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 mountains. 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 operations 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 magnitude 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 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 River 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 Mississippi 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 magnitude 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 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 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 such huge measures of heat are set free during the annual ope-
