38 ELEMENTARY FORESTRY. 



been very dry and the great continental plain, or at least por- 

 tions of it, must have bordered pretty closely upon a desert, 

 and the ''Great American Desert" may have been a reality. 

 It would seem then that the knowledge we are gaining- of the 

 unknown past, as well as the records of more recent years, 

 point to the recurrence of great fluctuations in the annual rain- 

 fall of this section, and it seems probable that such changes 

 follow series of years and that the recedence of our lakes may 

 be followed by periods of higher water. 



But the influence of the cultivation of the soil on water 

 supplies must be taken into account in this connection, for it 

 is undoubtedly true that man has changed the conditions of 

 the soil sufficiently to greatly influence the run-off. The 

 breaking up of large areas of prairie sod, with its low rate of 

 evaporation, and the planting of such land to agricultural 

 crops with a relatively hisrh rate of evaporation, has resulted 

 in a loss of soil water. Then the cultivated soil takes upmore 

 water than the sod-bound prairie slopes, so that it does not 

 have so good an opportunity to collect in lakes and swamps 

 which often supplied the water of wells. And further, the 

 straightening and cleaning out of water courses, and the 

 draining of swamps in the effort to get arable land, has had a 

 similar effect on subsoil water supplies. 



HOT WINDS. 



The hot winds of the plains which so often cause serious 

 injury to farm crops in Kansas, Nebraska aud the Dakotas 

 have been ascribed to the arid staked plains, whence taking a 

 northeasterly direction they draw all the moisture from the 

 vegetation with which they come in contact. The view has 

 also been presented that they have their origin on the Pacific 

 ('oast, ascend the Rocky Mountains, lose their moisture and 

 descend on the eastern slopes. But all theories that ascribe 

 their origin to a distant source are inadequate to explain their 

 phenomena. For instance, all who are acquainted with these 

 winds know that they blow only during very dry weather when 

 the earth is heated very hot, that a good rain speedily brings 

 them to an end,- and that they blow only during the day time, 

 commencing about 9 A. M. and continuing until sundown. 

 This daily movement is often constant for several weeks, 



