Dec. 4,1916 Evaporation of Moisture from the Soil 441 
data on evaporation for an hour from a wet soil with a surface of 27 
square inches: 
20 feet from hedgerow the evaporation was 10.3 c. c. 
150 feet from hedgerow the evaporation was 12.5 c. c. 
300 feet from hedgerow the evaporation was 13.4 c. c. 
At 300 feet the evaporation was 30 per cent greater than at 20 feet and 
6.7 per cent greater than at 150 feet, due largely to the vessels closer to 
the hedge having protection from the wind. 
McDonald (12) gives this terse summary: 
Evaporation depends upon the temperature of the evaporating surface, the dryness 
of the air, and the velocity of the wind. The hotter the day, the greater the evapora¬ 
tion; the drier, the greater the evaporation—the ceaseless sucking up of moisture. 
Bowie (1) claims that loss due to wind is caused by the more intimate 
contact of the air with the moist soil surface. He says: 
With average wind velocities of from 2.4 to 4.0 miles an hour, and with an aver¬ 
age water temperature of 70 F.,the increased evaporation rate due to wind was 
about 0.5 per cent a day for each mile of wind. 
Payne (13), in giving the advantage of windbreaks in retarding 
evaporation, shows that a sod wall 4 feet high and 20 feet long, running 
east and west, reduces the loss from buckets placed in the ground level 
with the field. On the north side, buckets 1, 3, 5, 7, and 10 rods dis¬ 
tant lost in 62 days moisture the equivalent of 677, 633, 700, 703, and 
712 tons to the acre, respectively; on the south side, buckets 1, 3, 5, 7, 
and 8 rods distant gave losses of 647, 686, 738, 764, and 761 tons to the 
acre, respectively. 
In work done by Principi (14) the conclusions reached were— 
That evaporation is most rapid from the materials which have the largest pore 
Spaces, and that it remains almost the same whether it arises from a free water sur¬ 
face or from thin films covering the particles of wet material. 
Woolny (21) claims that capillarity ceases when the diameter of the 
particles is more than 2 mm. and that it varies with smaller particles 
in proportion to their fineness—the finer the particles, the greater the 
lifting power, but the slower the movement. 
In regard to moisture movement through a column of dry soil not in 
direct contact with the moisture but with a saturated atmosphere be¬ 
tween, Buckingham (2, p. 9-18) says that moisture escapes probably by 
pure diffusion and that the loss in this way is proportional to the square 
of the porosity, following the same law as the diffusion of air and carbon- 
dioxid gas through soils. 
Whitney and Cameron (18) showed that loams over a saturated 
atmosphere gave a greater diffusion of moisture through the pore spaces 
than through those of clay. 
