38 PRINCIPLES OF AMERICAN FORESTRY. 



instances are very rare in which they have continued for 

 long distances through forests, and it is probably true that 

 forests have a tendency to prevent their formation and 

 perhaps entirely break up those of lesser violence. M. 

 Becquerel is said to have found by careful study that in 

 some parts of Central France hail-storms show a marked 

 disinclination to enter forests, and yet occasionally they do 

 so, but nothing of this sort has been noticed here. 



FOREST INFLUENCES ON FOGS AND CLOUDS. 



The influence of forests on fogs and clouds has frequently 

 been mentioned. The fog seems to linger in the woods 

 after it has cleared off elsewhere. Trees also act as con- 

 densers and gatherers of dew, hoar-frost, and ice ; the latter 

 phenomenon is especially remarkable in the so-called ice- 

 storms, where the accumulation of ice on the trees is so 

 great as to break them down. The load of ice on some 

 large trees is probably a ton or more. In this case the tree 

 acts simply as an inorganic body. 



IMPROVEMENT OF LAND ON WHICH TREES GROW. 



As has been shown, trees add large amounts of soluble 

 mineral matters to the soil through the fall and decay of 

 their leaves. In the same way they add large amounts of 

 humus to the land, which helps to keep the soil porous and 

 yet makes it more retentive of moisture and gases. The 

 roots of trees often penetrate deep into the soil and bring 

 up plant-food that would not be reached by agricultural 

 crops. A part of this is returned to the surface soil by the 

 yearly fall of the leaves and in the twigs and branches that 

 are left on the ground when the tree is cut down. The 

 roots deepen the soil, and by their decay furnish plant-food 

 to the soil and leave channels through which water and air 



