RAINS AND RIVERS. 103 



deiu' of tlieir operations, let us inquire how mucli rain falls annually 

 upon the water-sheds of one of these streams, as of the Mississippi ; 

 how much is carried off by the river; how much is taken up 

 by evaporation; and how much heat is evolved in hoisting up 

 and letting do^vn all this water. In another chapter we shall 

 inquire for the springs in the sea that feed the clouds with rain for 

 these rivers. If we had a pool of water one mile square and six 

 inches deep to be evaporated by artificial heat, and if we wished 

 to find out how much would be required for the pm-pose, we 

 should learn fi:om Mr. Joule's experiments that it would require 

 about as much as is evolved in the combustion of 30,000 tons of 

 coal. Thus we obtain (§ 271) our unit of measure to help us in 

 the calculation ; for if the number of square miles contained in 

 the Mississippi Valley, and the number of inches of rain that fall 

 upon it annually be given, then it wiU be easy to tell how many 

 of such huge measures of heat are set free during the annual opera- 

 tion 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. The area of the Mississippi Yalley is said by physical 

 Its area, and the la- ffeoOTaphcrs to embrace 982,000 SQuarc miles; and 



tent heat liberated o o i : . i ^ 



during the processes upou cvcry squarc milc there IS an annual average 

 of condensation ^am-fall of 40 iuches. Now if we multiply 982,000 

 by the number of times 6 will go into 40, we shall have the 

 number of om' units of heat that are annually set free among the 

 clouds that give rain to the Mississippi Yalley. Thus the imagina- 

 tion 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 Yalley with water is as 

 much as would be set free by the combustion of 30,000 tons of 

 coal midtiplied 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 pubhshed 

 by the Sm^geon General's Office at Washington in 1855, is the 

 authority on which we base om- 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 

 ui Tennessee, the annual fall of rain there to be 49 inches, the 

 annual evaporation 43, and the quantity of water that annually 



