194 REPORT oe THE FORESTRY COMMITTEE 



the air by the forest, may be compared to clouds of exhaust steam thrown into 

 the atmosphere and must necessarily play an important part in the economy of 

 nature. If the southern and southeastern winds, in their passage toward the 

 north, northwest, 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 the 

 enormous quantities of moisture given off by them, the precipitation in the Cen- 

 tral States and the prairie region would probably be much smaller than it is now. 

 For the central interior region of the United States is the battle ground of two 

 titanic forces — one harmful, the other beneficial. The beneficial one takes the 

 form of the mild and humid summer winds from the Gulf of Mexico and the At- 

 lantic Ocean, which at their height extend into the continent as far north as North 

 Dakota, as far west as the foothills of the Rocky Mountains, and as far east as 

 New England, and during the prevalence of which the rainfall in the eastern 

 United States is heaviest. The other and harmful force is made up of the warm 

 Chinook winds which blow out of the northern Rocky Mountains and the dry 

 westerly winds of the upper Mississippi and the western lake region, both of 

 which carry in their wake serious injury to orchards and fields. The Central 

 States and the prairie region are geographically at the point where the battle be- 

 tween the two forces is fiercest, and the victory is now on one side, now on the 

 other. When the humid southerly winds extend their influence far into the inte- 

 rior of the continent and overpower the dry continental winds,-the Central States 

 and prairie region, the granary of the United States, produce large crops. When 

 the dry winds overpower the humid southerly winds, there are droughts and crop 

 failures. 



As soon as the moisture-laden winds from the Gulf reach the land and en- 

 counter irregularities they are cooled and begin to lose part of their moisture in 

 the form of precipitation. As long as the air currents remain saturated with 

 moisture the slightest cooling or iregularity of the land that causes them to rise 

 will result in precipitation. But as they move inland and become drier the re- 

 maining moisture is given off with difficulty, and precipitation decreases. The 

 sooner the humid air currents over land are drained of their moisture the shorter, 

 of course, is the distance from the ocean over which abundant precipitation falls. 

 If precipitation over land depended solely on the amount of water brought by the 

 prevailing winds directly from the ocean, rainfall would, of course, be confined 

 only to a narrow belt close to the sea. Not all the water that is precipitated, how- 

 ever, is lost from the air current. A large part of it is again evaporated from the 

 land into the atmosphere. The moisture-laden air currents, therefore, soon lose 

 the moisure which they obtain directly from the ocean, but in moving further into 

 the interior absorb the evaporation from the land. Hence the further from the 

 ocean the greater is the proportion which evaporation from the land forms of the 

 air moisture. In fact, at certain distances inland practically all the moisture of the 

 air, or at least as great a part as that formed originally by the water evaporated 

 direct from the ocean, must consist of that obtained by evaporation from the land. 



In the case of the central and plain States, then, what would be the eflfect 



