Theory of the Winds. 2T 



gradient in the lower three thousand feet of the troposphere 

 varies throughout the day. Up to the level o£ the strato- 

 sphere the temperature gradient is more equable, but varies 

 from place to place and from time to time, owing to the 

 varying humidity &c. of the convection currents. 



The stratosphere contains from one third to one fourth of 

 the total weight of the atmosphere. 



The stability of such an arrangement of temperatures and 

 pressures has been clearly indicated by Helmhoitz * in 1888. 

 He remarks: "It is well known how very differently the 

 propagation of changes of temperature in the air goes on 

 according as heat is added or withdrawn above or below. 



"If the lower side of a stratum of air is warmed, as occurs 

 at the surface of the earth, by the action of solar rays, then 

 the heated stratum of air seeks to rise. This is effected very 

 soon ail over the surface in small, tremulous, and flickering- 

 streams such as we see on any plane surface strongly heated 

 by the sun; but soon these smaller streams collect into larger 

 ones when the locality affords opportunity, especially on the 

 side of a hill. The propagation of heat goes on relatively 

 rapidly throughout the whole thickness of the atmospheric 

 layer, and when it has uniform heat throughout its whole 

 depth, and is therefore in adiabatic equilibrium, then also 

 the newly added air seeks de novo to distribute itself through, 

 the entire depth. 



" The same process occurs with like rapidity when the 

 upper side of a stratum is cooled. 



"On the other hand, when the upper side is warmed and 

 the lower side cooled, such convection movements do not 

 occur. The conduction of heat operates very slowly in huge 

 dimensions, as I have already explained above. Radiation; 

 can only make itself felt to any considerable extent for those 

 classes of rays that are strongly absorbed. On the other 

 hand, experiments on the radiation of ice and observations 

 of nocturnal frosts show that most rays of even such low 

 temperatures can pass through thick layers of clear atmo- 

 sphere without material absorption. 



" Therefore a cold stratum of air can lie for a long time on 

 the earth, or equally a warm stratum remain at an altitude 

 without changing its temperature otherwise than very 

 slowly." 



According to Gold, the reason why the troposphere exists 

 is that the earth, sea, and the solid matter, vapour and cloud, 

 in the lower atmosphere heats the adjacent air, warms it and 

 causes it to rise. During its ascent it radiates this heat, and, 



* Smithsonian Miscellaneous Collections, No. 843, 1893. 



