172 



which is about 90100 feet, or between 17 and 18 miles, if 

 convective equilibrium existed and if the gaseous laws had 

 application to so low temperatures and densities. It has 

 always appeared to the Author to be most improbable that 

 there is any limit to our atmosphere ; and no one can suppose 

 that there is a limit at any height nearly so small as 17 or 18 

 miles. It is difficult to make even a plausible conjecture as 

 to the effects of deviations from the gaseous laws in circum- 

 stances of which we know so little as those of air at very low 

 temperatures ; but it seems certain that the other hypothesis 

 involved in the preceding equations is violated by actions 

 tending to heat the air in the higher regions. For at 

 moderate elevations above the surface, where we have 

 air following very strictly the gaseous laws, the rate of 

 decrease of temperature would, according to equation (4), be 



•41 V T 1° /. . 



be .TrfrT pel' foot, that is to say, ^ per foot, since 



T 



H = 26224 X 7.^1, or 1° cent per 329 feet. Now, the actual 



274 



decrease, according to Mr. Welsh, is 1° cent in 530 feet, or 

 not much more than half that according to convective 

 equilibrium. 



It seems that radiation, instead of partially accounting 

 for the greater warmth of the air below, as commonly 

 supposed, may actually diminish the cooling effect, in going 

 up, which convection produces. In fact, since direct con- 

 duction is certainly insensible, we have only convection 

 and radiation to deal with, except when condensations of 

 moisture, &c., have to be taken into account. In fair and 

 cloudless weather, then, the lower and lowest air being on the 

 whole warmer (the lowest being of course at the same tem- 

 perature as the earth's surface), it is perfectly certain that 

 the upper air must gain heat by radiation from the lower — 

 and that the convective difference of temperature must be 

 diminished by the mutual interradiation. 



