a large
vitrified drain pipe in the place of the one shown in the cut. These
vitrified pipes may be perforated in the manner described for the common
tile.

[Illustration: Fig. 37 - TILE SILT-BASIN.]

Fig. 37 - TILE SILT-BASIN.

In laying the main line _C_, (Fig. 21,) an underground basin of brick
work, (Fig. 35,) or its equivalent, should be placed at stake 7, because
at that point the water, which has been flowing on an inclination of 1.09,
2.00 and 2.83 per 100, continues its course over the much less fall of
only 0.56 per 100.

If, among the tiles which have passed the inspection, there are some
which, from over burning, are smaller than the average, they should be
laid at the upper ends of the laterals. The cardinal rule of the tile
layer should be _never to have a single tile in the finished drain of
smaller size, of more irregular shape, or less perfectly laid, than any
tile above it_. If there is to be any difference in the quality of the
drain, at different points, let it grow better as it approaches the outlet
and has a greater length above depending upon its action.

*Covering the Tiles, and Filling-in the Ditches.*--The best material for
covering the tiles is that which will the most completely surround them,
so as to hold them in their places; will be the least likely to have
passages for the flow of _streams_ of water into the joints, and will
afford the least silt to obstruct the drain. Clay is the best of all
available materials, because it is of the most uniform character
throughout its mass, and may be most perfectly compacted around the tiles.
As has been before stated, all matters which are subject to decay are
objectionable, because they will furnish fine matters to enter the joints,
and by their decrease of bulk, may leave openings in the earth through
which streams of muddy water may find their way into the tiles. Gravel is
bad, and will remain bad until its spaces are filled with fine dirt
deposited by water, which, leaving only a part of its impurities here,
carries the rest into the drain. A gravelly loam, free from roots or other
organic matter, if it is strong enough to be worked into a ball when wet,
will answer a very good purpose.

Ordinarily, the earth which was thrown out from the bottom of the ditch,
and which now lies at the top of the dirt heap, is the best to be returned
about the tiles, being first freed from any stones it may contain whi