THE PLANETS. 



119 



(Earth and Moon together 



Mars 



Jupiter and his satellites Tor4-8T 



Saturn CT^r-a" 



Uranu^ 2 TFO s 



Neptune TT 1 TT 



The mass -Q^J, which Le Verrier found, by means of his 

 sagacious calculations, before the actual discovery of Neptune 

 by Galle, is greater, although remarkably near to the truth. 

 The arrangement of the principal planets, according to their 

 increasing masses, is, when leaving out the small ones, the 

 following : 



Mercury, Mars, Venus, Earth, Uranus, Neptune, Saturn, 



Jupiter ; 



thus, like the volumes and densities, entirely different from the 

 order of succession of the distances from the central body. 



6. Densities of the Planets. By applying the above quot- 

 ed volumes and masses, the following numerical relations are 

 obtained for the densities of the planets (according as the 

 earth or water is taken as unity) : 



In the comparison of the density of the planets with water, 

 the density of the Earth serves as a basis. Reich's experi- 

 ments, made in Freiberg with the torsion balance, ga*e 

 5'4383 : very nearly the same as the analogous experiments 

 of Cavendish, which, according to the more accurate calcula- 

 tions of Francis Baily, gave 5'448. The result of Baily's 

 own experiments is 5 1 660. It will be seen from the above 

 table that Mercury, according to Encke's determination of 

 mass, comes very near to the other planets of medium mag- 

 nitude. 



This table calls to mind forcibly the classification, several 

 times mentioned by me, of the planets into two groups, which 

 are separated from each other by the zone of the small plan 



