SUN-SPOTS ON TERRESTRIAL CLIMATES. 57 



atmospheric matter, which are due to the variations in the 

 source of heat. 



Speaking generally, it maybe affirmed that the radiations 

 of obscure heat are more largely absorbed by the gases and 

 vapors of our atmosphere than those of luminous heat, 

 and the great differences in the mere luminosity of the 

 spots, penumbra, and photosphere justify the assumption 

 that the radiations of a sun-spot will (to use the expressive 

 simile of Tyndall) lose far more by atmospheric sifting than 

 will those from the photosphere. 



But the spot areas will be none the less effective on 

 terrestrial climate on that account. A given amount of 

 heat arrested by the earth's atmosphere will have even 

 greater climatic efficiency than if received upon its solid 

 surface, inasmuch as the gases are worse radiators than the 

 rocks, and will therefore, cceteris paribus, retain a larger 

 proportion of the heat they receive. 



I have long ago endeavored to show* that the depth of 

 the photosphere, from the solar surface inwards, is limited 

 by dissociation ; that the materials of the Sun within the 

 photosphere exist in a dissociated, elementary condition ; 

 that at the photosphere they are, for the most part, com- 

 bined. This view has since been adopted by many eminent 

 solar physicists, and if correct, demands a much higher 

 temperature within the depths revealed by that withdrawal 

 of the photospheric vail which constitutes a sun-spot. 



If I am right in this, and also in supposing the spot- 

 radiations to be so much more abundantly absorbed than 

 those of the photosphere, and if in spite of this higher tem- 

 perature of the spots, the surface of the earth receives from 

 them the lower degree of heat measured . by Professor 

 Langley, another interesting consequence must follow. The 

 excess of spot-heat directly absorbed by the atmosphere, 

 and mainly by the water dissolved or suspended in its upper 

 regions, must be especially effective in dissipating clouds 

 and checking or modifying their formation. The meteoro- 

 logical results of this may be important, and are worthy of 

 careful study. 



* " The Fuel of the $un," Chapters iv. to r. 



