THERMAL CONDITION OF THE EAKTIL 



clouds and the surface, warm the lower strata of the atmosphere 

 through which they are so frequently transmitted. 



185. Effect of contact of air and earth. But the atmosphere 

 also receives heat from the earth, by the contact of its lowest strata 

 with those parts of the terrestrial surface, which have a higher 

 temperature. When these lowest strata are thus warmed by 

 contact, they ascend as heated air does in a chimney, air having a 

 lower temperature flowing in to take their place. 



It is by these several means therefore, much more than by the 

 direct influence of the sun, that the temperature of the atmosphere 

 is maintained and regulated. 



186. Thermal effects of an uniform surface. If the whole 

 surface of the earth consisted of one homogeneous material, 

 whether solid or fluid, and were in one uniform condition, its 

 powers of reflection and absorption, and consequently its power 

 of radiation, would be everywhere the same. In that case 

 the thermal influence of the sun, governed by no other con- 

 ditions than those of its altitude and continuance above the 

 horizon, would necessarily be the same at all points of the same 

 parallel of latitude, since, at all such points, the sun's altitude 

 arid the length of the day are necessarily the same. The equator 

 and each parallel of latitude would then be isothermal lines, a 

 word which expresses a line upon the surface of the earth, all 

 points of which have the same mean temperature. Not only 

 would the mean temperatures of all points on the same parallel be 

 the same, but the extreme, temperatures of summer and winter 

 would likewise be the same. In other words, all places having 

 the same latitude would, under the conditions supposed, have 

 identically the same climate. They would have the same average 

 annual temperature, and would be subject to the same vicissitudes 

 of seasons. 



In comparing parallel with parallel, the average annual tem- 

 perature, as well as'the extremes of the seasons, would regularly 

 increase with the latitude. 



187. Why this regularity does not prevail. But the con- 

 ditions which would produce such climatic phenomena do not exist 

 on the terrestrial surface. Instead of consisting of one uniform 

 material, that surface is diversified, first, by land and water ; 

 and secondly, the land itself is still more diversified in its 

 character, according as it is more or less clothed with vegetation, 

 and even where it is destitute of vegetation, it varies according 

 to the quality of the rocks or other strata which compose its 

 uncovered surface. The reflecting, absorbing, and therefore 

 radiating powers of the land, are infinitely diversified. In general 

 the foliage of vegetation is a powerful radiator, while the naked 



183 



