824 SPECIAL RESULTS IN THE UEANOLOGICAL 



to the more exact re-calculation of Francis Baily, the very 

 similar result of 5*448, and Baily 's own experiments 5 '660. 

 We see that the density of Mercury, according to Encke's 

 determination of its mass, is near that of the other planets 

 of medium magnitude. 



The above table reminds us again of the division, to which 

 I have already repeatedly alluded, of the planets into two 

 groups separated from each other by the zone of the small 

 planets. The differences between the densities of Mars. 

 Venus, the Earth, and even Mercury, are very small, and 

 there is nearly as great a similarity between those of the far 

 less dense remoter planets Jupiter, Neptune, Uranus, and 

 Saturn. The density of the Sun (0'252), that of the Earth 

 being taken as =1 (1'37, therefore, when Water is taken 

 as =1) is a little greater than the densities of Jupiter and 

 Neptune. The following is the order of succession of the 

 sun and planets arranged according to increasing density 

 (526) . Saturn, Uranus, Neptune, Jupiter, Sun, Yenus, 

 Mars, Earth, Mercury. 



Although the densest planets are, on the whole, those 

 nearest to the sun, yet, taken individually, the densities of 

 the several planets are by no means proportional to their 

 solar distances, as Newton was inclined to assume ( 527 ). 



7. Sidereal period of revolution and rotation round 

 the axis. We shall content ourselves here with giving the 

 sidereal, or true periods of revolution of the planets in rela- 

 tion to the fixed stars or to some particular point in the 

 heavens. In the interval of time occupied by such a revo- 

 lution the planet performs 360 complete degrees round the 

 sun. The sidereal revolutions are very distinct from the 

 tropical and synodical, of which the former relate to the 

 return of the vernal equinox, and the latter to the difference 



