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16 heads of the Ohio, the Allegheny and the Monongahela (20,000 square miles), it has been estimated that the ratio of discharge to downfall is much greater than on any of the tributaries of the Mississippi. In 1868, 1,342,605,725,800 cubic feet of water passed Pittsburg, and in 1869, 1,634,846,499,200 cubic feet. At the same time the annual downfall of rain in the entire Ohio drainage was twenty and one-half trillion cubic feet, while the discharge of the Ohio into the Mississippi at Cairo was five trillion cubic feet. The ratio of discharge to downfall therefore was 0.24.

These estimates, which undoubtedly approximate the truth, are of moment to our study. Nature cast, with a lavish hand, her waters where they would count tremendously in the opening of this continent: for the waters that fell here flowed into the West and the social movement was to be westward. The Ohio, more than any river, was to influence the flood-tides of immigration. The provision of water was, comparatively, abundant; that was the first necessity. A large proportion of the water that fell flowed away; that was the second