130 The Control of Mosquitoes 



grow rapidly, and also raises the grade of the ditch 

 bottora. 



Ditches in soft soil and having a low grade are 

 the most expensive to maintain, and may become 

 a prolific source of Anopheles. Vegetation on the 

 sloping sides of open ditches prevents the banks 

 from scouring, but when the grass grows long it 

 falls into the water, retards the current, and often 

 assists in making conditions favorable to mosquito 

 propagation. Streams and ditches are treated 

 similarly; by straightening the channel and re- 

 grading parts of the bottom, we confine the normal 

 water flow and increase its velocity, which removes 

 the hiding-places of larvae. 



TILE DRAINS 



When rock, hard-pan, or other impervious 

 material underlies the surface soil, the ground 

 water not being able to penetrate it follows down- 

 hill on its upper surface, and where the impervious 

 stratum comes to the surface of the ground or 

 close to it, there may be a source of water. When 

 small quantities of water appear in this way on the 

 ground surface, they are called "seepage outcrops." 

 The line where water seeps out of the ground may 

 be short, or may extend along the entire side of a 

 hill, and become the source of a hillside swampy 



