120 The Control of Mosquitoes 



than a foot deep. The dried surface soil is at first 

 merely resting on liquid mud and if a person stands 

 on one of the solid portions it frequently sinks 

 beneath him. When the crust becomes thicker, the 

 cracks are more numerous, and so are the numbers 

 of Anopheles developing in the cracks after showers. 

 To control similar conditions we installed surface 

 drainage to assist the drying. This had to be 

 accomplished without drowning the laborers. 

 Large sheets of waste galvanized iron roofing were 

 thrown forward and heavy planks used to bridge 

 the space between these sheets and put only 

 light pressure on the thin crust. Shallow ditches 

 were then dug by laborers standing on the planks. 

 They do not prefer work of this nature! This 

 preliminary ditch rapidly dries the soil at its sides 

 so that it will bear a man's weight if he does not 

 stay too long in one place, and it can then be made 

 deeper. The process is repeated until the desired 

 depth and grade are obtained. When the cracks 

 do not become filled by the action of rains, as 

 soon as the crust is sufficiently strong to support 

 a mule, an iron rail is dragged over the surface, to 

 level and fill the cracks. When the material 

 contains much sand or gravel, the cracks do not 

 form. 

 About one half of the area on which Colon is 



