THE PLACETS. 447 



own experiments is 5 660. It will be seen from the abo\o 

 table, that Mercury, according to Encke's determination of 

 mass, comes very near to the other planets of medium 

 magnitude. 



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 

 planets. The differences of density which are presented by 

 Mars, Venus, the Earth, and even Mercury, are very slight ; 

 almost equally similar among each other, but from 4 to 7 

 times less dense than the former group, are the planets more 

 distant from the Sun, Jupiter, Neptune, Uranus, and Saturn. 

 The density of the Sun (0-252, if the Earth is taken as 

 1-000; therefore in reference to water 1'37), is but little 

 more than the densities of Jupiter and Neptune. Conse- 

 quently, the planets and the Sun 8 * must be arranged according 

 to their increasing density, in the following order : 



Saturn, Uranus, Neptune, Jupiter, Sun, Venus, Mars, 



Earth, Mercury. 



Although, upon the whole, the densest planets are nearer to 

 ^he Sun; still, when they are considered individually, their 

 density is by no means proportional to the distances, as 

 Newton was inclined to assume. 38 



34 The Sun (which Kepler considered to be magnetic, pro- 

 bably from enthusiastic admiration for the divina inventa of 

 his justly famous contemporary, William Gilbert, and whose 

 rotation in the same direction as the planets he maintained 

 long before the Sun-spots were discovered), Kepler declares, 

 in his Comment, de motibus Stella Martis (cap. 23), and i i 

 Astronomice pars optica (cap. 6), to be " the densest of all 

 cosniical bodies ; because it moves all the others which belong 

 to his system." 



** Newton, De Mundi Systemate, in OpuscuUs, torn. ii. p. 17: 

 " Corpora Veneris et Mercurii majore Solis calore magis con- 

 cocta et coagulata sunt. Planetse ulteriores, defectu caloris, 

 carcnt substantiis illis metallicis et mineris ponderosis quibus 

 Terra refcrta est. Densiora corpora quae Soli propiora: ea 



