102 THE PHYSICAL GEOGRAPHY OF THE SEA. 



Its area, and the la- squaie mile there is an annual average rain-fall of 40 



tent heat liberateci ... ^-_ . , - • i /-ino /\/-irv i j^i 



diu-ing the process- mclies. JNow u we muiUplj yo2,(JUU bj the num-' 

 thei-e. ber of times 6 will go into 40, we sliall have the num- 



ber of our units of heat that are annually set free among the 

 clouds that give rain to the Mississippi Yallej. Thus the imag- 

 ination is startled, and the mind overwhelmed with the announce- 

 ment that the quantity of heat evolved from the vapors as they 

 are condensed to supply the Mississippi Yalley 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 Eegister, compiled by Lorin Blodget, and publish- 

 ed by the Surgeon General's Office 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, 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. It is estimated that the tributaries which the Mississippi 

 Annual discharge of I^ivcr reccivcs bclow Mcmphis increase the volume 

 the Mississippi River. ^£» ^^.g ^g^^^pg about ouc 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 evap- 

 orated from this river-basin annually. All the coal that the pres- 

 ent 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 are the sources of man's mechanical 

 powers when compared with those employed by nature in moving 

 machinery which brings the seasons around and preserves the har- 

 monies of creation ! 



275. The amount of heat required to reconvert these 513 cubic 

 Physical adaptations, milcs of raiu-watcr into vapor and bear it away, 

 had accumulated in the Mississippi Yalley faster than the earth 



