582. THE PHILOSOPHY OF FARMING. 
stratum be not thoroughly impermeable, depth of 
drainage is most important ; because the drainage 
then not only extends farther laterally, but acts 
more effectually. Drains carried along the sides 
of slopes or inclining grounds nearly on a complete 
level, frustrate to a great extent the object for 
which they are intended; since, instead of con- 
veying away the surface water as soon as possible, 
they are calculated to detain it long as possible. 
Besides, through want of a current in the flow of 
the water, mud soon accumulates at the bottom of 
such drains from depositions therein from the 
stagnant water, so as ultimately to fill them up 
and choke all passage. And even to drain flat 
surfaces—levels where practicable should be so 
brought up to them, as to allow the drains being 
laid deep and a current however slight within 
allowed. It is essential, therefore, both to be 
understood and remembered, that those drains 
which by their depth extend their influence 
widest, and allow the quickest percolation and 
the readiest discharge of water from the surface 
are ever most efficient and desirable. 
In some districts, the clay of the subsoil is as 
impenetrable as the most continuous and unstra- 
tified rock. No water can be found within it, 
