274 ENGINEERING ON THE FARM 



soil and causes it to accumulate. As the slope increases, 

 the velocity of the water down the slope is increased, there 

 being a tendency for the water to collect in depressions. A 

 hard, sloping soil will run off a large percentage of water and 

 concentrate it upon lands having less slope. The flow of 

 water from the surface is materially retarded by the vege- 

 table growth. 



Maps and surveys. The basis for all drainage improvement 

 is an accurate map of the area to be drained. Such a map, 

 known as a topographic map, is made by an engineer after 

 a careful survey, and shows all local improvements, the 

 boundaries of lands to be benefited by the drainage, and the 

 boundary of the watershed. It should also show the eleva- 

 tion above a fixed point of all sloughs, lowlands, and the 

 tops of ridges through which it may be necessary to construct 

 the outlet channel. On this map the best location for the 

 proposed drains can be laid out, and their grade, size, and 

 approximate cost determined, after which the ditch or 

 trench may be staked out, with such minor changes as are 

 found necessary by conditions on the ground. 



Contractors. In drainage construction it is important that 

 the work be intrusted to persons who are properly equipped 

 and experienced in this class of work. Ditch construction 

 and tiling is a business in itself, the same as carpentry and 

 masonry work, and, if it is to be carried on as efficiently 

 as those classes of work, contracts must be let in the same 

 manner. The bidder's record for past work should be taken 

 into consideration as well as his bid. A low initial cost may 

 be made very expensive by poor judgment or careless work 

 done by the contractor. 



Method of drainage. One of the first things to be 

 determined is the method of drainage to be used. Gener- 

 ally the choice will be between open-ditch and tile, or a 

 combination of the two. The character of the ground, 

 surface slopes, crops raised, and value of land all have a 

 bearing upon the subject. However, in any locality an 



