146 The Control of Mosquitoes 



changed, it is not advisable to install permanent 

 work that must soon be destroyed or covered up. 



Under the conditions existing on the Isthmus, 

 the proper maintenance of open ditches is often a 

 difficult task. It is necessary at all times to keep 

 them free from obstructions. Even a small twig 

 caught by an irregularity on the bank will cause 

 other matter to collect and form a temporary 

 dam. When this happens in a ditch of low grade, 

 it may residt in the deposit of silt, sand, and clay 

 for one or several hundred yards above the 

 obstruction. 



In a deep ditch regrading and cleaning are 

 expensive. The current velocity may be increased 

 on the side of the ditch opposite an obstruction, 

 undermining that bank completely, and deposit- 

 ing large amounts of material at points lower down. 



Where a ditch passes through soft material, the 

 channel frequently becomes out of alignment, and 

 constant cleaning and regrading tends to widen it. 

 During dry periods it may develop into a semi- 

 stagnant pool, in which algae forms rapidly, making 

 a new small temporary ditch necessary within the 

 larger one. The small one has a tendency to close 

 up, and is often destroyed by the first shower. If 

 cattle walk in the ditch, the water in each hoof- 

 print must be separately oiled. In general, as the 



