ration alone, would
render the whole country barren; but Nature herself has kindly done the
work of draining upon a large proportion of our land, so that only a
healthful proportion of the water which falls on the earth, passes off
at the surface by the influence of the sun.

If the subsoil is of sand or gravel, or of other porous earth, that
portion of the water not evaporated, passes off below by natural
drainage. If the subsoil be of clay, rock, or other impervious
substances, the downward course of the water is checked, and it remains
stagnant, or bursts out upon the surface in the form of springs.

As the primary object of drainage is to remove surplus water, it may be
well to consider with some care

THE SOURCES OF MOISTURE.

_Springs._--These are, as has been suggested, merely the water of rain
and snow, impeded in its downward percolation, and collected and poured
forth in a perennial flow at a lower level.

The water which falls in the form of rain and snow upon the soil of the
whole territory of the United States, east of the Rocky Mountains, each
year, is sufficient to cover it to the depth of more than 3 feet. It
comes upon the earth, not daily in gentle dews to water the plants, but
at long, unequal intervals, often in storms, tempests, and showers,
pouring out, sometimes, in a single day, more than usually falls in a
whole month.

What becomes of all this moisture, is an inquiry especially interesting
to the agriculturist, upon whose fruitful fields this flood of water
annually descends, and whose labor in seed-time would be destroyed by a
single Summer shower, were not Nature more thoughtful than he, of his
welfare. Of the water which thus falls upon cultivated fields, a part
runs away into the streams, either upon the surface, or by percolation
through the soil; a part is taken up into the air by evaporation, while
a very small proportion enters into the constitution of vegetation. The
proportion which passes off by percolation varies according to the
nature of the soil in the locality where it falls.

Usually, we find the crust of the earth in our cultivated fields, in
strata, or layers: first, a surface-soil of a few inches of a loamy
nature, in which clay or sand predominates; and then, it may be, a layer
of sand or gravel, freely admitting the passage of water; and, perhaps,
next, and within two or three feet of the surface, a stratum of clay, or
of sand or gravel cemented with some oxyd of