108 PHYSICAL GEOGllAPHY OF THE SEA, AND ITS METEOROLOGY. 



ration of condensing the rain for our hydrographic basin And 

 then, if we could tell liow many inches of this ram-water are 

 again taken np by evaporation, we should have the data for de- 

 termining 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 

 condensaiion i/.ere.-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 ram-tall ot 

 40 inches. Now if we multiply 982,000 by the number of times 

 6 will cro 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 ot 

 heat evolved from the vapours as they are condensed to supply 

 the Mississippi Valley with water is as much as ^^oul^\|^^ ^^^ 

 free by the combustion of 30,000 tons of coal multiphed 6,540,000 

 times Mr Joule, of Manchestei-, is our authority for the heat- 

 ing power of one pound of coal; the Army Meteorological Ee- 

 o'ister, 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 ram; and 

 the annals of the National Observatory show, according to the 

 observations made by Lieutenant Marr at Memphis m Tennes- 

 see 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 Biver.—U is estimated 

 that the tributaries which the Mississippi Eiver 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 ram 

 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, 

 durino- its combustion, give out as much heat as is rendered 

 latent^ammally in evaporating this water. Utterly msignihcant 



