170 On Ihe Dijudon of Heat 



elastic medium. The jusf view of the operation of the atmo- 

 sphere in diffusing heat, is however more compHcated; other 

 conditions connected with it are to be taken into account, and 

 lead, I behove, to a very different result. 



The air heated at any part of the earth's surface, may, as it 

 ascends, impart, especially at first, a jjortion of heat to the air 

 with which it comes in contact. But as it rises, becoming sub- 

 ject to less pressure, it expands; by this its capacity for heat is 

 augmented, and its temperature, therefore, falls proportionally. 

 As it recedes from the surface, with its temperature thus con- 

 stantly falling, from its increasing capacity, its tendency to part 

 with heat is always becoming less; and as it must continue to 

 rise in the atmosphere until it attain an equilibrium of tempe- 

 rature with the air aroimd, any tendency to communicate heat 

 to that air nmst at length cease. If it retained its high tempe- 

 rature, or if it were to lose this gradually only from the commu- 

 nication of its heat, it might be conceived to convey caloric on- 

 wards ; but its capacity for caloric increasing from its rarefaction 

 as it ascends, it is enabled to retain the excess of heat it had 

 received, without having a corresponding elevation of tempera- 

 ture. It is only when it returns towards the surface, in conse- 

 quence of that circulation which the constant ascent of portions 

 of heated air establishes in the atmosphere, that this is evolved* 

 As it descends, it becomes progressively subject to greater pres- 

 sure, its capacity gradually diminishes ; and continuing to do 

 so as it falls, it gives out in the same gradual manner the excess 

 of heat which it contains. Thus, for every portion of heat con- 

 veyed by the ascent of a stratum of heated air from any part of 

 the circumference of the globe, a corresponding portion of heat 

 is given out by a descending stratum at some other part ; and 

 as this communication of heat from the atmosphere will happen 

 principally at the colder parts of the earth's surface, both as the 

 descent of the air will be there greatest, and the disposition to 

 receive heat also greatest, the whole forms an admirable arrange- 

 ment to counteract local inequalities of temperature, to diffuse 

 heat equally over the globe, and to prevent any dissipation of it 

 beyond the sphere of the atmosphere. 



It is thus, I conceive, demonstrated, that by the principal 

 mode in which heat is propagated through the atmosphere, — that, 

 by the motion of the heated portion of air, it can only be with^ 

 drawn to comparatively a short distance from the surface of the 

 earth, and that there is a constant return of it. By direct com- 

 munication its conveyance must be equally limited ; the con- 

 ducting power of an aeriform body is extremely imperfect, and, 

 there is every reason to believe, becomes less as the fluid be- 

 couies more rare ; and in the higher regions of the atmospiiere 



the 



