166 MOSQUITO ERADICATION 



large enough to allow a sufficient number of drops to fall to form 

 and maintain the film, and no more, it being borne in mind that 

 the drip will decrease at night and on cool, cloudy days and 

 increase on hot, sunny days. Sometimes, it has been found that 

 a more even flow is obtained by introducing a short, soft wick 

 into the outlet. 



"A crude, but cheap and easily made, drip-can," says LePrince, 1 

 "consists of a 5-gallon can, such as is used for shipping illuminating oil. 



A hole is made in its bottom with a 2 or 3-inch 

 Co/*" round nail. A wad of loose cotton is wrapped 



Bo/hm of can •! • 



around the nail just below its head. The nail is 

 then pushed through the hole on the inside of the 

 can, and ... oil is put in the can, which is 

 suspended or placed on a stand over the ditch. 

 Fig" 11.3 —Detail By pulling the point of the nail downwards and 

 of drip-regulating gently pushing it upward, the flow of oil may be 

 device - decreased or increased, as desired." 



Frequently, a 5-gallon can, having a metal discharge tap, is 

 used. The tap can generally be adjusted to give within a few 

 drops of the number desired to be discharged per minute. 



USE OF DRIP-CANS 



The quantity of oil that should be allowed to drip out depends 

 upon the spread of the oil, the size and alignment of the stream, 

 its grade and the absence or presence of obstructions in it. 

 LePrince 1 suggests from 10 to 20 drops of oil a minute for water 

 surfaces 1 foot wide. He continues: 



"For economical control, a trial should be made at each ditch or 

 stream where a drip-can is used to determine the desired rate of flow. 

 The drip-can should be regulated accordingly. In many cases, the drip 

 need be operated continuously for only 1 or 2 days of each week. With 

 larger streams, it may be found necessary to operate the drip con- 

 tinuously, day and night. On long streams or ditches, it is at times 

 necessary to use several drip-cans. They are then so located that the 

 next drip-can is installed approximately at the point where the effect 

 of the drip at the source disappears. 



"Continuous dry weather may make it become necessary to discon- 

 tinue the use of some drips or to change their location. At such seasons, 

 pools will be left isolated at the sides of a stream and will have to be 

 filled or separately treated with oil by other methods. When the stream 



1 "Control of Malaria: Oiling as an Anti-mosquito Measure," U. S. 

 Public Health Service, 1915. 



