634 EVERYDAY SCIENCE 



on the surface of the water. It will spread in an even film over 

 the entire surface. On marshes or large ponds, where weeds and 

 intervening dams of mud prevent the film of oil from spreading 

 over the entire surface of the water, it is best to use a sprayer. In 

 either case, use about 1 pint for approximately every 20 square 

 feet of water surface. 



This film of kerosene kills all eggs at the surface of the water, 

 suffocates the larva or "wigglers," by cutting off their air supply, 

 and destroys all adult female mosquitoes that try to lay their eggs 

 on the surface of the water. 



It takes about a week or ten days for the oil to evaporate from 

 the surface of the water, and at least 10 days after that before a 

 new generation of mosquitoes can be hatched. It is a safe plan, 

 therefore, to apply kerosene to the surface of all stagnant pools 

 about twice a month. In covered tanks, cesspools, etc., one appli- 

 cation a month is sufficient, because evaporation does not take 

 place so rapidly from such unexposed places. In heavy soil, cow 

 tracks and other small depressions may hold water long enough to 

 hatch a generation of mosquitoes. After every rain, such de- 

 pressions should be drained or else sprayed with kerosene. 



Fish. Where pools are used for the watering of stock, kerosene 

 cannot be used, of course. In such cases, the remedy lies in stock- 

 ing the ponds with top minnows or sunfish. These fish feed on the 

 larva of the mosquito. If there are no other fish in the pond, the 

 top minnow may be used. If the pond is stocked with larger fish, 

 the sunfish, sometimes called "pumpkin-seed," is to be preferred 

 because it is able to protect itself by means of its rays against larger 

 fish. Do not neglect to drain cow tracks around such ponds, or 

 else spray them with kerosene often enough to prevent mosquitoes 

 breeding in them. 



Screening. Water tanks, rain barrels, cisterns, and other re- 

 ceptacles for water for the household , cannot be treated with kero- 

 sene. Careful screening of all the openings of these receptacles 

 is the only remedy. The only effective screening against mosqui- 

 toes is the 16-mesh screen 16 wires to the inch. No one argues for 

 less than a 14-mesh screen, and most authorities insist on a 16-mesh. 



