p.11

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11 1859

season will cause a rise of the waters not only in consequence of the increased quantity of rain, but by the diminished evaporation; and a succession of such seasons may cause a continuance of the high waters for several years. On the otherhand a dry season or several dry seasons together may in consequence of the diminished amount of falling water and the increased evaporation cause a corresponding depression in the surface of the lakes for another series of years.

The quantity of water annually discharged by a river in this part of the world is found to be about one half of the mean annual quantity of rain &c. that falls upon its basin or upon the surface of the country chained by the river and its tributaries. But the annual quantity of water precipitated in the form of rain, snow, hail, &c., is subject to very considerable variations. if we confine our observations to a single point it will be found to be almost double in a year of excessive rains. What it is in a season of drought. It has been calculated that the addition of three inches to the annual amount of rain over the country drained by the upper lakes would require the rise of the

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