4 FOEESTS, WOODS, AND TEEES 



growing period of five months." This shows how much 

 moisture is passed into the air by a wooded area. Forests 

 thus enrich with moisture the winds that pass over them, 

 and contribute to the humidity of the regions into which 

 the prevailing winds pass. The usefulness of large forests 

 in the interior of great continental regions like the United 

 States, in regard to rainfall, is thoroughly explained by Dr. 

 E. Zon, in an important article on " Forests and Eainfall " 

 in Science, xxxviii. p. 63 (1913). He advocates the pro- 

 tection of such forests ; whereas swamps, which contribute 

 less to the moisture content of the air, ought to be drained. 

 When a forest is cut away, he recommends that the cleared 

 ground should be devoted to intensive cultivation, as after 

 trees, crops contribute most to the moisture of the air. 

 Vezozkii (4) holds similar views in regard to Eussia, namely, 

 that the forests ought to be preserved in the regions which 

 abound in moisture and lie in the route of the air currents 

 carrying moisture. 



The influence of forests upon local precipitation has been 

 determined by observations taken at Nancy in France over 

 33 years. These show an excess of precipitation in the 

 forested areas of 12 to 23 per cent. As Huffel says, the 

 forest always increases the frequency and abundance of 

 atmospheric precipitation. It rains more in the centre of 

 a great forest than at the margin, and more on the edge of 

 the forest than at some miles distant over agricultural land. 

 This action of the forest in increasing rainfall is much more 

 marked in the mountains and at high altitudes than in the 

 low and level plains. This is well shown by the following 

 table, taken from over 200 stations during a period of ten 

 years in Prussia : 



Altitude of Stations 



