whether natural or artificial, for
the use of a town or city, a survey is first taken of the district of
territory which naturally is drained into the reservoir, and thus the
number of square miles of surface is ascertained. Then the rain-tables
are consulted, and the fall of rain upon the surveyed district is
computed. The ascertained proportion of rain-fall, which usually goes
off by evaporation, is then deducted, which leaves with sufficient
accuracy, the amount of water which flows both upon the surface, and
through the soil, to the reservoir. With proper deductions for waste by
freshets, when the water will overflow the reservoir, and for other
known losses, a reliable estimate is readily made, in advance, of the
quantity of water supplied to the reservoir.

Now, these reservoirs Nature has placed in all our valleys, in the form
of lakes and ponds, and the drainage into them is by natural springs and
streams; and the annual amount of the water thus naturally flowing into
them may be readily computed, if the area within their head-waters be
known. If the earth's surface were, like iron, impervious to water, the
rain-water would come in torrents down the hill-sides, and along the
gentle declivities, into the streams, creating freshets and inundations
in a few hours. But instead of that, the soft showers fall, often on the
open, thirsty soil, and so are gradually absorbed. A part of the
rain-water is there held, until it returns by evaporation, to the
clouds, while a part slowly percolates downward, finding its way into
swamps and springy plains, and finally, after days or weeks of
wandering, slowly, but surely, finds its outlet in the stream or pond.

If now, this surplus of water, this part which cannot be evaporated, and
must therefore, sooner or later, enter the stream or pond, be, by
artificial channels, carried directly to its destination, without the
delay of filtration through swamps and clay-banks; the effect of rain to
raise the streams and ponds, must be more sudden and immediate.
Agricultural drains furnish those artificial channels. The flat and
mossy swamp, which before retained the water until the Midsummer
drought, and then slowly parted with it, by evaporation or gradual
filtration, now, by thorough-drainage, in two or three days at most,
sends all its surplus water onward to the natural stream. The stagnant
clay-beds, which formerly, by slow degrees, allowed the water to filter
through them to the