MARY WHITNEY. 



205 



generally announced sometime in advance by an agita- 

 tion of the photosphere, which manifests itself as faculae. 

 Sometimes a group of small dark points will appear, 

 change rapidly and disappear, without developing into 

 a typical sun-spot group. 



The nucleus or umbra of spots exhibits rapid changes. 



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Sun-spots, three views of one group, July, 1883. 



Diagram of monthly variation in number of sun-spots, 1883-1885, p. 211. 



The above group, selected from the belt of July, 1883, 

 illustrates this. The changes in form, number and ar- 

 rangements are very striking. Umbras multiply either 

 by the formation of new pores in the same penumbra, or 

 by subdivision of the primary umbra into two or more 

 parts. The method of this division I do not detect with 

 the glass I use, but I can plainly see that in many cases 

 it is division and not new formation. 



Larger telescopes, however, show that when this sub- 

 division takes place a mass of the brilliant matter of the 

 photosphere projects itself across the umbra, Some- 

 times it is thrown across very rapidly. Halley has 

 watched the full process of such a bridging of the nu- 



X89 



