rnt tiles being, like pale
bricks, quite pervious, and hard-burnt tiles being nearly or quite
impervious. The amount of pressure upon the clay in moulding also
affects the density and porosity of tiles.

Water should enter at the bottom of the tiles, and not at the top. It is
a well-known fact in draining, that the deepest drain flows first and
longest. A familiar illustration will make this point evident. If a cask
or deep box be filled with sand, with one hole near the bottom and
another half way to the top, these holes will represent the tiles in a
drain. If water be poured into the sand, it will pass downward to the
bottom of the vessel, and will not flow out of either hole till the sand
be saturated up to the lower hole, and then it will flow out there. If,
now, water be poured in faster than the lower hole can discharge it, the
vessel will be filled higher, till it will run out at both holes. It is
manifest, however, that it will first cease to flow from the upper
orifice. There is in the soil a line of water, called the "water-line,"
or "water-table;" and this, in drained land, is at about the level of
the bottom of the tiles. As the rain falls it descends, as in the
vessel; and as the water rises, it enters the tiles at the bottom, and
never at the top, unless there is more than can pass out of the soil by
the lower openings (the crevices and pores) into the tiles. It is well
always to interrupt the direct descent of water by percolation from the
surface to the top of the tiles, because, in passing so short a distance
in the soil, the water is not sufficiently filtered, especially in soil
so recently disturbed, but is likely to carry with it not only valuable
elements of fertility, but also particles of sand, which may obstruct
the drain. This is prevented by placing above the tiles (after they are
covered a few inches with gravel, sand, or other porous soil) compact
clay, if convenient. If not, a furrow each side of the drain, or a
heaping-up of the soil over the drain, when finished, will turn aside
the surface-water, and prevent such injury.

In the estimates as to the area of the openings between pipes, it should
be considered that the spaces between the pipes are not, in fact, clean
openings of one-tenth of an inch, but are partially closed by earthy
particles, and that water enters them by no means as rapidly as it would
enter the clean pipes before they are covered. Although the rain-fall in
England is m