A HISTORY OF SCIENCE 



to the relation between sun-spots and the weather in 

 India the field of observations having been confined 

 to that territory are fully elaborated. A remarkable 

 feature of the recent work in that connection has been 

 the proof, or seeming proof, that the temperature of 

 the sun fluctuates from year to year. At times when 

 the sun-spots are numerous and vigorous in their ac- 

 tion, the spectrum of the elements in these spots be- 

 comes changed. During the times of minimum sun- 

 spot activity the spectrum shows, for example, the 

 presence of large quantities of iron in these spots of 

 course in a state of vapor. But in times of activity 

 this iron disappears, and the lines which previously 

 vouched for it are replaced by other lines spoken of as 

 the enhanced lines of iron that is to say, the lines 

 which are believed to represent the unknown substance 

 or substances into which the iron has been decomposed ; 

 and what is true of iron is true of various other elements 

 that are detected in the sun-spots. The explanation 

 of this phenomena, if Professor Lockyer reads the signs 

 aright, is that during times of minimum sun-spot ac- 

 tivity the temperature of the sun-spots is relatively 

 cool, and that in times of activity the temperature be- 

 comes greatly increased. One must come, therefore, 

 to speaking of hot spots and cool spots on the sun ; al- 

 though the cool spots, it will be understood, would 

 hardly be considered cool in the terrestrial sense, since 

 their temperature is sufficient to vaporize iron. 



Now the point of the recent observations is that the 

 fluctuations in the sun's heat, due to the periodic in- 

 crease and subsidence of sun-spot disturbances such 

 fluctuations having been long recognized as having 



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