All About Engines

342 All About Engines occurs when the moist winds blow over mountain ranges. Here, in regions of lower atmospheric pressure, the air expands, and, in expanding, cools. And as the quantity of water vapour that air can carry depends upon the temperature, the seaward slopes of mountain ranges are bathed with moisture. The rain which falls upon the land becomes sepa- rated, generally speaking, into three portions. The first sinks deeply into the earth’s crust, forming underground waters which feed springs and wells, and often form the source of streams and rivers. The second remains for a time on the surface and is re-evaporated directly or through the breath- ing of plants. The third runs off the surface, feeds streamlet and river, and finally reaches the sea. It is a wonderful fact, that in this great cycle there is no waste. The water circulates con- tinuously from sea to air and down to earth, and back again to air or sea. It is the third portion of which man makes use to produce power. He builds great walls or dams across the valleys, and prevents the water running away until it has paid toll. From these reservoirs he conducts it through pipes or channels to water wheels or turbines, so that in fall- ing from a higher to a lower level it may do the work of which he stands in need. A cubic foot of water weighs 62-5 lb., and leaving friction out of account, every cubic foot falling from a height of 10 feet will perforin 625 ft.-lb. of work. This work may be used