ESSAY 



least 30 would probably appear, on some still 

 and serene night, between a small thermometer 

 placed with its bulb naked*, on the middle, or 

 leeward side of a stratum of a downy substance, 

 occupying a space upon a grass field, or bed of 

 snow, one or two square yards in extent, and a 

 similar thermometer inclosed in a case of gilt 

 paper, and suspended in the air a few feet above 

 the other. Two thermometers, thus placed, 

 would, I think, be sometimes found even in 

 this country to differ not much less than 30. 

 I have myself never made any such experiments 

 with a downy substance, which had a surface of 

 more than a few square inches, or in a very cold 

 night, when the atmosphere was clear and calm, 

 and the scene of observation remote from large 

 masses of building. 



But even a cold of 30 appears not to be the 

 greatest, that can be thought to occur, from the 

 radiation of heat to the heavens, at night, by 

 substances on the surface of the earth. For 

 experiments by Mr. Pictetf, Mr. Sixf, and I 

 may add by myself, establish that, in exception to 

 the common rule, the heat of the atmosphere in 

 clear and calm nights increases with the distance 



* The effect would, perhaps, be a little increased, by cover-* 

 ing the bulb with a very thin layer of lamp-black, 

 f Essai sur le Feu, c. x. 

 J Phil. Trans. 1784, and 



