4 FORESTS, WOODS, AND TREES 
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. 
R. Zon, in an important article on “ Forests and Rainfall ” 
in Science, xxxvili. 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 Russia, 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 percent. 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 
Below | 330 to | 780 to | 1970 to | 2300 to | 3000 to 
300 ft. | 650 ft. | 1300ft. | 2300ft. | 2600 ft. | 3250 ft. 
Rainfall in inches of 
stations in the forest. | 25°9 | 26°2 | 29-4 | 42°9 | 55°5 | 69°9 
Rainfall in inches of 
ordinary stations . . | 25°5 | 22-9 | 27-4 | 36-0 | 38°6 | 37°9 
