Page:Popular Science Monthly Volume 31.djvu/149

Rh in producing this result. But whether the chance of air-pollution is greater from sewers or from cesspools is not so easy to determine. There are a few records, however, which will help to clear this point. In Berlin, the distribution of cases of fever has been found to be one to each 9·3 unsewered, one to each 49·3 sewered houses. In Dantzic the mortality from typhoid fever dropped to less than one fourth the old rate after the introduction of the sewers in 1872, and has been still lower during the last five years. In Cincinnati the water-supply is abundant, but the sewerage is imperfect, and the death-rate from typhoid is increasing. The city of Mexico enjoys a water-supply of forty-four gallons a day to each person, but there are no sewers, and the fever mortality is very high.

Various Kinds of Rivers.—M. Woeikoff, the Russian meteorologist, considering rivers from a climatological point of view and with regard to their sources of supply, makes several types or classes of them. The first are those which derive their waters from the melting of snows in plains or regions of not more than three thousand feet elevation; such rivers exist only in the extreme north. Next are rivers fed by the melting of snows in the mountains. Instances are the Amou and Syr Barias, the Tarim, and the upper Indus. In their lower course these rivers traverse regions where it seldom rains, or rains only in winter. High waters occur in them at fixed periods, and the maximum height depends on the quantity of snow on the mountains. They are utilized in the plains of their lower course, where cultivation would not otherwise be possible, in vast systems of irrigation. A third class of rivers depend on rains, usually tropical and monsoon rains, and reach their maximum in the hot season. They are best represented by the Congo and the Orinoco, in whose valleys snow never falls. They are low in winter, or the dry season, and reach their maximum stage in summer, when immense quantities of rain fall into them. Some of the tropical rivers, like the Amazon, are partly fed by melting snows, but in very small quantity; for the snow exists only on mountain-tops above twelve thousand feet in height. To this third class belong also the Nile, the Ganges, and Brahmapootra, and the great rivers of China. That the rivers of China, Mantchooria, and the Amoor region possess the common feature with tropical rivers of having their freshets in summer is a testimony to the prevalence of the monsoons in their regions of supply. To a class that receive most of their waters from rains, but are swollen periodically by the melting of the snows, belong most of the rivers of Western and Northern Siberia, European Russia, Scandinavia, Eastern Germany, and the Northeastern United States. The southern hemisphere has no such rivers. A fifth class of rivers depend on rain-water, are of constant flow, and are highest in the cold season, without being subject to sudden freshets. They are found in the eastern part of the United States, in New Zealand, and in South America beyond latitude 40°. Other rivers receiving their waters from rain, and being highest in the cold season, are marked by great differences between high and low water. They predominate in Southern Europe, and are exemplified by the Po and Tiber, and by some rivers in the United States. Other types may be constituted of rivers which become dried up or lost in their course, and of those which exist as streams for only a part of the year. They are found in desert regions.

Improvement of our Climate.—Mr. John C. Goodridge, Jr., has suggested a project for modifying the climate of the Atlantic coast by closing the Strait of Belle Isle, and advances the theory that this scheme is feasible as a problem in physical geography capable of an engineering solution. He argues that it is shown by charts that the great body of the "cold wall" comes to us through that strait. Newfoundland deflects the remainder of the Arctic current to the southeast. Here, pressing against the Gulf Stream, it veers southward in the form of a loop, and finally, running under it, goes on toward the equator. That part of the Gulf Stream that passes our shores has a course directly north and a little west, is deflected slightly toward the east by the coasts of South and of North Carolina, and thence turns more to the north again, when it is deflected by the cold current returning from the pole. When this