102 COSMOS. 



moving josmical bodies and fixed. See Cosmos, vol. iii., p. 

 115.) 



The number of the principal planets has been exactly doub- 

 led since the first volume of Cosmos appeared,* so excessive- 

 ly rapid is the succession of discoveries, the extension and per- 

 fection of the topography of the planetary system. . 



2. Classification of the Planets in two Groups. — If the 

 region of small planets situated in the solar system betiveen 

 the orbits of Mars and Jupiter, but, on the whole, nearer to 

 the former, is considered as a separating zone — as it were, an 

 intermediate group — then, as has already been remarked, those 

 planets which are nearest to the sun, the interior (Mercury, 

 Venus, the Earth, and Mars), present several resemblances 

 among each other, and contrasts with the exterior planets 

 (Jupiter, Saturn, Uranus, and Neptune), or those which are 

 more remote from the sun, beyond this separating zone. Of 

 these three groups, the intermediate one occupies a space 

 scarcely equal to half the distance of the orbit of Mars from 

 that of Jupiter. Of the space between the two great princi- 

 pal planets, Mars and Jupiter, that part which is nearest to 

 Mars is, as far as has hitherto been observed, the most close- 

 ly filled ; for if, in the zone which the asteroids occupy, the 

 two outermost, Flora and Hygeia, are examined, it will be 

 found that Jupiter is more than three times further from Hy- 

 geia than Flora is from Mars. The most distinctive features 

 of this intermediate group of planets are the great inclination 

 and eccentricity of their interlacing orbits, and the extreme 

 smallness of the planets. The inclination of the orbits to- 

 ward the ecliptic increases in that of Juno to 13° 3', in that 

 of Hebe even to 14° 47', of Egeria to 16° 33', of Pallas even 

 to 34° 37' ; while in the same intermediate group it falls as 

 low, in the orbit of Astrea, as 5° 19', in that of Parthenope 

 to 4° 37', and that of Hygeia to 3° 47'. The whole of the 

 orbits of the small planets having inclinations smaller than 

 7° are, to go from the large to the small, those of Flora, Me- 

 tis, Iris, Astrea, Parthenope, and Hygeia. Nevertheless, none 

 of these orbits attain such a small degree of inclination as 

 those of Venus, Saturn, Mars, Neptune, Jupiter, and Uranus. 

 The eccentricities partly exceed even that of Mercury (0-206) ; 

 for Juno, Pallas, Iris, and Victoria have 0-255, 0'239, 0232, 

 and 0-218, while Ceres (0-076), Egeria (0-086), and Vesta 

 (0-089) have orbits less eccentric than Mars (0*093), without, 



* Cosmos, vol. i., p. 92. Compare also Encke, in Schumacher' s Astron 

 Nachr., vol. xxvi., 1848, No. G22, p. 347. 



