192 



ESSAY 



many hours \ yet these circumstances are found 

 to occur in substances attractive of dew, when 

 laid on the surface of the earth, in a still and 

 serene night, and are in perfect agreement 

 with the doctrine of heat, now universally ad- 

 mitted to be just. 



To render this more easy of apprehension, 

 let a small body which radiates heat freely, and 

 possesses a temperature, in common with the 

 atmosphere, higher than 32, be placed, while 

 the air is clear and still, on a slow conductor of 

 heat lying on the surface of a large open plain, 

 and let a firmament of ice be supposed to exist 

 at any height in the atmosphere ; the conse- 

 quence must be, that the small body will, from 

 its situation, quickly become colder than the 

 neighbouring air. For, while it radiates its 

 own heat upwards, it cannot receive a sufficient 

 quantity from the ice to compensate this loss ; 

 little also can be conveyed to it from the earth, 

 as a bad conductor is interposed between them^ 

 and there is no solid, or fluid except the air, to 

 communicate it laterally either by radiation or 

 conduction. This small body, therefore, unless 

 it shall receive from the air, nearly as much 

 heat as it has emitted, which, considering the 

 little that can be communicated from one part 

 of the atmosphere to another, in its present 

 calm state, must be regarded as impossible, will 



