134 LAND DRAINAGE AND IRRIGATION 



20. Explain why it is often best to lay two lines of tile in a wet draw between 



two areas of upland. 



21. How do tile work? Can land be too thoroughly drained? Explain. 



22. Explain and illustrate by use of a diagram what is meant by a system 



of tile drainage, mains, laterals, and sub-mains. 



23. What are the systems of tile drainage commonly used? Illustrate each 



by diagram. 



24. Why is it best to have one instead of many outlets to a system of 



tile drainage? 



25. What is the smallest ditch that should, under most conditions, be con- 



structed? Explain. 



26. Is drainage profitable? Give an example. 



27. What are vertical drains? Describe by use of a diagram the conditions 



under which a vertical tile drain can work. 



28. How is it possible to drain low-lying and flooded lands? 



29. Describe a drainage system which you have seen installed. 



30. What is the meaning of irrigation? To what kind of climate is irri- 



gation confined? 



31. Give the objects of irrigation. 



32. What are some of the sources of irrigation water? 



33. Name and describe the methods of irrigation. 



34. Do irrigated farms require less attention than farms not irrigated? 



35. Why do some irrigated lands need drainage? 



36. What are some of the special problems that confront the irrigation farmer? 



37. Is irrigation farming profitable? 



38. Who invented the art of irrigation? 



39. Describe an irrigation system which you have seen. 



