490 
On Deep Draining. 
uhich have come under my notice may throw light on a subject of 
so much interest and imjjortance to agriculture. 
As the efficacy of draining must depend upon the condition of 
the soil in which it is executed, so, in determining the depth to 
which the drain should be cut, it will be necessary first to ascer- 
tain in what measure the soil is pervious to water. In blue clays 
this will in some measure be indicated by the change of colour 
caused by the action of the air, which, by peroxidising the natural 
protoxide of iron in the soil, changes the colour from blue to red 
or yellov/ ; and in all soils the perforation by roots, &c. renders it 
more or less pervious to water for many feet from the surface. 
In flat unbroken ground, as the soil is saturated with the rain the 
water will rise gradually on a level throughout ; should there be 
any depression in the surface it will form pools, the level of whose 
surface will coincide with the subterranean level, thus. 
B 
Surface of Soil 
is 
"5 A 
'Water Xevcl 
J>6 
If a drain be dug at A, the level will assume an inclination, the 
angle at which it declines being greater or less in proportion as the 
soil is pervious to or retentive of water; in this, as in other cases 
of subterranean wafer flowing to a vent, " the inclination which its 
surface assumes will represent the amount of friction, or resistance, 
wliich the water encounters in its passage through the soil." 
Thus in the various chalk strata, through the fissures of which 
water flows with considerable freedom, this inclination is found to 
vary from 15 to 80 feet per mile ; in other materials, such as sand 
or gravel, though loose, it is usually much greater, and in retentive 
clays will necessarily be greater still. It is with reference to the 
amount of this angle of inclination that all draining operations must 
be conducted. If, for instance, a soil is equally porous to the depdi 
of 5 feet, it is obvious that a drain to that depth would drain it 
more effectually, and to a greater distance, than one of 4. 3, or 
less than that, and that the subterranean level would settle to a 
surface declining uniformly towards the drain. But as all clay 
soils are, from the above-mentioned causes, more porous at their 
surface than below, the angle at which various drains will act, 
and the extent of soil they will drain, will vary as they sink lower 
into the soil, though the deepest drain will invariahlg command- 
the greatest amount of soil. Thus, if a soil become less porous in 
proportion to its depth (5 feet), as indicated in the diagram below 
