NOTE ON THE DIRECT EFFECT OF SUK-SPOTS. 109 



sun-spot will (to use the expressive simile of Tyndall) lose far 

 more by atmospheric sifting than will those from the photo- 

 sphere. 



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 inward, is limited by dis- 

 sociation ; 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, combined. 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 veil 

 which constitutes a sun-spot. 



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

 tions to be so much more abundantly absorbed than those of 

 the photosphere, and if in spite of this higher temperature 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 meteorological results of this 

 may be important, and are worthy of careful study. 



In thus venturing to question some of Professor Langley's 

 inferences I am far from underrating the interest and impor- 

 tance of his researches. On the contrary, I regard the quanti- 

 tative results he has obtained as especially valuable and oppor- 

 tune, in affording means of testing the above-named and other 

 speculations in solar physics. Similar observations repeated 

 at different elevations would decide, so far as the lower regions 

 are concerned, whether or not there is any difference in the 

 quantity of heat imparted by the bright and obscure portions 

 of the Sun to our atmosphere. If the differences already 

 observed by Professor Langley vary in ascending, a new 

 means will be afforded of studying the constitution of the in- 



* " The Fuel of the Sun," Chapters iv. to x. 



