patterns, but the most common in use to-day is one
which is octagonal outside and circular inside. They are about one foot
in length and may be had from two to six inches inside diameter. The
ordinary size for laterals is four-inch diameter, while the mains into
which these laterals discharge are generally of six-inch diameter. These
tiles are laid in trenches about fifteen feet apart, although in porous
soil, such as coarse sand or gravel, this distance may be increased to
twenty feet. If the tiles are laid more than four feet below the
surface, this distance may be increased, and if the tiles are five feet
deep, the distance apart of the several lines may be fifty feet.

The grade of the line must be carefully taken care of, and while it is
possible to lay a line of tile with a carpenter's level and a
sixteen-foot straightedge, it is much safer to have an engineer's or
architect's level and set grade stakes, as in regular sewer work. A fall
of one fourth of an inch to the foot is a proper grade, although a
greater slope is not objectionable. It is sometimes desirable in soft
ground to lay down a board six inches wide in the bottom of a trench on
which to rest the tile, but, unless the ground is very soft, this is not
necessary. Care must be taken, however, if the board is not used, to
have the bottom of the trench very carefully smoothed so that a
perfectly even grade in the tile is maintained. There are three ways of
laying out a line of trench as shown in the following sketches (Fig. 4).
It is usually sufficient to run parallel lines of tile from fifteen to
fifty feet apart over the area which it is desired to drain, and let the
ends of these lines enter a cross line which shall carry off the water
led into it. This cross line should be six inches in diameter as a
general rule, unless there is more than a mile of small drains, in which
case the size of the cross pipe ought to be increased to eight inches.
This cross line then becomes the main outlet, and great care must be
taken to see that it has a perfectly free delivery at all times of the
year. In cities and sometimes in small villages it is possible to
discharge this outlet pipe into a regular public sewer, provided the
sewer is deep enough, and provided the municipal ordinances allow such a
connection. Otherwise, the outfall must be carried to a natural
depression.

[Illustration: FIG. 4.--Modes of laying out drains.]

In level ground, the problem of findin