42 PRINCIPLES OF AMERICAN FORESTRY. 



further, the straightening and cleaning out of watercourses, 

 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 w T hich so often cause serious 

 injury to farm crops in Kansas Nebraska, and the Dakotas 

 have been ascribed to the arid "staked" plains, whence, 

 taking a northeasterly direction, they draw r 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 Coast, ascend the Rocky Mountains, lose their 

 moisture, and descend on the eastern slopes. But all the- 

 ories that ascribe their origin to a distant source are inade- 

 quate 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 daytime, commencing 

 about 9 A.M. and continuing until sundown. This daily 

 movement is often constant for several weeks, showing 

 that there is evidently some connection between them and 

 the course of the sun. For these reasons and others which 

 would require too much space to give here, the best authori- 

 ties unite in attributing them to local origin. 



Mr. George C. Curtiss describes the process of the produc- 

 tion of a typical hot wind as follows: "The necessary con- 

 ditions are those of the 'warm wave/ namely, a diminishing 

 pressure to the northward, producing southerly winds 

 which initially elevate the temperature above the normal. 

 A cloudless sky favors an intense insolation, as a result of 

 which the dry ground is soon raised to an extreme tempera- 

 ture, and the air is heated from it by radiation, reflection, 

 and conduction. The resulting diminution of density due 



