The Campaign at Havana 7 



to control were drained or filled in, and the area 

 under control thus increased. Each place drained 

 or filled became one less to inspect and treat. 



As time went on, the anti-Anopheles work 

 consisted chiefly of draining and filling depressions 

 in fields where water collected. The use of what is 

 known as the "herring bone" system of ditching, 

 the training of streams t'o more direct courses, and 

 the reducing of the average wetted section of 

 ditches, rapidly decreased the necessity for regular 

 application of oil and for frequent inspection. 



The geological formation and topographical 

 conditions were favorable. The watercourses and 

 areas to be treated were often distant from one 

 another, yet the streams were of fair grade, and 

 most of the areas needing treatment were located 

 so that they could be drained. In some cases the 

 impervious surface clay rested upon a coral for- 

 mation. The latter was of a porous nature, and 

 absorbed a large quantity of water in a relatively 

 short time. In some places where there were 

 ponds, and surface drainage was out of the ques- 

 tion, they were connected by one or more ditches, 

 and led to shallow wells in the coral formation, 

 where the water was absorbed. When one well 

 was not sufficient, two or more were mads and 

 connected, to increase the absorbent surface. 



