26 PKINCIPLES OP AMERICAN FORESTRY. 



ket, yet the protection they afford is different in degree 

 and may become practically none. It will also be con- 

 ceded that it makes a great difference whether the cover be 

 placed before or behind the wind. Just so with the influ- 

 ence 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, espe- 

 cially 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 

 precipitation or distribution of atmospheric water; influ- 

 ence upon conservation of available water supplies; influ- 

 ence upon the distribution or 'run-off' of these supplies. 



INFLUENCE UPON PRECIPITATION. 



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

 ciably 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 dis- 

 cussion has, so far, baffled solution based upon definite and 

 strictly scientific observation. Yet new evidence is accumu- 

 lating all the time which apparently shows that under cer- 

 tain 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 disap- 

 pearance of our forests, will come a much more rigorous 

 climate and a decrease in rainfall. But the records of the 



