AT THE EARTH'S SURFACE. 417 



face of the earth, and that there is a constant return of it. In- 

 direct communication its conveyance must be equally limited ; 

 the conducting power of an aeriform body is extremely imper- 

 fect, and there is every reason to believe becomes less as the 

 fluid becomes more rare ; and in the higher regions of the at- 

 mosphere, the subtilty of the medium is so great, that beyond 

 a certain height it cannot be supposed to be the vehicle of the 

 conveyance of heat, far less that it can convey it into the 

 boundless fields of vacuity. 



But, farther, were the conducting power of the most perfect 

 kind, no effect could arise from it in the conveyance of caloric 

 beyond a certain height. The communication of heat from 

 one part of a mass of matter to another, or from one body to 

 another, depends, as Mr Playfair justly observes, on differ- 

 ence of temperature, and if there is no difference, there will 

 be no such communication. Now, a little reflection will shew, 

 that this completely limits the diffusion of heat through an 

 elastic fluid, receiving it under such conditions as our atmo- 

 sphere. The air heated at the surface expanding as it rises, 

 but at the same time, from this expansion falling in tempera- 

 ture, must come at length to be in equilibrium, both in densi- 

 ty and in temperature, with air at a certain height ; its farther 

 ascent will then cease, and its temperature being the same 

 with that of the air around it, it will yield none of its heat. 

 It is only in its descent, as a fresh portion of air rises, that it 

 will evolve caloric, and it will continue uniformly to do so, as 

 it returns to the surface of the earth. 



We thus trace a curious provision to prevent any discharge 

 of heat bythe atmosphere into the interplanetary space ; the 

 excess at particular regions is only withdrawn, is more equally 

 distributed over the surface, but is ultimately communicated 

 to the earth ; and there is no other apparent arrangement by 



which 



