SOIL IMPROVEMENT BY DRAINAGE. 65 



sub- soil comes near the surface and causes a discharge 

 of the water which has passed into the soil on the 

 higher land. In time of heavy rains land, such as 

 has been described, becomes full of water, and be- 

 comes dry slowly by the aid of evaporation from the 

 surface and the slow passage of water through the re- 

 tentive soil and sub-soil. 



If an open ditch be cut in such a soil, with the 

 bottom having a uniform slope to a stream or other 

 free outlet, it is evident it will rapidly carry off the 

 water until the line of saturated soil, near the ditch, 

 is on a level with its bottom. As the water cannot 

 move freely through the soil the line of satura- 

 tion gradually rises on either side, until at a dis- 

 tance depending on the depth of the ditch and the 

 character of the soil, no appreciable effect is produced 

 by the ditch. 



If the ditch were cut in very retentive soil, it could 

 carry off the water very slowly. If the surface soil be 

 open, but the ditch be cut into the impervious sub- 

 soil, it is evident the water can only enter it by flow- 

 ing along the top of the compact sub -soil until the 

 'ditch is reached. If the surface slope up on either 

 side of the ditch, it will affect the soil to a grerter 

 distance than if the surface be level. 



If a line of tile be placed in the bottom of the ditch, 

 with their ends so close together that dirt cannot be 

 washed in, and care is taken to see that ther^ is a uni- 

 form fall toward a good outlet, and the ditch be filled 

 up, the action will be much the same as in the case of 

 the open ditch. 



The efficiency of tile drains depends on many things, 

 as the character of the rain-fall, the retentiveness of 



