82 COSMOS. 



the centre of that planet than our moon is from the Earth. 

 The first or innermost of Saturn's satellites is nearer to its 

 central body than any other of the secondary planets, and pre- 

 sents moreover the only instance of a period of revolution of 

 less than twenty-four hours. Its distance from the centre of 

 Saturn may, according to M'adler and Wilhelm Beer, be ex- 

 pressed as 2'47 semi-diameters of that planet, or as 80,088 

 miles. Its distance from the surface of the main planet is 

 therefore 47,480 miles, and from the outermost edge of the 

 ring only 4916 miles. The traveller may form to himself an 

 estimate of the smalmess of this amount by remembering the 

 statement of an enterprising navigator, Captain Beechey, that 

 he had in three years passed over 72,800 miles. If instead of 

 absolute distances we take the semi-diameters of the principal 

 planets, we shall find that even the first or nearest of the 

 moons of Jupiter (which is 26,000 miles further removed 

 from the centre of that planet than our moon is from that of 

 the Earth) is only six semi-diameters of Jupiter from its centre, 

 whilst our moon is removed from us fully 60-^ semi-diameters 

 of the Earth. 



In the subordinate systems of satellites we find that the 

 same laws of gravitation which regulate the revolutions of 

 the principal planets round the Sun, likewise govern the 

 mutual relations existing between these planets among one 

 another, and with reference to their attendant satellites. The 

 twelve moons of Saturn, Jupiter, and the Earth, all move like 

 the primary planets from west to east, and in elliptic orbits, 

 deviating but little from circles. It is only in the case of our 

 moon, and perhaps in that of the first and innermost of the 

 satellites of Saturn (0*068) that we discover an eccentricity 

 greater than that of Jupiter ; according to the very exact 

 observations of Bessel, the eccentricity of the sixth of Saturn's 

 satellites (0'029) exceeds that of the Earth. On the extremest 

 limits of the planetary system, where, at a distance nineteen 

 times greater than that of our Earth, the centripetal force of 

 the Sun is greatly diminished, the satellites of Uranus (which 

 have certainly been but imperfectly investigated) exhibit the 

 most striking contrasts from the facts observed with regard to 

 other secondary planets. Instead, as in all other satellites, of 

 having their orbits but slightly inclined towards the ecliptic, 

 and (not excepting even Saturn's ring, which mai'be regarded 



