On the Heat in the Suns Rays. 



377 



Art. XXX. — On the Heat in the Sun's Bays ; by Elisha Foote. 



(Read before the Amer. Association for the Advancement of Science, Aug. 23, 1856.) 



The experiments here detailed were instituted for the pur- 

 pose of investigating the heat in the Sun's rays. 



Two instruments have been used for this purpose. One was 

 Leslie's differential thermometer. Both bulbs of it were black- 

 ened by holding them in the smoke of burning pitch. When 

 experimenting one was shaded, the other was exposed to the 

 direct action of the sun's rays ; and as both were thus equally 

 subject to all other influences, the result was not affected by 

 them. 



Generally, however, I have found it more convenient to use two 

 mercurial thermometers, and note their difference. Two small 

 and very delicate instruments were procured as nearly alike as 

 possible. The stems of both were attached to the same plate 

 about two inches apart, and the scales were marked upon it in 

 juxtaposition, so that the eye could see the indications of both 

 at the same time. Both bulbs were blackened as in the other 

 instrument. It was used in the same manner. The tempera- 

 tures in the sun and in the shade were noted, and their difference 

 was taken as equivalent to the indications of the differential 

 thermometer. 



The question that first arises is, does the difference between 

 the shaded and exposed bulbs afford a correct measure of the 

 heat in the sun's rays ? To this point I would ask attention be- 

 fore proceeding to the experiment. 



The theory of the differential thermometer was accurately in- 

 vestigated by Leslie. In one of the foci of two parabolic re- 

 flectors he placed a tin canister which was heated or cooled by 

 putting in liquids of different temperatures or frigorific mixtures. 

 In the other, the heat was received on one of the bulbs of his 

 differential thermometer : and under all circumstances, the in- 

 dications of the instrument were found to be accurately propor- 

 tional to the differences between the temperatures of the canis- 

 ter and those of the surrounding air. 



I have varied these experiments by keeping the canister at the 

 uniform heat of boiling water in different temperatures of the 

 air, and by substituting other sources of heat, and have always 

 found the results to accord with those obtained by the distin- 

 guished philosopher to whom I have referred. 



The principles of radiation lead to the same result ; for while 

 the differential thermometer receives heat from the canister, it at 

 the same time radiates it to surrounding bodies, and that in pro- 



SECOND SERIES, VOL. XXII, NO. 66. NOV., 1856. 



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