Page:Physical Geography of the Sea and its Meteorology.djvu/132

106 all be regarded as belonging to one hydrographic basin, for a canoe may pass inland from any one to either of the others with out portage. Of these hydrographic basins, three, including an area of 3,916,000 square miles, are American; six, which contain an area of 3,772,000 square miles, belong to Asia, one to Africa, and none to Europe. The three largest rivers of Asia, the Yenisei, Obi, and Lena (2,104,000 square miles), discharge their waters into the Arctic Ocean; their outlets are beyond the reach of the 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 asunder, the harmonies of the earth would be destroyed, and that beautiful adaptation of physical forces to terrestrial machinery, by which climates are regulated, would lose its adjustment, and the seasons would run wild, like a watch without a balance.

271. Heat required to lift vapour for these rivers.—We see these majestic streams pouring their waters into the sea, and from the sea we know those waters 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 marching down to the sea, taken up by the winds and the sun, and borne away again through the invisible channels of the air to the springs among the hills. This operation is perpetually going on, yet we perceive it not. It is the work of that invisible, 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 1½ in. thickness. Heat is the agent that distils for us fresh water from the sea. It pumps up