PORTION OF THE COSMOS. THE SATELLITES. 341 



all known satellites. It is, however, safer to designate 

 them merely as the smallest luminous points. At present 

 there appears more reason to believe that the smallest of all 

 planetary bodies, meaning thereby both primary planets and 

 satellites, are to be sought for among the small or telescopic 

 planets ( 541 ). 



The density of satellites is by no means always inferior 

 to that of their primary planets, as is the case in our Moon 

 (whose density, compared to that of the Earth, is as 0*619 

 to 1), andin Jupiter's 4th satellite. The densest of these satel- 

 lites, the 2d, is, on the other hand, denser than Jupiter ; 

 while the 3d and largest appears to have the same density 

 as the planet itself. Nor do the masses increase with the 

 distance : if the planets have arisen from revolving rings, 

 peculiar causes, which may perhaps ever remain hidden from 

 us, must have occasioned in the various cases larger or 

 smaller, and denser or rarer, accumulations around a nucleus. 



The orbits of satellites belonging to the same group have 

 very different excentricities. In the system of Jupiter, the 

 orbits of the 1st and 2 d satellites are almost circular; while 

 of those of the 3d and 4th the excentricities amount to 

 0-0013 and 0-0072. In the system of Saturn, the orbit of 

 the satellite which is nearest to the planet (Mimas) is con- 

 siderably more excentric than the orbit of Enceladus, or 

 than that of Titan, which has been so accurately determined 

 by Bessel. The excentricity of this, the 6th satellite 

 of Saturn, and the largest and earliest discovered, is only 

 0-02922. According to all these data, which are deserving 

 of considerable confidence, Mimas is the only satellite whose 

 orbit is more excentric than that of our Moon (0-05484). 



