These facts and principles are worthy of
examination, and they are set forth below, somewhat at length, especially
with reference to _Absorption_ and _Filtration_; _Evaporation_;
_Temperature_; _Drought_; _Porosity_ or _Mellowness;_ and _Chemical
Action_.
ABSORPTION AND FILTRATION.--The process of under-draining is a process of
absorption and filtration, as distinguished from surface-flow and
evaporation. The completeness with which the latter are prevented, and the
former promoted, is the measure of the completeness of the improvement. If
water lie on the surface of the ground until evaporated, or if it flow off
over the surface, it will do harm; if it soak away through the soil, it
will do good. The rapidity and ease with which it is absorbed, and,
therefore, the extent to which under-draining is successful, depend on the
physical condition of the soil, and on the manner in which its texture is
affected by the drying action of sun and wind, and by the downward passage
of water through it.
In drying, all soils, except pure sands, shrink, and occupy less space
than when they are saturated with water. They shrink more or less,
according to their composition, as will be seen by the following table of
results obtained in the experiments of Schuebler:
1,000 Parts of Will Contract 1,000 Parts of Will Contract
Parts. Parts.
Strong Limey 50. Pure Clay 183.
Soil
Heavy Loam 60.
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