National Geographic : 1952 Aug
ment with chemicals that precipitate out the salts, or by sand filtering. Through millions of years earth's water sup ply has followed a regular cycle-evaporating countless times from the oceans, rising aloft to condense and form clouds in the sky, falling again as rain, then quickly running back to the oceans or seeping slowly downward into the earth. In some cases it may remain in the ground thousands of years before reaching surface water bodies or evaporating. In others, it by-passes the cycle either by falling as rain directly into the sea or by evaporating far in land before it ever reaches the oceans. Evaporation from the oceans has been esti mated at 80,000 cubic miles of water a year more than 88 million billion gallons. Other tons of water return to the air by transpira tion-as vapor given off from the leaves of plants after the water has been absorbed from the ground by the roots and used in the grow ing process. Where Our Water Originated Many scientists hold that when the earth was new it had a great deal more water in its atmosphere than now, and that this fell as rain to fill the oceans; additional water was locked up in the rocks of the interior. Others believe that all the earth's water came from within, by way of hot springs. Wherever it came from, we still have it all. Of this huge supply of water, however, only a fraction is available for man's use in lakes, rivers, and natural underground reservoirs. More than 95 percent of our planet's water is in the oceans, and much of the rest is frozen in the great icecaps of Greenland and Antarc tica, virtually all of it useless to man unless he can find a cheaper way to desalt the sea or melt the glaciers. <- "Little Drops of Water . .." Tiny raindrops, multiplied by the billion, can do tremendous damage to unprotected soil. Their bullet like impact loosens earth, which running water washes away. Ground cover's importance as an erosion con trol is illustrated below: soil capped by rocks or sticks stands in pedestals; elsewhere rain has taken a two-inch bite. 272 U. S . Soil Conservation Service _n ^-e .- - _ it_11a.v"W -- as 1. m-. CI--rrffAMAi "