128 MOSQUITO ERADICATION 



rendering unnecessary previous oiling of hoof-prints, etc., along 

 their margins in the wet season. Another considerable economy 

 resulted from elimination of the necessity for periodical cleaning 

 of grass and weeds from open ditches. 



TILE DRAINS VERSUS OPEN DITCHES 



Ditches in which drainage tiles are to be installed must follow 

 grade exactly; hence, they should be laid out by instrument or 

 other accurate method and the bottom finishing checked from 

 time to time. Where the fall is less than 2 inches to 100 feet, very 

 careful work is required. 



Tile drains generally should be laid from 2 to 4 feet deep, 

 depending upon character of soil and other local conditions. As 



the ditches in which they are laid 

 are filled up as soon as the laying 

 is completed, they need be of no 

 particular shape, such as is re- 

 quired in the case of open ditches; 

 they may be dug with vertical sides, 

 if the nature of the soil will permit. 

 In digging ditches for tile clrain- 

 Fig. 89. — Cross-section of outlet age by hand, special tile spades are 



for a tile drain. i, ■, A , , £ i 



generally used. Another useful 

 instrument is the drain scoop, which comes in various sizes to suit 

 the size of tile. However, a very large proportion of the ditches 

 for agricultural tile drainage are dug by machinery today. 



A system of tile drainage may consist of a main, sub-mains, 

 laterals and sub-laterals, or only a main and a lateral. The main 

 is the drain through which all the water eventually flows. Some- 

 times, only a single line may be needed. 



A proper outlet for the main should be provided in all cases in 

 order to prevent caving and possible closing of the pipe. Prob- 

 ably the best protection is a concrete wall, into which the end of 

 the pipe is set. Where the outlet is low and liable to be sub- 

 merged in time of flood, a valve should be installed to keep the 

 water from backing into the drain. 



LAYING OUT A TILE DRAINAGE SYSTEM 



In laying out a tile drainage system, as few mains and as many 

 long laterals should be used as possible, as a rule, since, near the 

 mains, both the laterals and the mains drain the land. For this 



