"the doctor." The location of dwellings is often determined by the exposure of a site to this wind. For this reason, many native villages are placed as near the sea as possible. The houses of well-to-do foreigners often occupy the healthiest and most desirable locations, where the sea breeze has a free entrance, while the poorer native classes live in lower, less exposed, and less desirable places. A social stratification is thus determined by the sea breeze. In our own latitudes, exposure to sunshine is very important, as is well known, in determining house sites. Lugeon's study of one of the principal valleys between Martigny and the Rhone Glacier, has brought out some interesting facts in this connection. In this district the villages, with one or two exceptions, are on the sunny side. In fact, a distinction of classes results from this difference. There is developed what may be called an "aristocracy of the sun." The people on the sunny side are more prosperous and better educated, and look with some contempt upon the people on the shady side.
The trades, except where they blow onto windward coasts, or over mountains, are dry winds. On the lowlands swept over by the trades, beyond the polar limits of the equatorial rain belt (roughly between latitudes 20° and 30°) are most of the great deserts of the world. The interior of Africa has been out of contact with the civilized world largely because of the deserts to the north and south of it. Goods and passengers go around rather than across these deserts. In the desert, population gathers in oases, as on islands. Here the trails followed by the caravans converge like sailing routes at sea. There are small Arabian towns, built at oases, where the houses are almost crowded on top of one another, producing something not unlike the modern "sky scraper" of an American city, where land is scarce and expensive. The overflow of the Nile results from the rainfall on the mountains of Abyssinia during the northward migration of the belt of equatorial rains; and one of the most difficult problems in the construction of the Panama Canal, viz., the control of the floods of the Chagres River, is due to a similar cause.
The monsoons, reference to which was made a moment ago, are a special development of the general trade wind system. Monsoon regions have summer rainfall, and these rains are particularly heavy where the winds have to climb over high land. Thus, in India, the precipitation is heaviest at the head of the Bay of Bengal, where in the Khasi Hills, at a height of a little less than a mile above sea level, the rainfall averages between 35 and 40 feet a year. This is about ten to twelve times as much as the rainfall of New York City, and all this water falls in less than six months. Truly, at that place, "it never rains but it pours." In certain parts of India stores of provisions are laid in before the rains begin, the preparations being similar to those made on board a vessel bound on a long voyage. Special mention may
