382 



On the Heat in the Sun's Rays. 



Art. XXXI. — Circumstances affecting the Heat of the Sun's Rays ; 

 by Eunice Foote. 



(Read before the American Association, August 23d, 1856.) 



My investigations have had for their object to determine the 

 different circumstances that affect the thermal action of the rajs 

 of light that proceed from the sun. 



Several results have been obtained. 



First. The action increases with the density of the air, and 

 is diminished as it becomes more rarified. 



The experiments were made with an air-pump and two cylin- 

 drical receivers of the same size, about four inches in diame- 

 ter and thirty in length. In each were placed two thermometers, 

 and the air was exhausted from one and condensed in the other. 

 After both had acquired the same temperature they were placed 

 in the sun, side by side, and while the action of the sun's rays 

 rose to 110° in the condensed tube, it attained only 88° in the 

 other. I had no means at hand of measuring the degree of con- 

 densation or rarefaction. 



The observations taken once in two or three minutes, were as 

 follows : 



Exhausted Tuba 



| Condensed Tube. 



In shade. 



In sun. 



In shade. 



In sun. 



75 



80 



75 



80 



76 



82 



18 



95 



80 



82 



80 



100 



83 



86 



82 



105 



84 



88 



85 



110 



This circumstance must affect the power of the sun's rays in 

 different places, and contribute to produce their feeble action on 

 the summits of lofty mountains. 



Secondly. The action of the sun's rays was found to be greater 

 in moist than in dry air. 



In one of the receivers the air was saturated with moisture — 

 in the other it was dried by the use of chlorid of calcium. 



Both were placed in the sun as before and the result was as 

 follows : 



Dry Air. 



Damp Air. 



In shade. 



In sun. 



In shade. 



In sun. 



15 



75 



75 



75 



18 



88 



78 



90 



82 



102 



82 



106 



82 



104 



82 



110 



82 



105 



82 



114 



88 



108 



92 



120 



