WIND AND HAIL STORMS. 35 



supplies than its effect in retarding the run-off even though 

 its effect in preventing evaporation is very important. 



FOREST INFLUENCES ON WIND AND HAIL STOEMS. 



We have seen that the wind breaking power of the forest 

 is a very important factor in retarding evaporation, and in 

 preventing the drifting of sandy soil and snow. In the forest 

 the air may be rather still while in the open a piercing gale 

 may be blowing; in consequence there are no blizzards in a 

 wooded country. Tornadoes of great force have occasionally 

 broken down w r ide areas of timber in this section but 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 1his 

 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 act also as condensers 

 and gatherers of dew. hoar-frost and ice; the latter phenome- 

 non 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 



