punkah, immediately seeks shelter. It is certain that some thousand or fifteen hundred yards of water between a ship and a malarious coast may suffice to secure immunity to the crew. The experience of the notorious Walcheren Expedition proves this. A similar distance on land from a malaria source is probably approximately as effective. The diffusion of malaria by winds is generally extremely restricted. Inside, a city may be quite healthy, whilst outside the walls the country may be pestilential. One village may be sickly, whilst a neighbouring village may be healthy. Surely, if winds transport the malarial germ for any distance from its source, there would not be so great a difference in the relative salubrity of urban and suburban localities, nor of neighbouring houses and villages. Neither does the malaria germ ascend to any great height above the ground. Acting on the empirical observation of this fact, the peasants in many unhealthy spots in Italy and Greece are said to secure a remarkable degree of immunity by passing the night, during the fever season, on platforms raised on poles a few yards above the ground. It seems safe, therefore, to conclude that the horizontal and vertical diffusion of the malaria germ that is to say, of the mosquito is, as a rule, very restricted.
Influence of trees, houses, and lodging:—— The intervention of a belt of trees between a malarial swamp and a village is said to protect from malaria the houses on the leeward side of the trees. The trees may filter out the mosquitoes by affording them protection from winds. Open windows and doors, by giving ready access to mosquitoes, are sources of danger in malarial countries; for this and similar reasons sleeping on the ground, on the ground floor, or unprotected by a mosquito curtain, is dangerous.
Time of day in relation to infection.——Evidently in conformity with the habits of the mosquito, the time just before sunrise and just after sunset, and the night, have the reputation of being the most dangerous as regards liability to contract the infection. Although mosquitoes are most active during twilight and night, they bite readily enough during the daytime
