68 Temperature of the Terrestrial Globe.) SfC. 



would penetrate into the air, and its elastick force from within 

 Y^ould equal that from without, so that this power would he bal- 

 anced. It is, then, through the agency of cold that the superiour 

 layers of the atmosphere are deprived of their elasticity. Near its 

 superiour surface the temperature of the air should be that of the 

 liquefaction of this fluid, and the layer of liquid air should be of 

 such thickness as that its weight may equal the elastick force of 

 the inferiour air, upon which it rests. If the molecular force 

 should disappear in this exteriour layer, in consequence of the 

 mutual distance of the molecules, rendered very great by the ex- 

 treme rarefaction of the fluid, this layer could not support itself 

 upon that immediately beneath ; the gravity of its molecules to- 

 wards the earth could not be destroyed except we suppose a velo- 

 city of rotation and a centrifugal force communicated to them 

 greater than to those of this other layer ; and this experiencing no 

 exteriour pressure, should be considered the extreme layer of the 

 atmosphere, which can only lose its elasticity by liquefaction. 



We know not, accurately, the temperature necessary to a lique- 

 faction of atmospherick air, at its ordinary density, and still less in 

 the rarefied state of its superiour layers ; but there can be no doubt 

 that it is extremely low, and perhaps still more so in a case of feeble 

 density. This temperature, which is indispensable to a definite 

 termination of the atmosphere, is, it appears to me, the true cause 

 of the excessive cold of its superiour part, and of the decrease of 

 the heat of its successive layers, in proportion as we ascend from 

 the surface of the earth. This phenomenon, then, would still 

 take place if the atmosphere were perfectly in repose ; it is not 

 therefore owing, as has been sometimes supposed, to an ascen- 

 sional movement of the air, in which this fluid is dilated by the 

 diminution of pressure, and becomes cooled in consequence. 

 Those who have given this explanation have not observed that 

 this upward movement is accompanied by another, in the contrary 

 direction, and that in this double movement the masses of air 

 mingle together, and traverse each other, mutually, so that it would 

 be difficult to decide whether the final result should be an aug- 

 mentation or a diminution of the density and the mean tempera- 

 ture of the whole. But we must not lose sight of the fact that 

 this extremely low temperature of the superiour layer of the at- 

 mosphere is that of the air itself, and not that which would be 

 indicated by a thermometer placed therein. This might be much 



