158 FROM NEBULA TO NEBULA 



S. Why its temperature increases so uniformly as 

 we descend into its depths. 



9 Why, as geology shows, the earth's tempera- 

 ture has been approximately the same for millions of 

 years. 



10. Why the sun's temperature has also continued 

 so uniform. 



11. Why the oceans, as their marine life shows, 

 have preserved an equable temperature throughout evo- 

 lutionary time. 



12. The probable cause of Mercury's excessive 

 perihelion movement, arising from the sun's revolu- 

 tional movement and lopsidedness of figure. 



13. Why the whole universe does not concentrate 

 into a single ball. 



14. Why it hasn't "run down," or shown signs of 

 doing so. 



15. Why some stars are variable. 



16. What new stars are. 



17. Why nebulae, being but dust clouds from stel- 

 lar explosions, are so capriciously formed. 



18. Why some nebulae are hot and others cold, 

 according to remoteness of the explosions that begat 

 them. 



19. The origin of comets, and why their orbits are 

 so eccentric. 



20. Why, being formed of star debris, they lib- 

 erate gases as the result of internal friction on ap- 

 proaching the sun, thereby giving rise to coma, tail, etc. 

 Also why their tails, being their lightest part, are tidally 

 depressed to the rear. 



21. What it is that causes the sun-spots and their 

 periodicity. 



22. How explosions on the sun can account for 

 the terrestrial phenomena that occasionally accompany 

 them. 



