Attack by Drainage 147 



dry season advances, the cross section of the 

 wetted portion of a ditch grows smaller, and little 

 puddles capable of mosquito production are left 

 in the stream bed detached from the oiled water 

 in motion. All these depressions must be sepa- 

 rately oiled, and only an intelligent and inter- 

 ested laborer can be trusted to do this. As the 

 quantity of ditch water decreases, the rate of 

 growth of vegetation increases, and it is then nec- 

 essary to remove it, or smear it thickly with oil to 

 leave the film free from air holes. Then the work 

 must be thoroughly inspected to see that it 

 has been properly accomplished. A sudden show- 

 er may remove all the oil and the work have to 

 be repeated. 



When the grade becomes flat by removing the 

 vegetation, the ditch may gradually change into 

 a long stagnant pool, and the advantage gained by 

 velocity is lost. It can readily be seen how im- 

 portant it is to have for foreman in charge of the 

 ditching maintenance gang a man who knows the 

 necessity of working correctly, and who can be 

 held responsible for results. 



Spoiling the grade of ditches causes more costly 

 future oiling and ditch maintenance. Ditches 

 and streams should have a uniform grade, and as 

 straight a course as local conditions permit. Least 



