18 BULLETIN 502, U. S. DEPARTMENT OF AGRICULTURE. 
the crevices. Often where the overlying soil is thick, dense, and 
compact, the resistance offered overcomes the low pressure, and the 
water penetrates up into it but a little distance; but where the depth 
of soil is not great the water penetrates up through it and runs off 
the surface. More frequently, however, the friction of the soil over- 
comes the low pressure of the water before it quite reaches the sur- 
face, and a stationary condition would result but for a slight lateral 
movement of the water in the soil which causes it to spread over a 
more or less circular area larger than the outlet of the contributing 
feature in the shale. 
The condition just described is relieved by wells reaching into and 
tapping the water-bearing zone; for the water, seeking the path of 
least resistance, enters the larger openings formed by the wells and 
passes freely upward and outward. One well, or a number of wells, 
will not remove all of the water from the water-bearing stratum or 
area, but they tend to relieve the pressure and thus prevent the fur- 
ther rise of water to the small area of soil above. The relief of — 
pressure by this method is based on a well-known principle of hy- 
draulics, viz, that the pressure of flowing water in a confined medium 
decreases with the increase of velocity. 
It is well known that in any artesian area wells too closely spaced 
will interfere; that is, the discharge from each well will be reduced 
as the number of wells in the area is increased. In the drainage of 
shale lands the relief wells should be spaced so closely that this inter- 
ference practically overcomes the pressure between the confining 
strata. Otherwise, since these confining strata always are more or 
less imperfect, water will continue to rise into the subsoil and the 
land will remain water-logged, even where immediately adjacent to 
drains, whether open or covered. 
The relation between the tile drains and the relief wells in a 
drainage system can be summed up in the statement that the relief 
wells provide the desired drainage, while the tile drains merely 
provide outlets for the water developed by the wells. 
AREA OF INFLUENCE OF RELIEF WELLS. 
The area of influence of an ordinary relief well in shale is not large. 
A well may strike but one small crevice or water-carrying zone which 
is responsible for only a. very small spot in the seepage area and 
which is only one among many that are contributing water. While 
no rigid rule can be given for the spacing and location of wells, ex- : 
perience has shown that from two to six for each 100-foot length of 
trench will be necessary ordinarily. This does not mean, however, 
that a certain number of wells should be decided upon for a unit 
length of trench and then spaced evenly throughout that length. 
While the location of a well that will develop the maximum flow, or 
