System of Winds on the Earth. 501 



relative rest of the atmosphere is that of indifferent equilibrium 

 with the adiabatic scale of temperature appropriate to it. In 

 other words, setting aside friction, no work is required to take 

 a mass of air from one level to another, which means in this 

 case that the energy expended by the expansive action of the 

 air is balanced by the loss of heat in cooling, and inversely. 

 The condition of relative rest of the atmosphere depends 

 therefore on the maintenance of indifferent equilibrium, every 

 disturbance of which makes its appearance as a storage of 

 energy, having the tendency to re-establish it through the 

 motion of the air. These disturbances are to be explained 

 exclusively by the unequal heating of the strata of air by the 

 sun's rays and their unequal cooling by radiation of heat into 

 space. The sun's rays heat the surface of the earth, which 

 then heats the strata of air resting on it. The excess of 

 temperature thus brought about over the adiabatic tempera- 

 ture of the earth, which corresponds to the mean temperature 

 of the whole superincumbent column of air, forms a store of 

 energy like a bent spring, which can only be balanced again 

 through the spreading of the excess temperature of the 

 deepest to the overlying air-layers, and thus restoring the 

 disturbed indifferent equilibrium. Practically, this can 

 only be effected by air-currents. When the superheating is 

 only local, a rising of the superheated air will occur at some 

 locally favoured place, which will increase quickly with the 

 height, for the upflow in the so-formed natural chimney increases 

 with the height. This chimney differs from ordinary chimneys 

 not only in height, but essentially, in so far that it has elastic 

 walls, and that the pressure and density of the strata of air 

 inside and outside diminish with the height. The velocity of 

 the ascending air must therefore increase inversely as the 

 density, for at each instant equal masses of air must pass 

 through all sections of the chimney. Considering the small 

 height of the atmosphere as compared with the earth's radius, 

 no considerable increase of area with height takes place 

 within it, and hence the velocity of the currents of air with 

 up and down flow must increase and diminish absolutely with 

 the air-pressure of the place. Through this upflow a greater 

 portion of the sun's energy stored up in the air is therefore 

 changed into the vis viva of air in motion than would be the 

 case without such acceleration. The final results of the 

 upflow of air limited as to space which has been overheated 

 on the earth's surface, will be that this local air-current 

 ascends with accelerated velocity up to the very highest 

 regions of the air, that at the same time the strata of air sur- 

 rounding the upflow descend with diminishing velocity, and 



