The reasons for this action of draining become obvious, when its effects on the character of the soil are examined. There is always the same amount of water in, and about, the surface of the earth. In winter there is more in the soil than in summer, while in summer, that which has been dried out of the soil exists in the atmosphere in the form of a vapor. It is held in the vapory form by heat, which may be regarded as braces to keep it distended. When vapor comes in contact with substances sufficiently colder than itself, it gives up its heat,—thus losing its braces,—contracts, becomes liquid water, and is deposited as dew.
Many instances of this operation are familiar to all.
For instance, a cold pitcher in the summer robs the vapor in the air of its heat, and causes it to be deposited on its own surface,—of course the water comes from the atmosphere, not through the wall of the pitcher; if we breathe on a knife blade, it condenses, in the same manner, the moisture of the breath, and becomes covered with a film of-water; stone-houses are damp in summer, because the inner surface of their walls, being cooler than the atmosphere, causes its moisture to be deposited in the manner described;[2] nearly every night, in summer, the cold earth receives moisture from the atmosphere in the form of dew; a single large head of cabbage, which at night is very cold, often condenses water to the amount of a gill or more.
The same operation takes place in the soil. When the air is allowed to circulate among its lower and cooler, (because more shaded,) particles, they receive moisture by the same process of condensation. Therefore, when, by the aid of under-drains, the lower soil becomes sufficiently loose and open, to allow a circulation of air, the deposit of atmospheric moisture will keep it supplied with water, at a point easily accessible to the roots of plants.
If we wish to satisfy ourselves that this is practically correct, we have only to prepare two boxes of finely pulverized soil,—one three or four inches deep,—and the other fifteen or twenty inches deep, and place them in the sun, at midday, in summer. The thinner soil will soon be completely dried, while the deeper one, though it may have been previously dried in an oven, will soon accumulate a[pg 039] large amount of water on those particles which, being lower and better sheltered from the sun's heat than the particles of the thin soil, are made cooler.
We have seen that even the most retentive soil,—the stiffest clay,—is made porous by the repeated passage of water from the surface to the level of the drains, and that the ability to admit air, which plowing gives it, is maintained for a much longer time than if it were usually saturated with water which has no other means of escape than by evaporation at the surface. The power of dry soils to absorb moisture from the air may be seen by an examination of the following table of results obtained by Schuebler, who exposed 1,000 grains of dried soil of the various kinds named to the action of the air:
| Kind of Soil. | Amount of Water Absorbed in 24 Hours. |
| Common Soil | 22 grains. |
| Loamy Clay | 26 grains. |
| Garden Soil | 45 grains. |
| Brickmakers' Clay | 30 grains. |
The effect of draining in overcoming drought, by admitting atmospheric vapor will, of course, be very much increased if the land be thoroughly loosened by cultivation, and especially if the surface be kept in an open and mellow condition.
In addition to the moisture received from the air, as above described, water is, in a porous soil, drawn up from the wetter subsoil below, by the same attractive force which acts to wet the whole of a sponge of which only the lower part touches the water;—as a hard, dry, compact sponge will absorb water much less readily than one which is loose and open, so the hard clods, into which undrained clay is dried, drink up water much less freely than they will do after draining shall have made them more friable.
The source of this underground moisture is the "water table,"—the level of the soil below the influence of the[pg 040] drains,—and this should be so placed that, while its water will easily rise to a point occupied by the feeding roots of the crop, it should yield as little as possible for evaporation at the surface.