The human family have now lived together in communities more than six thousand years, yet they have not learnt to make their habitations clean. At last we are beginning to learn the lesson. When we shall have mastered it, we shall have conquered epidemics. Our duties, then, and our hopes in this respect, I shall proceed to show.
The principal constituents of the atmosphere maintain their equilibrium steadily over the whole surface of the globe. There is scarcely any difference in the relative proportion of its oxygen and nitrogen in the torrid zone and in the arctic regions. Whatever influence the atmosphere may have on climate must consequently depend on something adventitious to it and not in anything forming a part of it. Possibly therefore that something may be, in some degree, under human control.
The main constituents of climate are temperature and moisture, and these are the climatic conditions that exercise the greatest influence on epidemics.
Minor but still important conditions are the nature of the soil, the proportion of land that is cleared and under cultivation, the extent of forests, lakes, and rivers, the prevailing winds, the electrical state of the atmosphere, and so on.
The temperature is highest where the sun’s rays are vertical, or nearly so; where the sky is cloudless; where the day is longest; and where there is the smallest difference between the fervid noon-tide heat and the temperature of the short night.
The moisture is greatest where in addition to all the other sources of humidity there are periodical rains. In the countries subject to these rains, the entire extent of the level and low land is often covered a foot deeper with water than before the rain set in.
Elevated temperature and excessive moisture are combined in tropical countries; and they are concentrated in those parts of the tropics in which there are extensive forests having an undergrowth of luxuriant vegetation; in which the tides of the ocean penetrate deeply into the interior of the land, and mix with the waters of the rivers; and in which the rivers constantly overflow their banks and form marshes and swamps.
In tropical countries there are tracts such as these that extend in unbroken continuity hundreds of leagues. The western coast of Africa (the Bight of Benin) presents an unbroken area of upwards of 100,000 square miles, consisting of one vast alluvial and densely-wooded forest, irrigated by Atlantic tides, and intersected by numerous rivers and creeks, whose muddy banks are constantly overflowed.
In describing a tropical forest, Humboldt says, “Under the bushy, deep, green verdure of trees of stupendous height and size, there reigns constantly a kind of half daylight, a sort of obscurity, of which our forests of pines, oaks, and beech trees afford no example; forming a carpet of verdure, the dark tint of which augments the splendour of the aërial light.”