The Evolution of the Sciences 



perturbation which had agitated the sun's 

 atmosphere/' 



All sun spots have a similar history. Never- 

 theless, in all this infinite variety of changing 

 forms certain elements remain constant. In 

 the first place all spots are observed to contain 

 an almost entirely dark central part called 

 the nucleus, surrounded by a grey zone, the 

 penumbra, which, under high magnification, can 

 be dissolved into a series of elongated elements 

 alternately bright and dark, giving the impres- 

 sion of a furrowed slope surrounding a dark, 

 yawning hole. 



Another fact of the greatest importance is 

 that the spots revolve round the sun. Certain 

 spots, and especially those which are regular 

 and more or less round, develop very slowly, and 

 can be observed for several months, but they 

 do not remain always in the same place; they 

 may be seen crossing the solar disc from its 

 eastern to its western border. This semi-re- 

 volution has an average duration of fourteen 

 days, after which the perturbation passes on to 

 the invisible side of the sun and re-appears on 



the eastern edge after a further period of four- 



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