at the Surface of the Earth. 171 



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, further, 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 difference of tem- 

 perature ; and if tliere is no difference, there will be no such 

 communication. Now, a little reflection will show, that thii 

 completely limits the diffusion of heat through an elastic fluid, 

 receiving it under such conditions as our atmosphere. The air 

 heated at the surface expanding as it rises, but at the same time 

 from this expanion falling in temperature, must come at length 

 to be in equilibrium, both in density and in temperature, with 

 air at a certain height ; its further ascent will then cease, and 

 its temperture being the same with that of the air around it, it 

 will )-ield none of its heat. It is only in its descent, as a fresii 

 portion of air rises, that it will evolve caloric, and it will con- 

 tinue 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 by the atmosphere into the interplanetary space ; tlie 

 excess at particular regions is only withdrawn, is more equally 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 i)y which 

 this could have been attained, than by an atmosp'iere consti- 

 tuted as ours is, — expanding as it is heated, and falling in tem- 

 perature from augmentation of capacity as it expands. To ad- 

 mit of an unlimited conveyance of heat, a solid mass, however 

 •■ubtile, would have been required, or an atmosphere the capacity 

 of which for caloric should not increase with its rarefaction. 



The passage of heat is thus from the circumference towards 

 the centre of the globe, instead of the opposite direction, and it 

 is prevented from accumulating at the surface only by being con- 

 veyed into the mass of earth. "^ 



These conclusions may be applied to the present argument. 

 If the view I have stated be just, we have demonstration that the 

 atmosphere conveys no heat into unlimited space ; our planet, 

 in relation to the discharge of caloric from it, is bounded as it 

 were by a wall of non-conducting matter, and no arrangement 

 <an be imagined more perfect, by which the heat belonj;ing ^t 

 it could he confined. If the heat from the centre, Mr. Playfair 

 observes, on arriving at the circumference, were prevented from 

 proceeditig further, and if space or vacuity presented to heat an 

 unpenetrable barrier, then an accumulation of it at the surface, 



and 



