THE SUN'S HEAT. 



19 



of variation in the total heat-supply has, however, proved too small for 

 measurement with our present instruments, and science waits anxiously 

 for apparatus and methods of delicacy adequate to deal with the prob- 

 lem. We are as yet entirely uncertain whether, at the time of a sun-spot 

 maximum, the solar radiation is more or less powerful than ordinary. 



There has been a great deal of pretty vigorous discussion as to the 

 temperature of the sun, and that the subject is a difficult one is evi- 

 dent enough from the wide discrepancy between the estimates of the 

 highest authorities. For instance, Secchi originally contended for a 

 temperature of about 18,000,000 Fahr. (though he afterward lowered 

 his estimate to about 250,000) ; Ericsson puts the figure at 4,000,000 



Q 



a 



Fig. 3. Viollb's Actinometek. 



or 5,000,000 ; Zollner, Spoerer, and Lane name temperatures ranging 

 from 50,000 to 100,000 Fahr. ; while Pouillet, Vicaire, and Deville 

 have put it as low as between 3,000 and 10,000 Fahr. The intensest 

 artificial heat may perhaps reach 4,000 Fahr. 



The difficulty is twofold. In the first place, the sun can not prop- 

 erly be said to have a temperature, any more than the earth's atmos- 

 phere can. The temperature of different portions of the solar enve- 

 lope must vary enormously, increasing fast as we descend below the 

 surface ; so that in all probability there may be a difference of thou- 

 sands of degrees between the temperature at the upper surface of the 

 photosphere and that at the sun's center, or even at the depth of a 

 few thousand miles. 



