108 
the possibilities of evaporation within Signal Service shelters over 
the whole country for an average wind velocity. 
Daily evaporation= an 1.96 Pw+43.9 ( pupa) | 
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 90 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 Washington 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-SOIL 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 impunity 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.) 
