34 INDIRECT UTILITY OF FORESTS. 



Trees are considerably colder than the surrounding air 

 during the day, while during the night the difference almost 

 disappears, and in some cases they are even warmer at night. 

 The latter phenomenon is due to the rising sap which causes 

 the temperature of the trees, at any rate at the lower end, to 

 approach that of the soil. The thicker the trees, the more falls 

 their temperature below that of the surrounding air. 



d. Summary. 



The above data seem to justify the following conclusions : — 



(1.) The climate of forest countries is more equable than 

 that of open countries. 



(2.) The mean temperature of soil and air in forest countries 

 is somewhat lower than that of soil and air in open countries. 



(3.) The reduction of the temperature may act beneficially 

 in hot countries, but it may be injurious in countries removed 

 from the equator, where the temperature is already lower than 

 is good for the ripening of field crops. 



(4.) Vegetation awakens later in spring in well stocked 

 forests than in open ground. 



(5.) As forests moderate the extremes of temperature, plants 

 growing under the shelter of a forest crop are less liable to 

 8ufi"er from late and early frosts, or from drought, than plants 

 growing in the open. 



2. Effect of Forests on the Moisture of the Air and the 

 Movement of Water in Nature. 



Air can hold only a certain maximum quantity of vapour, 

 which increases and decreases with the temperature. When 

 the maximum has been reached, and more vapour is introduced, 

 a part becomes condensed. The absolute vapour in the air is 

 measured by its tension upon a column of mercury as repre- 

 sented in a barometer. The proportion of the absolute to the 

 maximum tension of vapour, which is possible at a certain 

 temperature and pressure, is called the rekitire humidity of 



