132 COSMOS. 



time, when the expression of a small Jovial world (Mundus 

 Jovialis) was frequently made use of, these analogies between 

 the subordinate systems and the solar system contributed 

 much to the more rapid and general diffusion of the Coper- 

 nican system of the world. They suggest the repetitions oi 

 form and position which is so frequently presented by organic 

 nature in subordinate spheres. 



The distribution of the satellites in the solar regions is so 

 unequal, that while the proportion of the moonless principal 

 planets to those which are accompanied by Moons is as 3 to 

 5, the latter belong, with the single exception of one, the 

 Earth, to the exterior planetary groups, situated beyond the 

 ring of the asteroids with interlacing orbits. The only satel- 

 lite which has been formed in the group of interior planets 

 between the Sun and the asteroids, the Earth's, Moon, has 

 a remarkably large diameter in proportion to that of its pri- 

 mary. This proportion is -^.g- ; while the largest of Saturn's 

 satellites (the sixth, Titan) is perhaps only T .j, and the larg- 

 est of Jupiter's satellites, the third, ^.g- of the diameter of 

 their primaries. A wide distinction must be drawn between 

 this consideration of a relative magnitude and that of an ab- 

 solute 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).* If the problem of tel- 

 escopic visibility depended only upon the diameter, and was 

 not, at the same time, determined by the proximity of the 

 disks 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. f 



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 0'619 of that of our Earth) and 



* Outlines, 548. 



t See Madler's attempt to estimate the diameter of Vesta (264 geo- 

 graphical miles) with a thousand-fold magnifying power in His Astro- 

 nomie, p. 218. 



