76 SOILS AND MANUKES 



receive them. It will be seen that 'in the higher latitudes 

 equal amounts of sunshine are spread over much larger 

 areas and have much smaller calorific effect per unit of 

 surface. Second, if the circle K T represent the outer 

 limit of the atmospheric envelope, the distances a' a, b f I, 

 c r c, etc., represent the depth of air traversed by the rays, 

 and it will be seen that they are much greater in the higher 

 latitudes. 



Influence of Altitude. The higher mountain peaks and 

 more elevated positions probably receive less heat from 

 the interior of the earth. The air, in the higher regions, 

 contains less water vapour and is less dense ; it has, there- 

 fore, for equal volumes, a much smaller capacity for heat. 

 Also it receives less heat by radiation from the surface of 

 the earth. The influence of altitude on the climate may 

 be seen from the fact that the line of perpetual snow is 

 found 



In latitude (equator) . . 18,000 feet above sea level. 



46 (Geneva) . . 9,000 



60 (St. Petersburg) . 4,000 . 



,, ,, 75 (Nova Zembla) . at the sea level. 



Influence of Aspect. For the reasons given above, in 

 the higher northern latitudes the soil receives more heat 

 from the sun when it has a southern slope or aspect. This 

 may be seen from the diagram (Fig. 12), in which A B C D 

 represents a sunbeam falling upon a flat portion of the 

 surface of the earth A D at an angle of 30. If A D be 

 horizontal, the sunshine falling upon it is represented by 

 the line B C ; but if A D be inclined to the northward at 

 an angle of 10 as e D the sunshine falling upon it will 

 be reduced to E C ; if the angle of inclination be 20 as 

 / D the sunshine will be reduced to F C. If A D be 

 inclined to the southward at an angle of 10 as h D the 

 sunshine falling upon it will be increased to H C ; if the 



