134 MOSQUITO ERADICATION 



2 by 12-inch sheathing. A flume was built from one side of the 

 shaft to the end of the ditch connecting with the lake. The out- 

 let thus formed proved sufficient to drain the entire lake and 

 keep it dry. 



Sometimes, if the fissure be small, there is a tendency for the 

 hole to silt up, especially when the water enters the hole directly. 

 This may be avoided by the use of a drain-head, designed to 

 keep out floating matter and the coarser sediment. Such a 

 drain-head may be an elongated, narrow box, laid as a pipe 



cover or e/oc" M ,ovfs/d(. 



ovrs/ae screen 



inside, screen 



Shaft- 

 Fig. 94. — Sketch of a drain-head for vertical drainage. 



from the ditch to the hole; the end toward the ditch should be 

 heavily screened and the other tightly closed; a hole in the 

 bottom near the closed end, through which the water is to enter 

 the shaft, also should be well screened; a door on the top of the 

 box will allow this latter screen to be cleaned from time to 

 time; the entrance screen of course may be kept clean from the 

 outside. 



STREAM RE-CHANNELING 



Contrary to the popular belief that mosquitoes breed only in 

 stagnant water, they may often be found breeding in abundance 

 in numerous flowing streams, since in most streams there are 

 many places where the current is almost imperceptible. 



Thus, in a stream in flat country, where the fall is slight, there 

 are apt to be many quiet pools, suitable for breeding. Even in 

 a stream with a considerable fall, obstructions may stop the 

 current or a soft bottom may wash out, resulting in a large, deep 

 hole. Furthermore, the numerous bends in most streams cause 

 a considerable loss of fall and hence of velocity. 



LePrince 1 says: 



"A stream should be made to have steep banks directly above 

 and below the flow line, uniform grade and width and a straight course, 



1 "Malaria Control: Drainage as an Anti-malaria Measure," U. S. 

 Public Health Service, 1915. 



