§ 272, 273. RAINS AND RIVERS. 201 



iiite idea may be conveyed as to the magnitude of the work and 

 tlie immense pliysico-mechanical power of this imponderable and 

 invisible agent called heat. Calculations have been made which 

 show that the great American lakes contain 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 yerfrs, 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 lakes at least twenty times over. 



272. That we may be enabled the better to appreciate the pow- 

 iiain-faii in the Mis- ^^ ^^'^^ ^hc majesty of the thermal forces of the sun, 

 sissippi Valley. ^^^^ comprchcud in detail the magnitude and grand- 

 eur of their operations, let us inquire how much rain falls annu- 

 ally upon the water-sheds of one of these streams, as of the Missis- 

 sippi ; 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 in- 

 quire 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 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 Yalley, and the number of inches of rain that fall 

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

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

 eration 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 determin- 

 ing the number of these monstrous measures of heat that are em- 

 ployed for that operation also. 



273. The area of the Mississippi Yalley is said by physical ge- 

 ographers to embrace 982,000 square miles ; and upon every 



