16 BULLETIN 502, U. S. DEPARTMENT OF AGRICULTURE. 
ward through the soil by capillary action carries salts in solution and 
by evaporation deposits them on the surface of the ground. ° The 
height to which capillary water will rise depends on the type of soil, 
its wetness and temperature, and, to some extent, on the kind of salts. 
High temperatures and certain salts increase the range of capillarity. 
In loose, sandy soils the rise is not great; in average soils of the arid 
West it probably ranges from 2 to 4 feet, and in clayey soils more 
than this. Of all the types of soils, the adobes encountered in the 
drainage of shale lands are perhaps most conducive to a high range 
of capillarity ; thus the necessity for deep drainage. 
One of the greatest advantages of deep drainage in this type of 
land is the increase of flow of the relief wells thus obtained: How- 
ever, there is a practical limit to the maximum depth, and it is 
believed that in the design of a drainage system the depth should be 
fixed with reference to that limit rather than by setting a minimum 
limit determined by capillarity, depth of root zone, etc. The prac- 
tical limit of depth, of course, would vary somewhat as between 
hand work and machine work. It would depend also upon the 
nature of the ground encountered. In the presence of these variable | 
factors it becomes impossible to fix a maximum depth, but the mini- 
mum should be not less than 6 feet and in many cases should be 7 or 
8 feet. 
PURPOSE OF RELIEF WELLS. 
In the drainage of many types of shale land the installation of 
relief wells is absolutely necessary for the success of the drainage 
system. A relief well is nothing more nor less than an artesian 
well. This does not mean necessarily that the water rises to the 
surface and overflows, since any well may be considered as artesian 
where the water rises to some extent after having been drilled 
through a comparatively impervious stratum into one carrying 
water; in other words, where the water enters the well under 
pressure. 
As mentioned before, the seepage areas in shale lands occur almost 
invariably where pressure conditions exist and the movement of 
the water is upward. In cases of extreme pressure this can be 
detected at the surface by the appearance of drops of water that 
have been forced up through the small pores of the soil. In but 
few cases, however, is it possible to place the drains deep enough 
to reach the supply of water that causes the saturation. Often the 
water-carrying zones of shale have been found at depths approxi- 
mating 30 feet. The cost of the installation of drainage lines 
becomes prohibitive long before this depth is attained, but unless 
the water-carrying medium is reached in some manner drains will 
be of little service, no matter how carefully they are located and 
constructed or how closely spaced. Cases are known where drain- 
