of pervious stone will form an effectual bar to the passage of water;
whereas, if it lay within a few feet of the surface, it would, in a season
of heat and drought become as pervious as a cullender. But when we have
got rid of the cold arising from the evaporation of free water, have given
a range of several feet to the roots of grass and cereals, and have
enabled retentive land to filter through itself all the rain which falls
upon its surface, we are not, in our present state of knowledge, aware of
any advantage which would arise from further lowering the surface of water
in agricultural land. Smith, of Deanston, first called prominent attention
to the fertilizing effects of rain filtered through land, and to evils
produced by allowing it to flow off the surface. Any one will see how much
more effectually this benefit will be attained, and this evil avoided, by
a 4-foot than a 2-foot drainage. The latter can only prepare two feet of
soil for the reception and retention of rain, which two feet, being
saturated, will reject more, and the surplus must run off the surface,
carrying whatever it can find with it. A 4-foot drainage will be
constantly tending to have four feet of soil ready for the reception of
rain, and it will take much more rain to saturate four feet than two.
Moreover, as a gimlet-hole bored four feet from the surface of a barrel
filled with water will discharge much more in a given time than a similar
hole bored at the depth of two feet, so will a 4-foot drain discharge in a
given time much more water than a drain of two feet. One is acted on by a
4-foot, and the other by a 2-foot pressure."

If any single fact connected with tile-drainage is established, beyond all
possible doubt, it is that in the stiffest clay soils ever cultivated,
drains four feet deep will act effectually; the water will find its way to
them, more and more freely and completely, as the drying of successive
years, and the penetration and decay of the roots of successive crops,
modify the character of the land, and they will eventually be practically
so porous that,--so far as the ease of drainage is concerned,--no
distinction need, in practice, be made between them and the less retentive
loams. For a few years, the line of saturation between the drains, as
shown in Fig. 11, may stand at all seasons considerably above the level of
the bottom of the tile, but it will recede year by year, until it will be
practically level, except imme