60 
PROPHYLAXIS OP MALARIA. 
they give fairly good results. Such drains are constructed as follows: 
A ditch is dug and the bottom covered with flat stones, the sides are 
formed by a layer of flat stones, and a cover stone is placed on top, 
after which the whole is covered with quite large stones and lastly 
with small stones. Regarding this type of drain Le Prince says: 
Where plenty of stone is available and must be removed from a field, this 
form of ditch is economical to make and, in the Tropics, much more economical 
to maintain than an open ditch is, while anopheles larvae do not occur 
therein * * *. We opened some of these drains when the flow of water in 
them was sluggish, but found no mosquito larvae therein. The advantage of 
this type of drain is that there is no first cost for material, the field stones are 
disposed of, and there is no cost of maintenance except when the ditch clogs 
up. A good grade is essential for blind drains, and they will stand on a 
fairly heavy grade. 
The cement-lined open drain .—This is the only form of open 
drain that should be adopted in the Tropics, and it should always be 
laid upon a considerable grade. Such drains should be semicircular 
in shape and constructed of stone or brick, lined with cement. If 
laid upon a good grade such drains will keep themselves clean by the 
flushing of the water and will not become filled with vegetation and 
algae. 
Subsoil drainage with tiles .—Le Prince says that “ the use of tile 
drainage is the most economical and permanent method of destroy¬ 
ing anopheles in the Tropics. The work costs from 16 to 20 cents a 
foot upward, according to the depth of the trench, material to be 
excavated, and distance that the tile and covering stone has to be 
carried." He recommends that the tile drains should not be given 
a flatter grade than 0.5 per cent, and states that those having as 
high a grade as 5 per cent gave perfect satisfaction if the tiles be 
covered with plenty of stone to hold them in place. This is the 
method of drainage adopted in the Canal Zone, where it has proven 
less expensive in the end than the open ditch and much more efficient. 
The question of the prevention of mosquitoes bv drainage is a 
purely economical one, as there is no question of its efficiency in any 
region where it can be applied. In the Tropics its application is 
much more difficult than in temperate regions, but the success of 
this prophylnctic method in the Canal Zone proves that even in the 
Tropics it can be used under the most difficult conditions and that 
its use is both economical and successful. In many of our posts a 
system of tile drainage would completely eradicate mosquitoes at a 
much less final cost than is now expended in screening and for 
mosquito nets. 
Filling .—In many localities the filling in of low ground and of 
small pools will serve to destroy most of the breeding places of 
mosquitoes, and this is often a very valuable prophylactic method. 
Removal of shelter .—In conjunction with drainage and filling in 
of the breeding places of mosquitoes, the removal of brush along 
