240 HEAT DECREASES WITH HEIGHT. SECT. XXV. 



evident that the accumulation of heat in the long days of summer, 

 which is but little diminished by radiation during the short 

 nights, is balanced by the small quantity of heat received during 

 the short days in winter, and its radiation in the long, frosty, 

 and clear nights. In fact, if the globe were everywhere on a 

 level with the surface of the sea, and of uniform substance, so as 

 to absorb and radiate heat equally, the mean heat of the sun 

 would be regularly distributed over its surface in zones of equal 

 annual temperature parallel to the equator, from which it would 

 decrease to each pole as the square of the cosine of the latitude ; 

 and its quantity would only depend upon the altitude of the sun 

 and atmospheric currents. The distribution of heat, however, in 

 the same parallel, is very irregular in all latitudes except between 

 the tropics, where the isothermal lines, or the lines passing 

 through places of equal mean annual temperature, are more 

 nearly parallel to the equator. The causes of disturbance are 

 very numerous ; but such as have the greatest influence, accord- 

 ing to M. de Humboldt, to whom we are indebted for the greater 

 part of what is known on the subject^ are the elevation of the 

 continents, the distribution of land and water over the surface of 

 the globe exposing different absorbing and radiating powers ; the 

 variations in the surface of the land, as forests, sandy deserts, 

 verdant plains, rocks, &c. ; mountain-chains covered with 

 masses of snow, which diminish the temperature ; the reverbera- 

 tion of the sun's rays in the valleys, which increases it ; and the 

 interchange of currents, both of air and water, whidli mitigates 

 the rigour of climates ; the warm currents from the equator 

 softening the severity of the polar frosts, and the cold currents 

 from the poles tempering the intense heat of the equatorial 

 regions. To these may be added cultivation, though its influence 

 extends over but a small portion of the globe, only a fourth part 

 of the land being inhabited. 



Temperature decreases with the height above the level of the 

 sea, as well as with the latitude. The air in the higher regions of 

 the atmosphere is much cooler than that below, because the warm 

 air expands as it rises, by which its capacity for heat is increased, 

 a great proportion becomes latent or absorbed, and less of it 

 sensible. A portion of air at the surface of the earth whose tem- 

 perature is 70 of Fahrenheit, if carried to the height of two 

 mites and a half, would expand so much that its temperature 



