LAND DRAINAGE PRACTICE. 37 



age. It may be done by the means which Nature has 

 provided, as is the case where the subsoil contains a 

 stratum of sand, gravel, or other permeable material, 

 into which water from above finds its way by gravity, 

 and thence passes to some water-course with which the 

 drainage stratum has free communication. This gives 

 the most complete drainage possible. When natural 

 underdrainage is wanting, or is defective, then artificial 

 drainage should be resorted to. Round drain-tiles are 

 the most suitable for this purpose, and are universally 

 acknowledged to furnish the most serviceable and com- 

 plete artificial means of draining the soil now known. 



Water enters the tile drain through the joints or be- 

 tween the ends of the tiles. Ordinarily it enters from 

 the bottom, being brought there by gravity and held to 

 the depth at which it may be in the tile by the lateral 

 pressure of soil water. Water from the surface presses 

 directly downward until it reaches a line where the soil 

 is saturated. It then moves only as the water below 

 the line of saturation is drawn off by the drain, when 

 it in turn passes downward and laterally until it reaches 

 the drain. The rapidity of this movement is measured 

 by the hindrance offered to the water, by the nature of 

 the soil particles, and by the ability of the drain to re- 

 move water as fast as it is brought to it. The line of 

 saturation rises and falls as the supply of water in- 

 creases or diminishes, receding during the time of least 

 supply into the lowest plane it can occupy. At the 

 time when the soil has just ceased to supply drainage 

 water to the tile, a very slight rainfall, or even the 

 change ot temperature incident to night-time, will 

 cause the drain to discharge. No water enters the tile 



