172 



FARM DEVELOPMENT 



ward from the surface of the ground to the surface of 

 the ground water, and then seeps sidewise into the 



Figure 74. Common drain tiles. 



stream running through the bottoms of the openings 

 through the row of tiles. Since the ground water out- 

 side of the tiles is little higher than that inside, the seep- 

 age movement is nearly horizontal. 



Survey for Construction 



While the general inspection of the land referred 

 to on a previous page might result in a choice of the 

 approximate location of the main drain and its 

 branches, it is necessary, on nearly level land, to 

 attend to the details for the exact location of the line 

 of the ditch. Where a slough or hollow with gentle 

 : slope is to be drained, it is often wise to supple- 



Figure 75. Union tiles. 



ment the "bird's-eye" survey with a series of levels taken 

 at intervals of 50 or 100 feet along the proposed line of 

 the ditch. In some simple cases all that is necessary 

 is to determine the height of the land at either end of 



