PORTION OF THE COSMOS. THE PLANETS. 297 



capable of evolving a faint light of their own, not derived 

 from reflection, as is sometimes evidenced by remarkable 

 phenomena appearing in the part of Yenus which is not 

 turned towards the sun, has been already remarked in the 

 first volume of the present work ( 493 ) . 



We propose to consider the planets in regard to their 

 number, the order of succession of their discovery, their 

 volume as compared with each other and with their distances 

 from the sun, and according to their relative densities, masses, 

 times of rotation, excentricities and inclinations of axis, as well 

 as to the characteristic diversity of those within and those 

 beyond the zone of the small planets. Among these sub- 

 jects of comparative consideration I have, in accordance 

 with the nature of my work, devoted particular care to the 

 selection of the most accurate numerical data for the epoch 

 at which these pages are printed i. e., the results of what 

 are supposed to be the best assured as well as the most re- 

 cent investigations. 



a. Primary Planets. 



1. Number and epoch of discovery. Of the seven cos- 

 mical bodies which, by their continually varying relative 

 positions and distances apart, have ever since the remotest 

 antiquity been distinguished from the " unwandering orbs" 

 of the heaven of the fixed stars, which to all sensible appear- 

 ance preserve their relative positions and distances un 

 changed, five only Mercury, Yenus, Mars, Jupiter, and 

 Saturn wear the appearance of stars: "quinque Stellas 

 errantes ;" while the sun and moon, from the size of their 

 disks, their importance to man, and the place assigned to 



