 place in times equivalent to those labeled "recent" in geological par- 

 lance. There is evidence that the axis of the moon has undergone 

 great displacements, and four great periods of change have been made 

 out. Finall}" they state that there is serious ground to believe that 

 there is an atmosphere of some sort remaining. 



It may readily be understood that with each increase of optical power 

 new satellites of the various planets have been discovered. Soon after 

 the discovery of Neptune a satellite was noted by Lassell. In 1846 

 both he and the eagle-eyed observer Dawes independently discovered 

 another satellite (Hyperion) of Saturn. Lassell was rewarded in the 

 next year ])y the discovery of two more satellites of Uranus; but, 

 strangest observation of all, in 1877 Hall discovered at Washington two 

 satellites of Mars some 6 or 7 miles only in diameter, one of them 

 revolving round the planet in seven and one-half hours at a distance of 

 less than 4,000 miles. As the day on Mars is not far diiferent in dura- 

 tion from our own, this tiny satellite must rise in the west and south 

 three times a day. 



Wonderful as this discovery was, it is certainly not less wonderful 

 when we consider it in connection with a passage in Gulliver's Travels, 

 so true is it that truth is stranger than fiction. Swift, in his satirical 

 reference to the inhabitants of Laputa, writes: "They have likewise 

 discovered two lesser stars or satellites, which revolve round Mars, 

 whereof the innermost is distant from the center of the primary planet 

 exactly three of his diameters and the outermost five. The former 

 revolves in the space of ten hours and the latter in twenty-one and a 

 half." 



The last discovery of this kind has been that of an inner satellite of 

 Jupiter by Barnard in 1892. 



The planets from Mercury to Saturn were known to the ancients. 

 I have alread}" referred to the discovery of Uranus by HerscheFs giant 

 telescope, not long before the century was born, and of Neptune, by 

 analysis, toward the end of the first half of the century. With regard 

 to what modern observations have done in regard to their physical 

 appearance, the first place in general interest must be given to Saturn 

 and Mars. 



Saturn has always been regarded as the most interesting of the 

 planetary family on account of its unique rings. Many sul)di visions 

 of the rings, and a dusky ring, first seen 1)}^ Dawes and Bond, have 

 been discovered during the last sixty years. 



