68 Temperature of the Terrestrial Globe, &c. 
would penetrate into the air, and its elastick force from within 
would equal that from without, so that this power would be 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 liquefagtion. 
We know not, accurately, the temperature necessary to a lique- 
faction of aittiiogphetick 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 
