SATELLITES. 89 



distance from the center of he tplanet is, according to 

 Madler and Wilhelrn Beer, 2*47 semi-diameters of Saturn, 

 or 80088 miles; from the surface of the planet, therefore, 

 only 47480, and from the outermost edge of the ring, only 

 4916 miles. The traveller may find pleasure in realising 

 to his imagination the smallness of this amount, by remem- 

 bering the statement of a distinguished navigator, Captain 

 Beechey, that, in three years, he had sailed over 72800 geo- 

 graphical miles. If we estimate distances, not in absolute 

 measure, but in semi-diameters of the primary planets, we 

 find that the first or nearest of Jupiter's satellites (which, in 

 absolute distance, is 26000 miles farther from the centre of 

 that planet than our moon is from the earth) is only six 

 semi-diameters of Jupiter from its centre, while our moon is 

 dista,nt from us fully 60^ semi-diameters of the Earth. 



In the subordinate systems of satellites or secondary 

 planets, we see reflected in their relations to their primary 

 planets and to each other, all the laws of gravitation which 

 regulate those primary planets in their revolutions round the 

 sun. The twelve moons attendant on Saturn, Jupiter, and 

 the Earth, all move, as do their primary planets, from west to 

 east, and in elliptical orbits differing little from circles. It 

 is only the earth's moon, and probably the first or inner- 

 most of the satellites of Saturn, which have orbits 

 more elliptic than that of Jupiter. The eccentricity of 

 the sixth satellite of Saturn, which has been so accurately 

 observed by Bessel, is 0'029, and is greater than that of the 

 Earth. Near the extreme limits of the planetary system, 

 where, at a solar distance nineteen times greater than that of 

 the Earth, the centripetal force of the solar orb is considera- 

 bly diminished, the satellites of Uranus (which have, it ia 



