Short of general overflow, the higher the subsoil water the greater the chance of a given locality supplying breeding-places for mosquitoes and therefore of proving malarious. Hence arises marked liability to epidemics of malaria on the subsidence of extensive floods; and hence the danger attending the raising of the level of the subsoil water by irrigation works, canals, embankments, and other engineering works.
Influence of rainfall.—— As regards the relation of the prevalence of malaria to rainfall there have been too many generalizations based on the limited experience of one or two districts. Thus, it is often said that the most malarious time of the year is at the end of the rains, when the soil is beginning to dry up. A wider view of the subject shows that, though applying to some places, this statement does not apply to all. There are localities where the fever curve is highest before the setting in of the rains. In some places, particularly in those that are low-lying, flat, and swampy, fevers of first invasion disappear almost entirely when the country becomes flooded. This apparent want of a universal and definite relationship of fever curve to rainfall indicates that the conditions determining the prevalence of malaria are highly complex, and that they are not by any means merely a matter of heat, moisture, and vegetation. In some places much rain will scour out the mosquito pools; in other places it will just fill them. The key to the explanation of the varying relation of malaria to rainfall is to be found in the influence of the latter on the local mosquito pools.
Influence of winds and atmospheric diffusion.—— It has been said that the wind can carry the malaria germ great distances, roll it along the ground like thistledown, and even force it to ascend high mountains. It is very doubtful, however, if the malarial mosquito can be transported, in this way, very far from its native pool. The mosquito does not ascend more than a few feet from the ground, and in high winds, or even in draughts of air, such as that from a
