Attack by Drainage 145 



only occurs in lined ditches having practically no 

 grade, and then only at periods of minimum flow. 

 It is rapidly removed by the application of a small 

 quantity of copper sulphate used at the head of 

 the ditch. Food and protection for Anopheles 

 larvae are absent, and they appear to avoid in- 

 stinctively ovipositing in places where these 

 conditions prevail. 



On fair grades there is no necessity for oiling or 

 treatment of a lined ditch. Inspection may be 

 made less often and as rapidly as one can walk. 

 Defects may be noted at a glance. If the lining 

 has been properly planned and installed, there is 

 biit little maintenance cost, and in most cases 

 none at all. 



Where ditches are practically without grade, 

 and the soil in the higher lands above the ditch is 

 heavily eroded by storm water, as the velocity 

 decreases, the depositing of matter in suspension 

 increases, but generally only a fraction of the 

 amount is deposited in a lined ditch that remains 

 in an open earth ditch. In fact, concrete lined 

 ditches often take care of themselves, and they 

 need very little attention. 



The disadvantage of concrete lined ditches is 

 the higher first cost, and in case of construction 

 work, where the topography is being constantly 



