108 



the possibilities of evaporation within Signal Service shelters over 

 the whole country for an average wind velocity. 



Daily evaporations: y 1.96y>,„-|-43.9(/?w — pa) 



His results in this respect are platted on chart No. VI of the 

 Monthly Weather Review, September, 1888, and show that the total 

 annual depth of evaporation has its maximum of over 1)0 inches in 

 southern Arizona, California, and New Mexico, whence it dimin- 

 ishes to a minimum of 20 inches annually in the northwest corner of 

 the State of AVashington and thence eastward to Maine. These fig- 

 ures, like his formula, take no account of the wind, because within 

 the Signal Service shelters the wind is reduced to a velocity far less 

 than that in the open air. These figures, therefore, represent the 

 evaporation in open air only when there is no wind above some 

 small limit — say 6 miles per hour but may be adapted to strong winds 

 by the use of the figures given in the first paragraph of this section. 



CULTIVATION DIMINISHES SURFACE-SOIIi EVAPORATION. 



The general effect of cultivation is to pulverize the upper soil ; 

 this protects the capillary roots from surface exposure, it breaks up 

 the capillary outlets of the moisture in the soil, checks the natural 

 evaporation that goes on at the surface, and thus preserves the water 

 within the soil for the use of the plants. Dr. E. L. Sturtevant's 

 observations show that the extent to which the water is thus con- 

 served by cultivation during the months from May to September, 

 1885, at Geneva, N. Y., may be thus expressed: With a rainfall of 

 14.42 inches the cultivated soil evaporated 1.4 inches less than the 

 uncultivated naked soil and 2.25 inches less than the soil covered 

 with sod. In other words, the efficiency of the soil to retain useful 

 water is increased by cultivation to an extent equivalent to 10 per 

 cent of the rainfall. If the capillary connections between the soil 

 in the neighborhood of the roots and the supply of moisture lower 

 down be broken no supply of moisture can come up from below, but 

 if the soil be well rolled the compacting will aid the capillary attrac- 

 tion and the plants will secure moisture from below. Again, when 

 weeds are allowed to grow freely the injury to the crops is not due 

 to robbing the soil of nutrition nor to their shading the ground, but 

 principally to their robbing the soil of its moisture. Those who can 

 with impimity allow weeds to grow must have soils containing an 

 excessive moisture, which they thus get rid of, while those who have 

 a comparatively dry soil must destroy the weeds in order to reserve 

 moisture for the use of their crops. (Agr. Sci., Vol. I, p. 216.) 



