DESTRUCTION OF LAEV^. 77 



can be sprinkled to advantage out of an ordinary watering pot with 

 a rose nozzle or, for that matter, pouring it out of a dipper or cup will 

 be satisfactory. In larger ponds, pumps with a straight discharge 

 nozzle may be used. A straight stream will sink and then rise and 

 spread until the whole surface of the pond can be covered without 

 waste. The English workers in Africa advise mopping the petroleum 

 upon the surface of the water by means of cloths tied to the end of a 

 long stick and saturated with kerosene. The use of such a mop may 

 be desirable, even where a straight discharge pump has been used, 

 in order to commingle two or more surface sheets of oil. In some of 

 his early work on Staten Island, Doctor Doty, the health officer of 

 New York, used a pump with a submarine discharge, throwing the 

 oil out at the bottom of a pool and allowing it to rise to the surface. 

 It seems that the idea was to destroy the insects feeding at the bottom 

 more quickly, but as most mosquito larvae rise to the top to breathe 

 about every minute, there is practically nothing to be gained by such 

 a method of distributing kerosene. 



The use of larvacides in tropical regions brings in certain new fea- 

 tures which complicate the problem of mosquito destruction to a cer- 

 tain extent. Colonel Gorgas and his corps of workers at Panama have 

 been using petroleum very extensively just as they did at Habana. 

 They find, however, that at Panama the rapid growth of vegetation 

 prevents the oil from spreading uniformly and that it can not make a 

 thin uniform film over the surface of water in which vegetation 

 grows. They find also that algae on the surface of the waters form 

 with the oil a dark scum, which collects at the bottom of shallow 

 pools. This scum later breaks up and floats about on the surface, 

 rendering succeeding oilings less efficacious and necessitating the use 

 of larger quantities of oil. They also find, that where vegetable 

 debris collects in a large body of water it will be blown about as a 

 mass, its location changing with the wind, and thus break the fUm of 

 oil. Mosquito larvae also hide in this vegetation, which protects 

 them from fish. The wind blows the oil to one side of the surface 

 and it evaporates very rapidly in the Tropics. During the rainy sea- 

 son it is washed away very rapidly before it destroys all of the larvae 

 and of course where the film is not perfect the larvae find free places 

 to breathe. The bulk of the oil and the cost of transportation in 

 rough territory for work on a large scale are disadvantages. In 

 their work they find that they must constantly occupy themselves 

 in removing vegetation before oil is applied, in order to prevent the 

 necessity of using excessive amounts of oil. They find that new 

 growths of algae appear to develop very rapidly after the oil has 

 imited with the previous crop and sunk to the bottom. 



In the course of the Panama work, as previously stated, phinotas 

 oil has been used, and has been foimd to have the following advan- 



