552 MELLONI ON THE NOCTURNAL COOLING OP BODIES. 



Moreover, the " serein" always falls in summer, or at the com- 

 mencement of autumn, at the close of warm, moist days, under 

 a sky slightly scorched and whitish {hale et blanchdtre). There 

 is every reason therefore for believing that the air is then saturated 

 with humidity up to a certain elevation, and that the upper por- 

 tion of this diaphanous vapour is transformed into vesicular va- 

 pour, by the cold which prevails in the higher regions of the 

 atmosphere. These vesicles, which are sufficiently rare and uni- 

 formly scattered as to cause only a slight tinge of white which 

 does not sensibly alter the proper colour of the atmosphere, will 

 therefore lose, with the last rays of the setting sun, the heat 

 which supplied the losses caused by their radiation into space ; 

 there will be a lowering of temperature, and the formation of 

 small drops, which, traversing in their descent the lower strata 

 of an atmosphere saturated with humidity, can suffer only a 

 feeble degree of evaporation, and hence will even reach the 

 surface of the earth. 



dusk may very probably play a certain part in the production of this pha^no- 

 menon. I make this remark, because, on looking at the moon through a good 

 glass, I have often observed its disc traversed by fragments of clouds whilst 

 the stars everywhere shone brightly, and the sky appeared perfectly clear. 



