472 AIELLONI ON THE NOCTURNAL COOLING OF BODIES. 



Thus, between the limits of these observations the cold becomes 

 greater in proportion as the cotton and wool which envelope the 

 thermometer become less dense and more voluminous. Now if 

 the action increases in proportion as the matter is less dense, 

 the cause of the excess of cold observed evidently does not lie in 

 the emissive or radiating power of this matter. Whence arises 

 then the frigorific superiority of wool and of cotton compared to 

 lamp-black? The solution of the question will become simpler 

 if we examine the different circumstances which concur in the 

 formation of the dew in meadows. 



And, entering now on the investigation of this interesting 

 phaenomenon, let us in the first place call to mind that the gi'ass 

 and kinds of earth and sawdust introduced into our open vessels 

 were cooled nearly as much as lamp-black in powder, and that 

 a free thermometer coated with this latter substance does not 

 descend more than 1°'8 below the temperature of the air. 



But if we wish to assure ourselves directly that nocturnal 

 radiation lowei's the temperature of the vegetables which cover 

 the soil, we have only to place, during calm and clear nights, 

 one of our thermometers supported by its metallic tripod, in 

 contact with the under surface of the grass or leaves of any 

 plant ; for if the instrument be then observed, it will be con- 

 stantly found lower than a thermometer with metallic armature 

 freely suspended alongside of it in the same hoi-izontal stratum 

 of the atmosphere. But the diffei'ence between the two thermo- 

 meters will never exceed 2°, whatever be the vigour and isolation 

 of the plant. And the feebleness of the action manifested must 

 not be attributed to the small mass of the leaf compared to the 

 mass of the apparatus ; for thermometers of very small size may 

 be employed, of cylindrical or spherical form, of greater or less 

 degrees of thinness or flattening, without any sensible variation 

 of the final temperature indicated, although the calorific equili- 

 brium is the more speedily established the smaller the reservoir 

 of the thermometer, as might be easily foreseen. Now this fact, 

 whilst completely removing all doubts of the truth of Wells' 

 principle as to the nocturnal radiation of bodies, leads neces- 

 sarily to the consideration of the theory of dew under a different 

 aspect to that generally taken by writers on physics. 



In order to incur no eri'or by general allusions, let us take a 

 particular example chosen from one of the best modern treatises. 

 In the last edition of the Traite dc Physique of M. Pouillet 



I 



