during very warm and dry summers, the heat extracts
all the humidity from the malarious soil, and thus procures for us a
freedom from the disease which is _purely hydraulic_. This may continue
for a long time (as happened in the Roman Campagna during the years 1881
and 1882), but may also be completely destroyed by a single shower. Nature
also sometimes renders a district healthy in a manner _purely
atmospheric_, by covering a malarious soil with earth which does not
contain the malarial ferment, or with a matting formed of earth and the
roots of grasses growing closely together in a natural meadow.

In the attempts of purification by suspending the malarial action, which
have been devised by man, the same thing has been done; that is to say, it
has been sought, to eliminate at least one of the three conditions
essential to the development of the specific ferment contained in the
infected soil. Naturally, they have not thought of bringing about a
thermic purification, such as nature produces in winter, because of the
impossibility of moderating the action of the sun; but they have tried
from all time to procure hydraulic or atmospheric purifications, and
sometimes to combine these together in a very happy way.

The hydraulic systems are very numerous, for the problem which is
presented, namely, that of depriving the ground of its humidity during the
hot season, necessitates different solutions according to the nature and
the bearing of the soil. 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, f