I70 The Control of Mosquitoes 



problem of keeping the oil film in place arises, for 

 on water surface of more than a few square feet 

 the oil film tends to drift to leeward under the 

 impulse of a moderate air current, and leaves much 

 water tmcovered. 



The tendency to drifting is very difficult to 

 overcome. Various devices are in use, all being 

 modifications of the "boom" principle used by 

 lumbermen to catch log drifts. The simplest 

 method is to anchor planks at intervals of five or six 

 feet at right angles to the prevailing wind move- 

 ment. Where the winds are variable and strong, 

 a wooden grille may be employed, with spaces 

 inversely proportional to the wind force. 



Stumps, brush, grass, and stones projecting 

 through the oil film tend to form oil-free rings 

 around themselves by their oscillation, and by the 

 breaking of the ripples against these obstructions. 

 All such obstructions should be removed from 

 areas to be treated with oil. 



The heavier oils in conjunction with algae form 

 a tangled heavy mass, and while some of these 

 masses sink, many do not, but remain on the 

 surface, and produce defects in the film by drifting 

 with the surface current. It is well to remove all 

 algas from water to be oiled. 



When heavy oil is poured repeatedly and liber- 



