INFLUENCE OF LATITUDE AND ASPECT 153 



is simply due to the different angle at which the rays fall 

 on the earth at different times of the day. 



The reason why so much depends on the angle at which 

 the sun's rays strike the earth will appear by an inspection 

 of Figure 4. Here three sunbeams of equal dimensions are 

 represented as falling on a flat surface, one vertically, another 

 at an angle of 30, and the third at an angle of 10. It will 

 be seen that the beam at 30 spreads itself over twice as 



FIGURE 5. 



much earth surface as the vertical beam, and consequently 

 only supplies half the heat per unit of area ; while the beam 

 at 10 spreads itself over more than five times the surface, 

 and thus supplies less than one-fifth of the heat per unit of 

 area which the same beam would furnish if falling vertically. 

 Another cause, but one far less important, of the diminishing 

 heating power of the sun's rays as it approaches the horizon, 

 is the greater atmospheric absorption of heat rays which then 

 takes place. In Figure 5 we have a representation of an 



