664 Recent Researches on Malaria. [November, 
all its moisture by evaporation, the production of malaria 
ceases, and decidedly malarious soils may remain harmless 
during the whole course of a hot dry summer. This may 
be called a natural hydraulic sanitation. But if the ground 
is then moistened by rain, of even brief duration, there may 
occur — so to speak — an explosion of malaria. 
With us a wet spring and summer, followed by heat, are 
favourable conditions for malaria. If before the commence- 
ment of the hot season the ground has been thoroughly 
soaked in rain, and if it is then exposed to an elevated 
temperature, so that it dries on the surface, the development 
of malaria begins. In some cases it is even found possible 
to show a diredt proportion between the extent of the en- 
demic and the quantity of rainfall. Thus it is known that 
both the malignity and the generality of malarial fever are 
direcTly as the rainfall in the past winter and spring. Not 
that the great quantity of moisture has any diredt influence, 
but that the moisture of the soils persists longer, and with 
it the endemic. 
A short cessation of the production of malaria in the soil 
does not suffice to destroy its cause, the disease-germs in 
the soil. They remain merely in a latent condition, and can 
again develop their destructive action as soon as the neces- 
sary conditions are reproduced. 
The attempts hitherto made for the sanitation cf malarious 
districts have been guided by the opinion that malaria 
originated only in swamps or swamp-like localities, and 
where masses of organic refuse were slowly undergoing de- 
composition. Since then it has been held sufficient lor 
sanitary purposes to promote and regulate the constant 
efflux of the water, thus eliminating one factor only. 
In fact thermic sanitation, such as is produced by Nature 
in winter, is impracticable, because we cannot diminish the 
action of the solar rays. [To do so would be suicidal, were 
it in our power.] Hence we can only act upon the two re- 
maining factors. 
The systems of hydraulic sanitation are very various, since 
the problem of removing the moisture of the soil during the 
hot season admits of various solutions, according to the lo- 
cality. Drains, open or covered, may answer the purpose 
of conveying large quantities of water from the malarial soil. 
In other cases the natural drainage of the soil sucks out 
and carries away the subterranean moisture. Thus the 
level of the “ ground-water ” is lowered, so that it can no 
longer reach and keep moist the malarial strata which are 
exposed to the action of the atmospheric air. 
