228 



PHYSIOGRAPHY 



poles, should receive more heat than any other part of the earth 

 in summer. 



2. Other things being equal, the surface of the land or water 

 gets most heat where the sun's rays fall most nearly vertically, 

 because the rays are there most concentrated, and because they 

 pass through a lesser thickness of the air, which absorbs some of 

 their heat. This is shown by Fig. 210. A given bundle of rays, 1, 



Equator 



\ \ 



\ \ 



Fig. 210. Diagram to illustrate the unequal heating due to the direction 

 in which the rays of the sun reach the surface of the earth. The broken 

 line may be taken to represent the outside of the atmosphere. 



falling vertically on the surface, is distributed over a given space, 

 while an equal bundle of rays, 2, falling obliquely on the surface, 

 is spread over a greater area, and therefore heats each part less. 

 Again, the oblique rays, 2, have passed through a greater thick- 

 ness of air, and more of their heat has been absorbed by it 

 before they reach the solid part of the earth. 



The greater the angle of the sun's rays, the greater the heat. The 

 angle at which the sun's rays reach the earth varies from place to 

 place, and from time to time at the same place. This is a result 

 of the inclination of the earth's axis, and is illustrated by Figs. 

 202, 207, and 208, which have been explained. In general, low 

 latitudes receive the sun's rays less obliquely than high latitudes. 



