RAINS AND RIVERS. 107 



out of the ocean all the water for our lakes and rivers, and gives 

 power to the winds to transport it as vapour thence to the moun- 

 tains. And though this is but a part of the work which in the 

 terrestrial economy has been assigned to this mighty agent, we 

 may acquire much profitable knowledge by examining its opera- 

 tions here in various aspects. To assist in this undertaking I 

 have appealed to the ten greatest rivers for terms and measures 

 in which some definite idea may be conveyed as to the magni- 

 tude of the work and the immense physico-mechanical power of 

 this imponderable and invisible agent called heat. Calculations 

 have been made which show that the great American lakes con- 

 tain 11,000 cubic miles of water. This, according to the best 

 computation, is twice as much as is contained in all the other 

 fresh-water lakes, and rivers, and cisterns of the world. The 

 Mississippi Eiver does not, during a hundred years, discharge 

 into the sea so large a volume of water as is at this moment 

 contained in the great northern lakes of this continent ; and yet 

 this agent, whose works we are about to study, operating through 

 the winds, has power annually to lift up from the sea and pour 

 down upon the earth in grateful showers fresh water enough to 

 fill the great American lakes at least twenty times over. 



272. Rain-fall in the Mississipj)i Valley. — That we may be 

 enabled the better to appreciate the power and the majesty of 

 the thermal forces of the sun, and comprehend in detail the mag- 

 nitude and grandeur of their operations, let us inquire how 

 much 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 down 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 evapo- 

 rated by artificial heat, and if we wished to find out how much 

 would be required for the purpose, we should learn from 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 will be easy to tell how many of 

 sucli huge measures of heat are set free during the annual ope- 



