INFLUENCE UPON PRECIPITATION. 25 



areas is claimed as one that, under certain conditions, is 

 potent; 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 insolation 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 mosquito 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 coniferous, 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 dis- 

 cussions, 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 pre- 

 cipitation or distribution of atmospheric water; influences up- 

 on conservation of available water supplies; influence upon 

 the distribution or "run off" of these supplies. 



INFLUENCE UPON PRECIPITATION. 



"Whether forest areas are, or are not, capable of appre- 

 ciably increasing precipitatation within their limits or on 

 neighboring ground is still a matter of dispute, and the com- 

 plexity of the elements which must enter into the discussion 

 has so far baffled solution based upon definite and strictly 

 scientific observation. 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 



