of heavy rainfall water may back up
into the main with no injury other than temporary failure to perform
its work, but continuous submersion will lead to deposits of silt that
may close the tile.

Locating Main and Branches.--There are various systems of drainage.
Wherever a branch or lateral joins the main, the means of drainage is
duplicated within the area that the main can drain, and the system
should call for the least possible waste of this sort. It usually is
best that the main take the center line of the low land, laterals being
used to bring the water to the main from both sides, but there is less
duplication of work when the main can be at one side of the wet land.
Branches of the main may be needed to reach remote parts of the area.

The Laterals.--Small lines of tile are used to bring the water to the
main when the wet land extends beyond the influence of the main. The
distance between these laterals depends upon their depth and the nature
of the land. A tight clay soil will not let water pass laterally more
than a rod or 20 feet, compelling the placing of the drains not over 40
feet apart, while an open soil may permit a distance of 60 or more feet
between laterals.

Size of Tile.--The size of the main depends upon the area that
eventually may be drained, the amount of overflow from higher land, the
nature of the soil, and the grade of the drain. It is a common mistake
to make the main too small because the drainage immediately
contemplated is less than that which will be desired when its value is
known. In the determination of the size the judgment of an expert is
needed, and if this cannot be had, the error should be on the side of
safety. If the main will not be required to carry overflow from other
land, and has a fall of 3 inches to 100 feet, one may assume that a
6-inch main will carry the surplus water from 12 to 20 acres of land,
and an 8-inch main will carry the water of twice that area. Some
drainage experts figure larger areas for such mains, but there is
danger of loss of crop when the rainfall is very heavy.

The laterals need not be larger than 3 inches in diameter when laid on
a good bottom.

Kind of Tile.--When clay tile is used, it should be well burned. Some
manufacturers offer soft tile for sale, as the loss from warping and
cracking is less in case of insufficient burning. The claim may be made
that the efficiency of soft tile is greater than that of the hard tile
whose porosity h