OF TEMPERATURE IN THE ATMOSPHERE. 127 



Welsh^ is 1° Cent, in 530 feet, or not mucli 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 

 temperature as the eartVs 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. 



There are difficulties connected with the radiation of air 

 and earth out into space, and of heat from the sun to air 

 and earth ; but I think a full consideration of all the cir- 

 cumstances must explain the smallness of the decrease of 

 temperature which observation shows. 



Dr. Joule having suggested that condensation of vapour 

 in upward currents of air might account, to a considerable 

 extent if not perfectly, for the smallness of the lowering of 

 temperature actually found in going up, I have added the 

 following investigation, in which the effect of condensation 

 is taken into account. 



If a quantity of air, dry or moist, is allowed to expand 

 from bulk v to bulk v + dv, it will do an amount of work 

 equal to pdv on the surrounding matter. Now, by the 

 principle established approximately by Dr. Joule, in his 

 experiments on air in 1844^, the change of temperature 

 which the mass will experience will be almost exactly 



* " On the Changes of Temperature produced by the Earefaction and 

 Condensation of Air," communicated to the Eoyal Society, June 20, 1844, 

 and published in the ' Philosophical Magazine,' 1845, ^^^t half year. 



