60 A PRIMER OF FORESTRY. 



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 ahiiost 

 any other substance, and so it happens that bare soil 

 or rock exposed to the ra3\s 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 ele- 

 ments 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 simpl}^ getting back 

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

 So we may sa}" roughly that the air about the tree dur- 

 ing its lifetime has been deprived of as much heat as 

 would be given off if the whole tree were burned. 



The effect of the cooler air of the forest is felt to 

 some distance in the open country. During the day, 

 in calm summer weather, when the air is warmer than 

 the tree tops, it is gradually cooled by contact with the 



