ration of condensing the rain for our hydrographic basin. And then, if we could tell how many inches of this rain-water are again taken up by evaporation, we should have the data for determining the number of these monstrous measures of heat that are employed for that operation also.
273 Its area, and the latent heat liberated during the processes of condensation there.—The area of the Mississippi Valley is said by physical geographers to embrace 982,000 square miles; and upon every square mile there is an annual average rain-fall of 40 inches. Now if we multiply 982,000 by the number of times 6 will go into 40, we shall have the number of our units of heat that are annually set free among the clouds that give rain to the Mississippi Valley. Thus the imagination is startled, and the mind overwhelmed with the announcement that the quantity of heat evolved from the vapours as they are condensed to supply the Mississippi Valley with water is as much as would be set free by the combustion of 30,000 tons of coal multiplied 6,540,000 times. Mr Joule, of Manchester, is our authority for the heating power of one pound of coal; the Army Meteorological Register, compiled by Lorin Blodget, and published by the Surgeon General's Office at Washington in 1855, is the authority on which we base our estimate as to the average annual fall of rain; and the annals of the National Observatory show, according to the observations made by Lieutenant Marr at Memphis in Tennessee the annual fall of rain there to be 49 inches, the annual evaporation 43, and the quantity of water that annually passes by in the Mississippi to be 93 cubic miles. The water required to cover to the depth of 40 inches an area of 982,000 square miles would, if collected together in one place, make a sea one mile deep, with a superficial area of 620 square miles.
274. Annual discharge of the Mississippi River.—It is estimated that the tributaries which the Mississippi River receives below Memphis increase the volume of its waters about one-eighth, so that its annual average discharge into the sea may be estimated to be about 107 cubic miles, or about one-sixth of all the rain that falls upon its water-shed. This would leave 513 cubic miles of water to be evaporated from this river-basin annually. All the coal that the present mining force of the country could raise from its coal measures in a thousand years would not, during its combustion, give out as much heat as is rendered latent annually in evaporating this water. Utterly insignificant
