“This reasoning applies to the most extensive agricultural operations as well as to the care of a potted plant. After water has soaked into the ground it should find some channel to carry it off; otherwise the roots will decay for want of air. That is why clayey soils, which retain water when they are once saturated, are unsuited to agriculture, while light soils, having sand mixed with the clay and thus readily allowing the water to drain off, are well adapted to it. For the same reason, again, a sandy subsoil accelerates vegetation, and a clayey subsoil retards it. A sandy subsoil offers the same advantage as a flower-pot open at the bottom, whereas a clayey subsoil is like a flower-pot closed at the bottom. In the first case the surplus of water drains off and the air has free access; in the second the superabundant moisture finds no outlet and the air cannot reach the roots. “Now let us suppose we have a marshy soil to deal with. Because of the stagnant water either on the surface or a little below it nothing can grow on this piece of ground except rushes or other hardy plants designed by nature for this kind of soil. Accordingly we proceed to dig a number of small ditches, of a depth somewhat greater than that attained by plant-roots, and we fill the bottom of these ditches with small stones, on which we finally throw back the earth we have removed. These underground ditches are suitably inclined, and all empty at the lower end into a main canal. The water saturating the soil collects in these ditches, filters through the layer of pebbles, and empties into the main canal, which carries it off to some river or other stream. Our marshy soil is now like the potful of earth with a hole at the bottom, the bit of broken tile, and layer of little pebbles: the air has free access and brings fertility with it. This operation of ours is called drainage, a word formed from ‘drain,’ which is both a verb and a noun. In the latter sense we apply it to the narrow ditch dug for carrying off superfluous water. “A drainage system like that just described is the simplest possible, but there is one serious objection to it: the layer of small stones soon becomes clogged with soil washed down by the water, and the latter can no longer run off. Hence it is customary to use fagots instead of stones, since they offer less obstruction. But still better results are obtained with earthenware conduits laid in the ditches. Sometimes these conduits take the form of drain-tiles such as are used on roofs, and they rest on sills or ground-pieces of the same material; or, again, they may be tubular in form, the successive sections loosely fitted together so that the water to be carried off may enter where the sections join.