TILLAGE AND EVAPORATION 153 
if two are opened, nearly twice as many may be ad- 
mitted in the same time; if more gates are opened, 
the passengers will be able to enter the train more 
rapidly. The water in the lower layers of the soil is 
ready to move upward whenever a call is made upon 
it. To reach the surface it must pass from soil grain 
to soil grain, and the larger the number of grains that 
touch, the more quickly and easily will the water 
reach the surface, for the points of contact of the soil 
particles may be likened to the gates of the railway 
station. Now if, by a thorough stirring and loosen- 
ing of the topsoil, the number of points of contact 
between the top and subsoil is greatly reduced, the 
upward flow of water is thereby largely checked. 
Such a loosening of the topsoil for the purpose of 
reducing evaporation from the topsoii has come to 
be called cultivation, and includes plowing, harrow- 
ing, disking, hoeing, and other cultural operations by 
which the topsoil is stirred. The breaking of the 
points of contact between the top and subsoil is un- 
doubtedly the main reason for the efficiency of cul- 
tivation, but it is also to be remembered that such 
stirring helps to dry the top soil very thoroughly, 
and as has been explained a layer of dry soil of itself 
is a very effective check upon surface evaporation. 
(See Fig. 35.) 
That the stirring or cultivation of the topsoil 
really does diminish evaporation of water from the 
soil has been shown by numerous investigations. 
