The Evolution of the Sciences 



corona supplies another valuable piece of in- 

 formation; it has revealed the rotation of this 

 atmosphere. We saw in the last chapter how 

 the displacement of the lines, when the eastern 

 and the western edges of the sun are viewed in 

 succession, enabled us to infer the rotation of 

 the photosphere. Applied by M. Deslandres to 

 the lines of the corona, this method has led to 

 interesting results, by showing that the gaseous 

 corona turns in the same direction as the photo- 

 sphere, and with a speed of the same order of 

 magnitude. 



Finally, direct observation, completed by 

 photography, has revealed a very' important 

 law of variations. At periods of maximum 

 solar activity the gaseous corona has a much 

 greater extension than during a minimum. Thus 

 in 1874, a year near a maximum, the line of 

 coronium was visible at an angular distance of 

 over forty-five minutes from the edges of the 

 photosphere, whereas in 1878, during a minimum, 

 it could not be seen beyond fifteen minutes, and 

 in the eclipse of 1900, when the solar activity 

 was also very weak, it could only be distinguished 

 five minutes from the edge. In 1905, on the 



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