96 CO3M JS 



and are at the same time the largest, most compressed at the 

 poles, and the least dense. According to the most recent 

 measurements of Madler, Uranus has a greater planetary 

 compression than any other of the planets, viz., ^l^cl- In our 

 Earth and her moon, whose mean distance from one another, 

 amounts to 207,200 miles, we find that the differences oi 

 mass* and diameter between the two are much less consider- 

 able than are usually observed to exist between the principal 

 planets and their attendant satellites, or between bodies of 

 different orders in the solar system. While the density of tho 

 Moon is five ninths less than that of the Earth, it would ap- 

 pear, if we may sufficiently depend upon the determinations 

 of their magnitudes and masses, that the second of Jupiter'g 

 moons is actually denser than that great planet itself. Among 

 the fourteen satellites that have been investigated with any 

 degree of certainty, the system of the seven satellites of Saturn 

 presents an instance of the greatest possible contrast, both in 

 absolute magnitude and in distance from the central body. 

 The sixth of these satellites is probably not much smaller than 

 Mars, while our moon has a diameter which does not amount 

 to more than half that of the latter planet. With respect to 

 volume, the two outer, the sixth and seventh of Saturn's satel- 

 lites, approach the nearest to the third and brightest of Jupi- 

 ter's moons. The two innermost of these satellites belong 

 perhaps, together with the remote moons of Uranus, to tho 

 smallest cosmical bodies of our solar system, being only made 

 visible under favorable circumstances by the most powerful 

 instruments. They were first discovered by the forty-foot 

 telescope of William Herschel in 1789, and were seen again 

 by John Herschel at the Cape of Good Hope, by Vico at Rome, 

 and by Lament at Munich. Determinations of the true di- 

 ameter of satellites, made by the measurement of the apparent 

 size of their small disks, are subjected to many optical diffi- 

 culties ; but numerical astronomy, whose task it is to prede- 

 termine by calculation the motions of the heavenly bodies as 

 they will appear when viewed from the Earth, is directed al- 



* If, according to Burckhardt's determination, the Moon's radius be 

 ft.2725 and its volume ^.^th, its density will be 0-5596, or nearly five 

 ninths. Compare, also, Wilh. Beer und H. Midler, der Mond, $ 2, 

 10, and Madler, Ast., $ 157. The material contents of the Moon are, 

 according to Hausen, nearly ^th (and according to Madler ^.^th) 

 th;it of the Earth, and its mass equal to-^p^d that of the Earth. In 

 the largest of Jupiter's moons, the third, the relations of volume to th 

 central body are -p-J^ili, and of mass --^th. On the polar flatten- 

 ing oHJrauus, see Schum., Astun. Nachr., 1844, No. 493. 



