148 The Control of Mosquitoes 



larvae will be found where steep banks stretch 

 above and below the normal flow Hne. The width 

 of ditches should be no more than is absolutely 

 essential, and the water in them should be kept in 

 motion. Prompt removal of obstructions and 

 attention to minor detail save much future ex- 

 penditure and reduce propagation. Laborers at 

 work cleaning or regrading a ditch frequently pile 

 the excavated mud in such a manner that sooner 

 or later it returns to its original position. This 

 practice is common along railroads, and laborers 

 should be closely watched to prevent it. 



Ditch maintenance gangs often make ditches 

 wider and deeper. On hillsides the banks of ditches 

 are apt to collapse from constant deepening of flood 

 water. This can be partially controlled by giving 

 a proper slope to the ditch sides, allowing grass to 

 grow on them, and preventing holes forming in the 

 bottom of the ditch. As soon as the soft spots are 

 formed, they must be filled with stone, thoroughly 

 tamped into place. When it is definitely known 

 that the erosion cannot be stopped, and that the 

 material carried away will be deposited in the 

 ditch at a point lower downstream, to be removed 

 at regular intervals, an estimate should be made 

 of the cost of constant removal compared with the 

 expense of lining the parts of the ditch being 



