454 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



when the sky was covered with clouds, or when a screen was 

 stretched horizontally at the distance of some feet from the ther- 

 mometers, so as to intercept entirely the view of the celestial vault. 



The experiments of Wells have been repeated by several 

 observers, and in particular by M. Pouillet. This philosopher in- 

 serted one of the thermometers into swansdown contained in a 

 vessel placed on the ground, and left the other suspended freely 

 at the height of 4 feet, in the same way as Wells and Wilson. 

 The lower thermometer, on certain nights, descended eight or 

 nine degrees below the upper one*. 



The differences of temperature between the two thermometers 

 employed in these various experiments are evidently owing to 

 the calorific radiation towards the upper regions of the atmo- 

 sphere ; and the simple fact of the quickness with which they 

 diminish or entirely cease, on the appearance of clouds or under 

 the mere influence of an obstacle opposed to the exchange of 

 heat between the thermometers and the sky, is a sufficiently 

 evident proof of it. 



Nevertheless if we examine them with attention, it is not dif- 

 ficult to convince ourselves that these differences do not represent 

 the excess of radiation of the lower thermometer above that of 

 the upper one. 



And, in fact, we know that the temperature of the air at dif- 

 ferent distances from the ground is not constant, but variable 

 with the height. In general, the heat increases during the day 

 as we approach the terrestrial surface ; but the contrary occurs 

 in calm and serene nights. This latter fact, which was first 

 observed by Pictet towards the end of the last century, and after- 

 wards confirmed by Six, Maveix and other experimenters, leads 

 evidently to the consequence, that in the experiments above- 

 mentioned, the thermometers nearest the ground acquire, by the 

 mere contact of the medium in which they are plunged, a tempe- 

 rature lower than that of the upper thermometers ; and that, 

 consequently, the difference between the two temperatures is not 

 entirely owing to radiation. Again, as glass is endowed with a 

 very great emissive power, naked thermometers cool quite as 

 much as the most radiating bodies, and do not indicate, under 

 a serene atmosphere, the true temperature of the air. 



Hence, to obtain comparable results, and to judge how much 

 a thermometer covered or enveloped with a given substance falls 

 » rouillet, Elemcnls dc P/iysique, llh edition, 1814, p. 610. 



