416 ON THE DIFFUSION OF HEAT 



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 around, any tendency to communicate heat 

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

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

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

 wards ; but its capacity for caloric increasing from its rarefac- 

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

 had received, without having a corresponding elevation of 

 temperature. It is only when it returns towards the surface, 

 in consequence 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 pressure, its capacity gradually diminishes ; and con- 

 tinuing 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 conveyed by the ascent of a stratum of heated 

 air from any part of the circumference of the globe, a corre- 

 sponding 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 arrangement to counteract local ine- 

 qualities 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 be withdrawn to comparatively a short distance from the sur- 

 face 



