Page:Popular Science Monthly Volume 23.djvu/666

648 ; and, 2. Making excavations. Which of these two is the more fruitful in producing malaria is not stated, nor is the modus operandi of either suggested. In Hong-Kong, an island consisting of little more than bare and barren rocks of weather-beaten granite, and whose soil contains but two per cent of organic matter, malarial disease was formerly unknown, and only became prevalent, as it is at present, after excavations had been made in digging granite for building purposes. So, again, tanks and pools of water—cess-pools, mill-ponds, reservoirs, and bilge-water on shipboard—appear to be specially productive of malaria. In Ceylon, the tanks of Candelay and Minery—the one twenty miles in circumference, the other twelve—have been considered the cause of malaria in that region (see Davy on "Diseases of the Army," p. 51, etc.). It is easy to comprehend how such pools, tanks, and excavations containing water may constitute mosquito nurseries, where the female may deposit her eggs and propagate, which would probably have been prevented in the absence of such water accumulations. How simply digging up the soil may contribute to the formation of malaria, or to the development of mosquitoes, without excavations, I am not able to explain.

9. "In certain countries it" (malaria) "seems to be attracted and absorbed by bodies of water lying in the course of such winds as waft it from the miasmatic source." That the malaria itself is absorbed by water is pure hypothesis. The known fact embodied in this ninth statement is really this: A body of water intervening between a healthy locality and a fever district will, provided it be sufficiently wide (three fourths of a mile or more) prevent the transmission of fever from the infected to the healthy locality, also provided, of course, that the prevailing wind does not blow the fever-generating element from one side to the other, as we have already seen it may do over a much wider space—probably five miles. This, again, is not difficult of possible explanation by the mosquito theory. All depends upon the answer to this question: Over how wide a sheet of water will the mosquito, in the absence of irresistible winds, attempt to fly? I am unable to answer this question positively. It may depend upon the degree to which the insect possesses far-sightedness, for, if it can not see land across a body of water three fourths of a mile wide, such a width would appear to its vision boundless as an ocean, and under those circumstances it might not voluntarily attempt to cross. Furthermore, the flight of the insect being mostly nocturnal, long vision would be all the more difficult. These suggestions need confirmation: they are tentative, but still sufficient to suggest the possibility of the protective influence of wide bodies of water being explicable on the mosquital basis.

10. "In proportion as countries, previously malarious, are cleared up and thickly settled, periodical fevers disappear." Here, too, we may remark that in such countries the land is cultivated, and' its