88 CELESTIAL PHENOMENA. 



visible only under peculiarly favourable circumstances, and 

 with the most powerful telescopes. After the sixth and 

 seventh of the satellites of Saturn, comes, in order of volume, 

 the third and brightest of Jupiter's. The diameters of satel- 

 lites deduced from measurements of the apparent magnitude 

 of their small disks are subject to many optical difficulties ; 

 fortunately, the calculations of astronomy, which shew the 

 movements of the heavenly bodies as they will appear to 

 us when viewed from the earth, depend much more on 

 motion and mass than on volume. 



The absolute distance of any satellite from the planet 

 round which it revolves is greatest in the case of the outer- 

 most (or seventh) of the satellites of Saturn, being above 

 two millions of geographical miles, or ten times the distance 

 of our moon from the earth. The distance of the outermost, 

 or fourth, satellite of Jupiter from that planet is only 

 1040000 miles; the distance between Uranus and his 

 sixth satellite (supposing the latter really to exist) amounts 

 to 1360000 miles. If we compare in each subordinate 

 system the volume of the central planet with the dimensions 

 of the orbit of its outermost satellite, we obtain a new series 

 of numerical relations. The distances of the outermost 

 satellites of Uranus, Saturn, and Jupiter, expressed in semi- 

 diameters of the respective central planets, are as 91, 64, 

 and 27 ; and in this mode of estimation, the outermost 

 of the satellites of Saturn appears to be only a little (-^ ) 

 farther from the centre of that planet, than our Moon 

 is from the Earth. The satellite, which is nearest to 

 its central planet, is undoubtedly the first or innermost 

 of Saturn, and it offers, moreover, the only example of a 

 period of revolution of less than twenty -four hours : its 



