ITS CHANGES AT THE SURFACE. 119 



equator, and how the southward slopes of moun- 

 tains receive the most heat in our latitudes. 



The earth-surface, thus warmed by the sun's 

 iniluence^ yields up again, by radiation, and by 

 conduction to the cooler matter in its neighbour- 

 hood, a portion of the heat which it receives. One 

 part of the heat derived from the sun is thus trans- 

 ferred to the underlying strata of the earth, 

 another part is taken up from the ground by the 

 air that rests upon it. Of the rays, which the 

 heated earth sends forth, and which have now 

 again to pass through the atmosphere, there also 

 remains behind a portion sometimes greater, 

 sometimes less ; the rest diffuses itself into space, 

 and is lost for any purpose of terrestrial heat. 



We receive heat most copiously at noon, when 

 the sky is cloudless. However, this point of time 

 is not that of highest temperature on the earth's 

 surface. For this latter, while it is taking in heat, 

 is always parting with it at the same time ; so that 

 the temperature must continue to rise, so long as 

 the influx of heat exceeds the escape ; and thus 

 the limit of increase of temperature must coincide 

 with the point of equilibrium of these two antago- 

 nist actions. Now there is scarcely any difference 

 between the direction and, consequently, between 

 the heating power, of the rays of the sun when it is 

 at the highest point, and that which it has for some 

 time after; but the excess of influx of heat is 



