22 
BULLETIN 190, IT. S. DEPARTMENT OF AGRICULTURE. 
would apply to lands underlain by a sand stratum at from 4 to 6 feet 
below the surface. The 
drains should cut through 
this sand stratum, which 
itself thus becomes a great 
sheet drain. Under this 
arrangement three lines of 
tile to the 40-acre tract, 
or less than 100 feet of 
tile per acre, will drain 
fairly tough clay soil. 
APPLICATION OF THE NATURAL 
SYSTEM. 
The natural system of 
Branch// 
\V % 
Plan 
Fig. 16. — Plan and section showing method of draining where source of seepage is a buried shale knoll. 
drainage is also applicable 
to irrigated lands. The 
principle involved in the 
application of this sys- 
tem is to assist the natu- 
ral drainage and expedite 
its movement. To do this 
the drains are laid in the 
natural depressions and 
the ground water lowered 
in those locations, so that 
the movement of water 
toward the depressions is 
made more rapid. The 
system is especially ap- 
plicable to lands under- FlG 17 ._ Sketcll map showing application of a ^^ 
lain by gravel which OCCUrS system of drainage. 
in undulating strata or beds. It has been found, however, that in the 
