140 PROGKESS IN ASTRONOMY. 



of science which had developed since Laplace published his famous 

 Exposition du Systeme du Monde. 



After all the meteorites in the parent swarm had been condensed 

 into the central gaseous mass that mass h^d to cool. So that we had 

 in the heavens not only stars more or less meteoritic in structure, of 

 rising temperature, but stars chiefly gaseous, of falling temperature. 

 It was obvious that representatives of both these classes of stars might 

 have nearly the same mean effective temperature and therefore more 

 or less the same spectrum. A minute inquiry entirely justified these 

 conclusions. 



So far has the detailed chemistry of the stars been carried in the lat- 

 ter years of the century that the question of stellar evolution has given 

 rise to that of inorganic evolution generally, the sequence in the phe- 

 nomena of which can only be studied in the stars, for laboratory work 

 without stint has shown that in them we have celestial furnaces, the 

 heat of which transcends that of our most powerful electric sparks. 

 In this way astronomy is paying the debt she owes to chemistry. 



THE SUN AND HIS SYSTEM. 



Although the outer confines of space have, as we have seen, been 

 compelled to bring their tribute of new knowledge by means of the 

 penetrating power possessed by modern telescopes, and the camera and 

 spectroscopes attached to them, the stud}^ of the near has by no means 

 been neglected, and for the reason that in astronomy especially we 

 must content ourselves in the case of the more distant bodies by sur- 

 mising what happens in them from the facts gathered in the region 

 where alone detailed observations are possible. 



Thus what we can learn about the sun helps to explain what we dis- 

 cern much more dimly in the case of stars. A study of the moon's face 

 we are compelled to take as showing us the possibilities relating to the 

 surface condition of other satellites so fai" removed from us that they 

 only appear as points of light. 



To begin, then, with the sun. Where a volume might be written a 

 few words must sufiSce. I have already stated that at the beginning 

 of the centur}" the prevailing opinion was that it was a habital)le globe. 

 It was limited to the fier}' ball we see. At the end of the century it 

 is a body of the fiercest heat, and the ball we see is only a central 

 portion of a huge and terribly interesting mechanism, the outer por- 

 tions of which heave and throb every eleven years. Spots, promi- 

 nences, corona, everything, feel this throbbing. 



Although the discovery of spots on the sun was among Galileo's 

 first achievements, it was reserved for the last half of the present 

 century to demonstrate their almost perfect periodicit3^ 



Thanks to the labors of Schwabe, Wolf, Carrington, and De la Rue, 

 Stewart and Loewy, we now know that ever}^ eleven years the spots 



