100 THE PHYSICAL GEOGRAPHY OF THE SEA. 



commercial world ; consequently they do not possess the interest 

 which, in the minds of men generally, is attached to the rest. The 

 three others of Asia drain 1,668,000 square miles, and run into 

 the Pacific; while the whole American system feed with their 

 waters and their commerce the Atlantic Ocean. These rivers, 

 with their springs, give drink to man and beast, and nourish with 

 their waters plants and reptiles, with fish and fowl not a few. 

 The capacity of their basins for production and wealth is without 

 limits. These streams are the great arteries of inland commerce. 

 Were they to dry up, political communities would be torn asun- 

 der, the harmonies of the earth would be destroyed, and that beau« 

 tiful adaptation of physical forces to terrestrial machinery, by 

 which climates are regulated, would lose its adjustment and run 

 wild, like a watch without a balance. 



271. We see these majestic streams pouring their waters into 

 Heat required to lift the sca, and from the sea we know those waters 

 ers. must come again, else the sea would be full. We 



know, also, that the sunbeam and the sea-breeze suck them up 

 again ; and it is curious to fancy such volumes of water as this 

 mighty company of ten great rivers is continually discharging 

 into the sea, taken up by the winds and the sun, and borne away 

 through the invisible channels of the air to the springs among the 

 hills that are the source of all rivers. This operation is perpetu- 

 ally going on, yet we perceive it not. It is the work of that in- 

 visible, imponderable, omnipresent, and wonderful agent called 

 heat. This is the agent which controls both sea and air in their 

 movements and in many of their offices. The average amount of 

 heat daily dispensed to our planet from the source of light in the 

 heavens is enough to melt a coating of ice completely encasing 

 the earth with a film 1|- in. in thickness.* Heat is the agent that 

 distills for us fresh water from the sea. It pumps up out of the 

 ocean all the water for our lakes and rivers, and gives power to 

 the winds to transport it as vapor thence to the mountains. And 

 though this is but a part 5f 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 va- 

 rious aspects. To assist in this undertaking I have appealed to 

 the ten greatest rivers for terms and measures in which some defi- 

 * Deduced from the experiments of Pouillet. 



