254 HEAT DECREASES WITH HEIGHT. SECT. XXVI. 



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 quan- 

 tity 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 equa- 

 tor, 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, how- 

 ever, in the same parallel, is very irregular in all lati- 

 tudes 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, according 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 absorb- 

 ing 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, 

 which mitigates the rigor of climates ; the warm cur- 

 rents from the equator softening the severity of the 

 polar frosts, and the cold currents from the poles tem- 

 pering 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 



