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SCIENCE 



[N. S. Vol. XXXVIII. No. 968 



being equal, than a similar area bare of 

 vegetation or covered with some herbaceous 

 vegetation. The amount of- water con- 

 sumed by forests is nearly equal to the 

 total annual precipitation ; in cold and 

 humid regions it is somewhat below this 

 amount and in warmer and dry regions it 

 is above it. 



This enormous amount of moisture given 

 off into the air by the forest, which may be 

 compared to clouds of exhaust steam 

 thrown into the atmosphere, must play an 

 important part in the economy of nature. 



If the present area occupied by forests in 

 the Atlantic plain and the Appalachian 

 region were instead occupied by a large 

 body of water, no meteorologist would hesi- 

 tate for a moment to admit that the water 

 surface has a perceptible influence upon 

 the humidity of the central states and 

 prairie region. Should not, therefore, for- 

 ests which give off into the atmosphere 

 much larger quantities of moisture than 

 free water surface, have at least a similar 

 influence upon the regions into which the 

 prevailing air currents flow. 



If the southern and southeastern winds, 

 in their passage toward the north, north- 

 west and northeast, in the spring and 

 summer, did not encounter the vast forest 

 areas bordering the shores of the Gulf of 

 Mexico and the Atlantic coast and those of 

 the southern Appalachian, and, therefore, 

 were not enriched with enormous quantities 

 of moisture given off by them, the precipi- 

 tation, in the central states and the prairie 

 region would undoubtedly be much smaller 

 than it is now. 



What would be the effect of complete or 

 even partial destruction of forests in the 

 Atlantic plain and in the southern Appa- 

 lachian Mountains upon the humidity of 

 the continental portion of the United 

 States? As the mean temperature in the 

 eastern part of the United States drops 



rapidly from south to north, the moisture- 

 laden air currents upon entering land 

 would be cooled off and rapidly drained of 

 their moisture within a comparatively 

 short distance from the ocean. The sandy 

 soil which is so characteristic of the south- 

 ern pine belt of the gulf and south Atlantic 

 States would rapidly absorb the rain which 

 would percolate into the ground, without 

 returning much of it into the atmosphere. 

 The rain falling upon the slopes of the 

 mountains would rapidly run off into 

 streams. While direct evaporation from 

 the ground not sheltered by forest cover 

 may become greater, yet the more rapid 

 run-off and the absence of transpiration by 

 trees would necessarily reduce the total 

 amount of water evaporated into the at- 

 mosphere. The land, were it even taken 

 up for agriculture, would not return such 

 large quantities of rain into the atmosphere 

 as the forests did. The inevitable result 

 would be that less moisture would be car- 

 ried by the prevailing winds into the in- 

 terior of the country, and therefore less 

 precipitation would occur there. Such is 

 the influence of forests in a level or a hilly 

 country. 



Whether forests in the mountains have 

 the same effect as forests in level countries 

 upon the precipitation of the regions into 

 which the prevailing winds that pass over 

 them blow, is difficult to determine. The 

 problem is more complicated for the rea- 

 son that high mountain chains exert an 

 influence upon the direction of the winds, 

 not only by presenting a mechanical ob- 

 struction to the free passage of the air, 

 but also on account of the difference in the 

 heating of the different slopes. A moist 

 current of air in passing over a mountain 

 chain undergoes several changes. It is 

 known that the air in ascending becomes 

 cooler. The temperature of not fully sat- 

 urated air decreases 1° F. for every 182 



