90 HOUSEHOLD INSECTS 



was used. In the first place, ditches with their strong 

 currents need a light oil that will spread as rapidly as 

 possible. In the second place, petroleum has cost us 

 just as much as the refined oil, even if bought by the barrel. 

 For bodies of water that have no currents a heavy oil 

 might be superior to ordinary kerosene because the former 

 would not evaporate so quickly and thus need not be 

 applied so often. On the other hand, an oil too heavy 

 will not spread easily, but will gather in spots here and there 

 over the surface, thus losing its effectiveness. Howard 

 says, "so long as the oil flows readily and is cheap enough, 

 the end is gained, provided it is not too light and does not 

 evaporate too rapidly." Other investigators have found, 

 that what is known as "light fuel oil" is the most satis- 

 factory. 



Amount of oil to use. Careful experiments have shown 

 that, in general, one ounce of kerosene is sufficient for 

 every fifteen square feet of surface. One-half of a tea- 

 cupful for a barrel is amply sufficient. If the oil is 

 applied with a spray pump and fine nozzle, smaller 

 quantities will be used. 



How to apply oil. For small pools, ditches, and 

 tanks, a five-gallon knapsack spraying machine (Fig. 32) 

 is almost ideal. The sprayer should have ten or twelve 

 feet of hose attached and the hose should be furnished 

 with a good nozzle tied to a pole about six feet long. 

 With the pole the operator can reach both sides of a 

 ditch and all sides of a tank without changing position. 



In cases of barrels, cisterns, and cans, the oil may be 

 poured on or thrown on from a cup, dipper, or other re- 

 ceptacle. In fact, it may be applied to the surface in 

 many cases from a bucket sprinkler. 



