32 ■ PRACTICAL FORESTRY. 



flat, hilly, or mountainous, and the character of the surface covering. 

 These are all connected with the temperature in a special manner. 

 Lastly, it is afl^ected by the winds and the moisture of the atmosphere. 

 Now, it is clear that of all these factors of climate the forest can 

 influence only the wind, the moisture, and the surface covering; but 

 heat (with which the surface covering has so much to do), moisture, 

 and wind are the three things which change when we say that the 

 weather changes. These are just the points where a change due to 

 the forest would have most effect on daily life. The influence of the 

 forest is exerted upon them in two ways: 



First. The forest cover intercepts the rain and the rays of the sun, 

 checks the movement of the air, and reduces the radiation of heat at 

 night. 



Second. The waste from the trees and from certain plants which 

 grow only in their shade forms the forest floor, which has much to do 

 with the movement of water on the ground and within it. The influ- 

 ence of the forest cover and the forest floor appears in the temperature 

 of the air, the evaporation of water, the rainfall, and the course of the 

 rain water after it has reached the earth. 



EFFECT OF FOREST COVER ON TEMPERATURE. 



So far as the influence of the forest is concerned, the temperature 

 of the air is affected chiefly by the forest cover. The leaves, which 

 compose the greater part of the cover, contain from 50 to 70 per cent 

 of water. More heat is required to raise the temperature of a pound 

 of water one degree than for a pound of almost any other substance, 

 and so it happens that bare soil or rock exposed to the rays of the sun 

 becomes heated many times faster than the water in the leaves. 

 While the heated rock or soil was warming the air about it the forest 

 cover would still be absorbing heat and keeping the air below it cool. 

 The leaves of the cover also tend to cool the air by transpiration, which 

 is the evaporation of water from the leaves. This is true because 

 heat is required to change water into water vapor, and a part of the 

 sun's heat is taken up for this purpose. In these two ways the forest 

 cover acts somewhat like a surface of water. 



The growth of the tree itself also helps to cool the air. When the 

 leaves take carbonic-acid gas from the air they break it up and force 

 its carbon into new chemical compounds, which are then stored away 

 as new material in the tree. So with water and the other substances 

 upon which the plant feeds. But the elements are less at ease in 

 these new compounds, and heat is required to force them to make the 

 change. When we burn wood for fuel we are simply getting back 

 again the heat which was used to bring about this change. So we 

 may say roughly that the air about the tree during its Ufetime has 

 :-!58 



