Sometimes this is done by digging open or closing
ditches intended to draw away large bodies of water. At other limes a
system of drainage is established, by means of which the water is drawn
out of the earth and its level is depressed, so that the upper malarious
strata, exposed to the direct action of the air, are deprived of moisture
during the hot season. This system of drainage is not a modern invention;
the Italian monks understood it as well as, and even better than, we do.
In deep and loose soils they used sometimes, just as we do now, porous
clay pipes; but when the subsoil was formed of compact and nearly
impermeable matters, they employed a system of drainage, the extent and
grandeur of which astonishes us. It is that of drainage by cavities,
applied by the Etruscans, Latins, and Volsci to all the Roman hills formed
of volcanic tufa, the tradition of which I have found still preserved in
some countries of the Abruzzi.
We may sometimes establish a double drainage, from below and from above;
that is to say, to drain the subsoil, and at the same time increase the
evaporation of water from the surface of the ground. It is well known that
clearing off the forests of malarious countries has often proved an
excellent means of making lands salubrious which were before too damp;
for, by removing every obstacle to the direct action of the sun's rays
upon the ground, we cause an increase of evaporation from its surface, and
may thus be enabled to exhaust the superficial strata completely of their
water during the hot season.
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