Page:A Life of Matthew Fontaine Maury.pdf/183

Rh fresh water on the surface of the globe. . . . 'This great chain of lakes has been estimated to contain 11,000 cubic miles of fresh water.' To give an idea of the amount of this vast body of water, after explaining his mode of careful measurement of the Mississippi river at Memphis, the lecturer said, 'that the lakes contained more water than the Mississippi discharges into the Gulf of Mexico in one hundred years.' In other words, were all the waste from the lakes by evaporation and other causes to be cut off, and a sluice to be opened, the size of the Mississippi channel, it would flow a century in draining these vast inland seas.

"The lakes themselves contain a surface of 2000 square miles, and they drain a territory, or their water-shed, of 50,000 square miles.

"From the above figures it may well be imagined that this vast extent of fresh water maintains and subserves most important influences and purposes, as pertaining to the climate, temperature, and hygiene of their vast surrounding or tributary region.

"There is a difference, not universally well understood, between the effects on climate incident to their being fresh or salt. Were the lakes salt, like the Caspian, they would give to their region a warmer temperature; the winters, latitude remaining the same, would be milder, the summers more sultry. This arises simply from the effects of evaporation: fresh water, according to the observations of Professor Chapman at Montreal, evaporates fester than salt, carrying off more heat.

"The amount of the evaporation of the lakes was illustrated as follows. The Cataract of Niagara represents the excess of the precipitation, by rains upon the lakes and their watershed, over the evaporation.

"Thus in the lakes the evaporation may be represented as five or six times the water at Niagara, and to produce this