122 OUTLINES OF FORESTRY. 



passing along the surface of the earth from the 

 equator to the poles. Or, in other words, three 

 hundred and fifty feet skyward equals seventy 

 miles poleward. 



In summer, when the sun is more nearly over- 

 head, and when in our hemisphere the earth is 

 gaining rather than losing heat, the land areas, 

 and consequently the air over them, rapidly become 

 heated; while the water areas, and consequently 

 the air over them, remain comparatively cool. 



In winter, however, when the loss of heat is 

 greater than the gain, the land areas, and conse- 

 quently the air over them, rapidly become cooled; 

 while the water areas continue for a long time to 

 part with the great stores of heat that they have 

 taken in during the summer, and thus remain 

 comparatively warm. 



Similar differences are observed between the 

 temperature of the air over the land and water 

 areas during the daylight while they are exposed 

 to the sun's heat, and during the night when they 

 are throwing it off. 



There is another reason why the water areas are 

 heated less rapidly than the land areas : the heat 

 penetrates the water to a comparatively great 

 depth, is diffused through a great body of water, 



