420 ON THE DIFFUSION OF HEAT 



At the parts of the surface of the earth which are at still 

 lower temperatures, and at those where intense cold prevails, 

 the supposition of any radiation is of course still more preclu- 

 ded, and the negative even may be proved. When a solid bo- 

 dy, and the surrounding elastic medium, are at the same tem- 

 perature, there seems to be no discharge of radiant heat from 

 the former ; and still more when the temperature of the solid 

 is lower than that of the elastic medium, (and this is usually 

 the case in colder regions), it is rather disposed to absorb than 

 to emit caloric by radiation. The resistance, too, opposed by 

 the atmosphere to the discharge of rays having the weak pro- 

 jectile force which those emitted at such a low temperature 

 must have, would in a great measure prevent their escape, if 

 the power to radiate did actually exist. 



At the hotter parts of the earth's surface there may be some 

 emission of caloric by radiation, but it is not difficult to shew, 

 that the quantity of this cannot be equal to the quantity com- 

 municated by the solar rays ; for of the heat derived from the 

 latter source, a portion is absorbed by the earth, and conveyed 

 to the interior, as is apparent from the decreasing tempera- 

 ture, as we recede from the surface to a certain depth ; and 

 another portion is carried off by the ascending current of 

 heated air, and conveyed to colder regions, where it is also 

 absorbed. 



Thus, even from 'those parts of the surface of the earth 

 where the circumstances are those most favourable to radia- 

 tion, the quantity radiated cannot be equal to the quantity re- 

 ceived by the solar rays. Over the whole surface, the differ- 

 ence must be still greater, and instead of the conclusion, that 

 this planet discharges an excess of heat by radiation, there is 

 every reason to draw the opposite conclusion, — that part of the 

 heat which it receives from the sun is retained. 



The 



