Page:Popular Science Monthly Volume 23.djvu/665

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INSECTS AND DISEASE.

would carry it out to sea, the water will form a barrier to its farther progress seaward, for it is not a marine insect. Mosquitoes, therefore, accumulate on sea-coasts notably at some of our familiar summer resorts, Cape May, Atlantic City, etc.

5. Malaria "has an affinity for dense foliage, which has the power of accumulating it when lying in the course of winds blowing from malarious localities."

6. "Forests or even woods have the power of obstructing or preventing its transmission under these circumstances."

These last two propositions, embodying, first, the "accumulation," and, second, the "obstruction," of malaria by forests and trees, may be considered together. That a wind coming from a marsh (from, in fact, a mosquito nursery), and bearing a colony of mosquitoes, should be screened or sifted of its insect burden by passing through the foliage of a forest, or a belt of trees, is certainly far more comprehensible than the conception of a malarial vapor being so screened by virtue of its "affinity for foliage." And though, in the case of a single belt of trees, even the mosquital filter may appear imperfect, the insect, should it have been carried far, is probably anxious to settle, and may so vary its course by steering as to take the first opportunity of clinging to anything that may come in its way; and, having settled, we may readily conceive its shifting round to the leeward side of a leaf or branch, and there holding on until the wind sufficiently subsides to allow of safer flight. Thus mosquitoes, "like malaria, may both accumulate in, and be obstructed by, forests and trees.

The conduct, or rather the mechanical properties, of the mosquito, when carried by the wind, can hardly be better described than in the following verbatim quotation from Sir Francis Day, in his description of malaria. He says: "Malaria may be carried by the winds to places where it was not generated; it is obstructed by and hangs in the foliage of trees, or in mosquito-curtains; it subsides into low places, and may be blown over a hill, and, may be very virulent on the side opposite to that on which it was formed. In like manner it may be taken up the side of a hill, and, as a lull takes place in the atmosphere, consequent upon its weight it rolls down, and may thus envelop its base with a deadly belt of fever, for there, hanging in the leaves of the trees, it gradually sinks through them to the earth beneath, in which situations it is most dangerous to pass the night" (Sir Francis Day's work, p. 87).

7. "By atmospheric currents it" (malaria) "is capable of being transported to considerable distances, probably as far as five miles." So, certainly, is the mosquito.

8. "It" (malaria) "may be developed in previously healthy places by turning up of the soil, as in making excavations for the foundations of houses, tracks for railroads, and beds for canals." Here two things are confounded, viz.: 1. Turning up of the soil, as by plowing or dig-