nature of the
soil; but in ordinary circumstances, it would be, perhaps, about
forty-eight hours. Drains usually run much longer than this after a
heavy rain, and, in fact, many run constantly through the year, but they
are supplied from lands at a higher level, either near by or at a
distance.

If, on the other hand, the soil were perfectly dry, holding no water in
suspension, then there would be no water of drainage until the soil had
become saturated.

Evaporation is constantly carrying off great quantities of water during
the warm months, so that under-drained soil is seldom in the condition
of saturation, and, on account of the supply by capillary attraction and
by dew, is never thoroughly dry; but the same soil will, at different
times, be at various points between saturation and dryness, and the
water of drainage will be consequently a greater or less per centage of
the rain-fall.

An experiment made by the writer, to ascertain what quantity of water a
dry soil would hold in suspension, resulted as follows: A soil was
selected of about average porosity, so that the result might be, as
nearly as possible, a mean for the various kinds of soil, and dried by
several days' baking. The quantity of soil then being carefully
measured, a measured quantity of water was supplied slowly, until it
began to escape at the bottom. The quantity draining away was measured
and deducted from the total quantity supplied. It was thus ascertained
that one cubic foot of earth held 0.4826+ cubic feet of water, which is
a little more than three and one-half gallons. A dry soil, four feet
deep, would hold a body of water equal to a rain-fall of 23.17 inches,
vertical depth, which is more than would fall in six months.

The quantity which is not drained away is used for vegetation or
evaporated; and the fact, that the water of drainage is so much greater
in proportion to the rain-fall in England than in this country, is owing
to the humidity of that climate, in which the evaporation is only about
half what it is in this country.

The evaporation from a reservoir surface at Baltimore, during the Summer
months, was assumed by Colonel Abert to be to the quantity of rain as
two to one.

Dr. Holyoke assigns the annual quantity evaporated at Salem, Mass., at
fifty-six inches; and Colonel Abert quotes several authorities at
Cambridge, Mass., stating the quantity at fifty-six inches. These facts
are given by Mr. Blodget, and also the table