of Energy in the Atmosphere, 455 



lower layers of air and in evaporating water. The loss of 

 heat by radiation into space also takes place chiefly from the 

 solid and liquid surface of the earth, and only to a very small 

 extent directly from the mass of air. There are here two 

 important considerations to be taken into account. Whilst 

 the solar radiation, regarded as radiated from one point, falls 

 chiefly upon low latitudes, the radiation from the earth into 

 all parts of space is independent of latitude, and depends 

 only on the differences between the temperature of the radia- 

 ting portions of the earth's surface and that of space. Since 

 the radiation of the stars, which apparently gives heat to 

 space, is under precisely the same conditions for all parts of 

 the earth's surface as its radiation, it may be neglected, and 

 the temperature of space is to be taken as absolute zero. Of 

 the radiation it is further to be observed that the direct loss by 

 radiation of the higher and more rarefied layers of air must 

 be greater than that of the lower layers, since radiation is 

 greater in vacuo than in a space filled with air. 



This having been premised, the following conditions for 

 the equilibrium of the atmosphere may be stated : — 



1. The condition of equilibrium of the atmosphere at rest 

 is indifferent, the corresponding curve of temperature adia- 

 batic. This amounts to saying that the transference of a 

 mass of air from one height to another — disregarding any 

 loss from friction — is accompanied neither by loss nor gain 

 of work. 



2. A disturbance of the indifferent equilibrium of the at- 

 mosphere, corresponding to a local accumulation of energy, 

 is produced by the heating of the masses of air lying nearer 

 the earth's surface by means of the sun's rays above the 

 appropriate adiabatic temperature, as well as by the cooling 

 of the higher layers of air through increased radiation. The 

 heat employed in the evaporation of water increases this dis- 

 turbance of equilibrium in the same direction and ratio, since 

 aqueous vapour has a smaller specific heat than air, and since 

 the latent heat of the vapour condensed by the adiabatic cooling 

 of the air as it rises is used in the heating and expansion of 

 air. 



3. The energy accumulated in the disturbance of the neutral 

 equilibrium of the atmosphere by excessive heating of the 

 lower layers, and excessive cooling of the upper layers, must 

 distribute itself by means of ascending and descending cur- 

 rents of air. In accordance with Clausius's second law of 

 the mechanical theory of heat, the excess of heat of the ex- 

 panding air is chiefly transformed into the vis viva of moving 

 air ; to a smaller extent it is distributed in larger and com- 



2K2 



