6 EVOLUTION OF THE EARTH 



5. There is a rough progression of density, increasing both 

 ways from Saturn, the least dense of all the planets in the 

 system. 



As regards the planets themselves, we have : 



6. The plane of the planets' rotation, the plane of their 

 equators, roughly coinciding with those of the orbits (probably 

 excepting Uranus) . 



7. The direction of the rotation of the planets about their 

 polar axes the same as that of their revolution in their orbits 

 (excepting probably Uranus and Neptune). 



8. The plane of orbital revolution of the satellites of each 

 planet coinciding nearly with that of the planet's rotation, its 

 equatorial plane. 



9. The direction of the satellites' revolution in their orbits 

 also coinciding with that of the planet's rotation about its axis, 

 with exceptions in the case of the ninth satellite of Saturn and 

 probably the seventh of Jupiter. 



10. The largest planets rotating most swiftly. 



The sun, a member of the stellar system. The sun, as has 

 been stated, is but a star and a member of the stellar system. 

 What are the orders of magnitude in number, in size, in dis- 

 tance, in speed, in duration among these countless orbs, and 

 how do these relations enter into the problem of the origin of 

 the earth as one of that retinue of planets which attend upon 

 the sun? 



The luminous stars of our system are estimated to be more 

 than a hundred million in number. The number of the dark 

 stars is unknown. Giving no ray of light to reveal their exist- 

 ence, they may for all we know be as numerous or more 

 numerous than those in the radiant stages of their existence. 

 The few stars whose sizes are known range in diameter from 

 somewhat below a million to upward of ten million mije, s anol 

 more. 



Many of the stars are in reality double or multiple stars, 

 consisting of companions so close that the two or more appear 

 as one star to the naked eye, or even under the highest power 



