IM M. Melloni on (he Theory of Dew. 



indicates the result sought for, namely, the cooling of bodies 

 below the ambient medium. 



There is, however, one experiment of Wells' in which a 

 thermometer, wrapped about with wool, placed at the same 

 level with a free thermometer, gave a lowering of tempera- 

 ture of 50°*3. Here the wool was cooled two or three times 

 more than the lamp-black in my experiments ; but I know 

 that the emissive power of wool is not greater than that of 

 lamp-black* 



In order to determine the cause of this extraordinary cold 

 observed by Wells, it was necessary, in the first place, to 

 put the fact beyond doubt. With this view, having enveloped 

 one of my thermometers in a tuft of wool, I exposed it along 

 with two others of the same dimensions, one of which was 

 covered with lamp-black, the other retained its metallic bril- 

 liancy, the instrument descended, in a few minutes, double 

 the extent of the blackened thermometer. A fourth instru- 

 ment, enveloped in an equal quantity of wool, condensed and 

 pressed around the metallic cover by means of some rounds 

 of thread of the same substance, gave an intermediate cold 

 between the two preceding. I lastly prepared a fifth ther- 

 mometer with a double covering of fine flannel, and it sunk 

 still less than the fourth. I repeated these experiments by 

 substituting cotton for wool, and obtained results in every 

 respect analogous, I then perceived that the superiority of 

 cotton or wool over lamp-black, in the phenomena of noctur- 

 nal refrigeration, was owing to a certain modification intro- 

 duced into the emissive power of these bodies by the presence 

 of air among their interstices. 



But how does air increase the cold arising from radiation % 

 The answer is simple and obvious. We have known, for 

 many years, that the nocturnal coldness of bodies does not 

 vary with the temperature of the atmosphere. Thus, Cap- 

 tains Parry and Scoresby have found, that during the calm 

 and pure nights of the polar regions, the snow cooled about 

 9 degrees below a stratum of air raised from 1™ 30 to 1™ 60, 

 when the atmosphere was from — 25 or — 30 degrees, and when 

 its temperature is very near zero. M. Pauillet has seen 

 swan-down descend to about 7° under the temperatures of 



