464 COSMOS. 



largest of Jupiter's satellites, the third, |. T of the diameter 

 of their primaries. A wide distinction must be drawn 

 between this consideration of a relative magnitude, and that 

 of an absolute magnitude. The Earth's Moon, relatively so 

 large (1816 miles in diameter), is absolutely smaller than all 

 four of Jupiter's satellites (3104, 2654, 2116, and 1900 miles 

 in diameter). The sixth satellite of Saturn differs very little 

 in magnitude from Mars (3568 miles). 48 If the problem of 

 telescopic visibility depended only upon the diameter, and was 

 not at the same time determined by the proximity of the 

 discs of the primaries, the great distance and the nature of 

 the reflecting surfaces, it would be necessary to consider as 

 the smallest of the secondary planets, the first and second of 

 Saturn's satellites (Mimas and Enceladus), and the two satel- 

 lites of Uranus ; but it is safer to represent them merely as 

 the smallest luminous points. It has hitherto appeared more 

 certain that upon the whole, the smallest of all planetary 

 bodies (primaries and satellites), are to be found among the 

 small planets.* 9 



The density of the satellites is by no means always less 

 than that of their primaries, as is the case with the Earth's 

 Moon (whose density is only O619 of that of our Earth) and 

 the third satellite of Jupiter. The densest of this group of 

 satellites, the second, is even denser than Jupiter himself, 

 while the third and largest appears to be of equal density 

 with the primary. The masses also do not increase in at all 

 the same ratio as the distances. If the planets have been 

 formed from revolving rings, then the greater or less dense 

 aggregation round a nucleus must have been caused by 



48 Outlines, 548. 



49 See Madler's attempt to estimate the diameter of Vesta 

 (264 geographical miles) with a thousand-fold magnifying 

 power in his Astronomic, p. 218. 



