REMEDIES AND PREVENTIVES AGAINST MOSQUITOES. 7 
made to form a bed covering at night, and every camping outfit for 
work in tropical or malarial regions should possess such framework 
and plenty of mosquito netting as an essential part of the outfit. 
The size of the mesh in mosquito bars and window screens is 
important. Twenty meshes to the inch can be relied upon to keep 
mosquitoes out, but 15 to the inch admits some of them. 
SCREENING BREEDING PLACES. 
Where the rain-water supply is conserved in large tanks, as in 
cities in the Gulf States, screening is necessary and is now rather 
generally enforced. Rain-water barrels everywhere should be 
screened in the same way, except where fish are used to kill the 
early stages of mosquitoes. A cheap cover for a water barrel can 
be made by covering a large iron hoop with a piece of stout calico 
or sacking, free from holes, in such a manner that a good deal of 
sag is left in the material. 
SMUDGES AND FUMIGANTS. 
Anything that will make a dense smoke will drive away mosquitoes, 
and various smudges are used by campers. For household use a 
number of different substances have been tried. 
PYRETHRUM POWDERS. 
Pyrethrum powders, known to the trade as Dalmatian insect 
powder, Persian insect powder, buhach, and otherwise, are very 
effective when fresh and pure. Pure powders are the finely ground 
flower-heads of two species of composite plants of the genus Pyreth- 
rum. The essential principle seems to be a volatile oil that dis- 
appears with age and exposure. Many powders for sale in the drug 
stores are apparently diluted by the grinding of stems as well as 
flower-heads and in other ways. These powders are not so effective 
as pure powders. Pyrethrum powders are usually used dry, and are 
puffed or blown into crevices frequented by insects, or puffed or 
blown into the air of a room in which there are mosquitoes. The 
burning of the powder in a room at night is common practice. The 
powder is heaped up in a little pyramid which is lighted at the top 
and burns slowly, giving out a dense and pungent smoke. Often the 
powder is moistened and molded roughly into small cones, and after 
drying it burns readily and perhaps with less waste than does the dry 
powder. Mosquitoes are stupefied by the smoke and fall to the floor, 
where they may be swept up and burned. With open windows and 
constant currents of fresh air this fumigation is not especially effec- 
tive, and it is necessary, for protection, to sit in a cloud of smoke. 
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