SATELLITES. 87 



four, whereas the diameter of the largest of all known satel- 

 lites the sixth of Saturn is supposed to be one-seven- 

 teenth, and that of the largest of Jupiter's satellites the 

 third only one-twenty-sixth part of the respective diameters 

 of the planets round which they revolve. The planets most 

 rich in satellites are found among those most remote, of 

 greatest magnitude, least density, and greatest compression 

 According to the most recent measurements of Madler, 

 Uranus is the planet which has the greatest compression, 

 viz. 9^3. The Earth and her moon are 207200 miles 

 apart, and the differences of mass ( 40 ) and diameter in these 

 two bodies are much less than we are accustomed to meet 

 with elsewhere in the solar system, between bodies of dif- 

 ferent orders, or primary planets and their satellites. The 

 density of the Moon is -f- less than that of the Earth, while 

 the second satellite of Jupiter appears, if we may place suffi- 

 cient dependence on the determinations of magnitude and 

 of mass, to be even actually denser than the great planet 

 round which it revolves. 



Among the fourteen satellites concerning which investi- 

 gation has arrived at some degree of certainty, the system of 

 the seven satellites of Saturn offers the greatest contrasts, 

 both of absolute magnitude and of distance from the central 

 planet. The sixth satellite is probably but little smaller 

 than Mars (whose diameter is twice that of our moon), 

 while, on the other hand, the two innermost satellites (dis- 

 covered by the forty-foot telescope of "William Herschel in 

 1789, and seen again by John Herschel at the Cape of 

 Good Hope, by Yico at Rome, and by Lamont at Munich) 

 belong, perhaps, together with the remote moons of Uranus, 

 to the smallest cosmical bodies of our solar system, being 



