THE PHYSICAL PROPERTIES OF SOILS 75 



fall vertically upon it, i.e., when the plane of the surface 

 makes a right angle with the direction of the rays. Only 

 a comparatively narrow zone, lying between the parallels 

 of 23J north and south latitude, ever occupy that most 

 favourable position ; in every other latitude the sun's rays 

 always strike the earth at an angle which is more or less 



FIG. 11. 



acute according to the time of year. For this reason, and 

 also because more heat is absorbed by the atmosphere, 

 owing to the greater depth traversed by the sun's rays, 

 in the higher latitudes the soil receives less heat from the 

 sun. 



These effects are illustrated in the diagram (Fig. 11). 

 First, if the divisions of the straight line A B, B C, C D, 

 etc., represent equal amounts of sunshine, the corre- 

 sponding divisions a b, b c, c d, etc., will represent the 

 extent of the portions of the surface of the earth which 



