32 ELEMENTARY FORESTRY. 



and this factor, being under the control of human agency more 

 than any other possible modifier of climate, must therefore be 

 of greatest interest to us. It is clear, from what has been stated 

 so far, that the influences of the forest, if any, will be due mainly 

 to its action as a cover protecting the soil and air against insola- 

 tion and against winds. That the nature of a cover, its density, 

 thickness, and its proper position has everything to do with the 

 amount of protection it affords, everybody will admit. A mos- 

 quito net is a cover, so is a linen sheet or a woolen blanket, yet 

 the protection they afford is different in degree and may become 

 practically none. It will also be conceded that it makes a great 

 difference whether the cover be placed before or behind the 

 wind. Just so with the influence of the forest; it makes all the 

 difference whether we have to do with a deciduous or conifer- 

 ous, a dense or an open, a young low or an old high growth, 

 and what position it occupies with reference to other climatic 

 elements, especially to prevailing winds and water surfaces. In 

 the following discussions, when the word forest is used, unless 

 differently stated, a dense growth of timber is meant. 



"The question of forest influences on water supplies can be 

 considered under three heads, namely influence upon precipi- 

 tation or distribution of atmospheric water; influences upon 

 conservation of available water supplies; influence upon the dis- 

 tribution or 'run-off' of these supplies. 



INFLUENCE UPON PRECIPITATION. 



"Whether forest areas are, or are not, capable of appreciably 

 increasing precipitation within their limits or on neighboring 

 ground is still a matter of dispute, and the complexity of the 

 elements which must enter into the discussion has so far baf- 

 fled solution based upon definite and strictly scientific observa- 

 tion. Yet new evidence is accumulating all the time which 

 apparently shows that under certain conditions forest areas 

 obtain larger precipitations than open grounds, that is, they 

 may increase at least the amount of precipitation over their own 

 immediate and near lying areas. 



[In Minnesota popular opinion inclines to the belief that 

 there is a close connection between the existence of forests and 

 the rainfall of this section, and that with the disappearance of 



