138 HEAT OF LOWEST STRATUM OP AIR. 



by the collective effect of which a great part of 

 the heat of the torrid zone is dispersed over the 

 colder regions of our earth, so that the heat of the 

 former, and the cold of the latter, are mutually 

 mitigated. The most universal of these causes are 

 the peculiar distribution of the land and sea, the 

 direction and height of the mountain-chains, the 

 currents of the ocean and of the air, and evapora- 

 tion. 



Since these various agencies towards the distri- 

 bution and equalization of temperature over the 

 surface of the earth (to which are added numerous 

 local influences, and even many which are altered 

 in the lapse of time) are often of very dissimilar 

 importance under similar latitudes, it thus happens 

 that the places of equal annual temperature, lie in 

 latitudes frequently ten degrees apart. 



Humboldt was the first to draw attention to 

 these remarkable relations of temperature, by con- 

 necting the places of equal annual temperature by 

 lines drawn round the earth, which he called Iso- 

 therms. The number of points — for which, up to 

 the present time, the mean temperature of the year 

 has been determined — are too few to enable us to 

 gird the globe with a perfect system of isotherms. 

 However a sufficient number of observations have 

 been collected, at least for the northern hemisphere, 

 into which the greatest masses of land are com- 

 pressed, to indicate the main directions of these 



