MELLON! ON THE NOCTURNAL COOLING OF BODIES. 4/5 



braced by the beautiful experiments of M. Pouillet, I shall bring 

 forward his series of observations, which will serve us as a 

 starting-point in conceiving cleai'ly how it happens that the 

 stratum of air situated near the earth's surface always descends 

 nearly to the point of greatest moisture before the dew begins 

 to appear. 



These observations, inserted at page 610 of the second volume 

 of his Traite de Physique (4tli edition), have reference to the 

 cold produced by the radiation of swansdown, suitably isolated 

 by means of an apparatus which the author designates by the 

 name of actinometer. 



Putting aside everything superfluous to the object with which 

 we are at present occupied, we shall have the two reduced tables 

 here placed by the side of each other : — 



Table of Nocturnal Cooling under different Temperatures 

 of the Air. 



The data contained in these two tables prove that swansdown 

 is cooled about 7° (mean value) under the nocturnal influence of 

 a clear sky, whether the temperature of the air fall to nearly 

 zero or rise to 20° or more degrees. 



It is almost needless to add, that the observations of M. 

 Pouillet, and the determination by Parry and Scoresby of the 



