FOREST POLICY. 15 



than in the forest — during summer by lo per cent, and during winter 

 by 3 per cent, and during the year by 6 per cent. — owing to the lower 

 temperature of forest air. As a consequence, radiation is checked 

 during the night and sudden changes of temperature are mitigated in 

 the forest, the forest soil is kept more moist during the summer, and 

 more precipitations, dew and rain occur in the forest. 



(XVIII.) MOISTURE AND PRECIPITATION. 



From a theoretical standpoint we should assume that, the humid- 

 ity in the forest being greater than over the field, more precipitations 

 occur in the forest than in the field. 



European experiments relative to the influence of the forest on 

 precipitation are not conclusive. The position of the ombrometer 

 influences the result to a high degree. 



As a maker of rainfall, elevation is of largely greater importance 

 than presence of forests. In Europe the higher elevations are usually 

 covered with forests. Experiments showing the truth of the matter 

 can only be made in level country. 



(XIX.) EVAPORATION OF SOIL MOISTURE. 



There is a vast difference between the evaporation from field soil 

 and from forest soil, especially if the leaf litter is left on the ground 

 in the forest. 



In the latter case, after Ebermeyer, the evaporation from forest 

 soil is only i6 per cent, of the evaporation from field soil. When the 

 litter is removed by rake or fire, the proportion is 38 per cent. 



On the other hand, the evaporation from the crowns of the trees 

 is enormous. Forests, it may be said, are the greatest consumers of 

 water. Observations in the Russian steppes prove conclusively that 

 the level of the underground water beneath forests is lower than in 

 the surrounding country. Where there is water, we invariably find 

 forests, in nature, — but not vice versa. Cause and effect should not 

 be mixed. 



Evaporation largely depends on the velocity of the wind, which 

 is readily reduced by a shelter belt of forest. Hence, drought in the 

 prairies might be checked by sheltering screens of woodland placed, 

 gridiron-fashion, across country at proper intervals. (Compare 

 Green, p. 28.) 



(XX.) SPRINGS AND RIVERS. 



The following facts tend to increase the regularity, if not the 

 amount, of water running off from the forest: 



1. The greater porosity of the forest soil increases (proven by 

 German experiments) the permeability of the forest soil. 



2. The litter on the ground checks the superficial run-off of water 

 on the slopes. 



