90 THE BORDERLAND OF SCIENCE 



the equatorial velocity of the sun himself. It amounts, 

 in fact, to about 7-J- miles per second ! 



We do not propose to consider here at any length 

 the system of satellites over which Jupiter bears sway ; 

 but this preliminary sketch would be incomplete with- 

 out a few words on the subject. It is worthy of notice 

 that although our earth in some sort resembles the 

 outer planets in , being accompanied by a satellite, yet 

 the relation which our moon bears to the earth is 

 altogether different from that which the satellites of 

 the outer planets bear to their respective primaries. 

 Our moon is by no means a minute body by comparison 

 with the earth, and compared with Mars or Mercury 

 she may be regarded as having very respectable dimen- 

 sions. We may, indeed, look upon the moon as a fifth 

 member of the inner family of planets, a member 

 inferior to the rest, doubtless, but still not so far 

 inferior to Mercury as Mercury is inferior to the earth. 

 In the case of the outer planets, however, and especially 

 in Jupiter's case, moons hold . an utterly subordinate 

 position. Taking the accepted measurements, we find 

 the largest of Jupiter's moons less than the 16,000th 

 part of its primary as respects bulk, while its mass or 

 weight is less than the 11,000th part of Jupiter's.* 



* It is not uncommonly stated in our text-books of astronomy, that 

 the density of Jupiter's moons is far less than Jupiter's density ; and 

 Lardner goes so far as to say that ' the density of the matter composing 

 these satellites is much smaller than that of any other body of the 

 system whose density is known.' But this is a mistake. All the satel- 

 lites, save one, are of greater density than Jupiter, and that one the 

 innermost is denser than Saturn, Uranus, or Neptune. 



