INLAND DRAINAGE 



101 



located by instrument in the field, stakes being set at 50- or 

 100-foot intervals, with the cut at each stake marked thereon. 



Even if the project is not large enough to justify this amount of 

 preliminary work — as will often be the case — it may frequently 

 be necessary in low, fiat country to run various lines of levels to 

 ascertain the fall available and the best location for the main 

 ditches. 



Where the country is hilly or rolling, the eye alone may be 



Fig. 65. — A common type of anti-mosquito ditch. 



sufficient for open-ditch work, although in all cases of doubt, the 

 level should be used. 



As a general rule, open ditches in anti-mosquito work need not 

 be graded with the accuracy of a sewer trench, though there 

 should be no low spots. With lined ditches and sub-surface 

 drains, however, this is not the case; stakes should be set for 

 these in all cases. 



Owing to the fact that mosquitoes do not develop in standing 

 water for several days, anti-mosquito ditches do not always have 

 to be as large as ditches for agricultural drainage and other 

 purposes. Thus, if a ditch will carry off the standing water in 



