DAMS TO dam (originally, to stop up). It to 0,stFuct or restrain the flow of a liquid., In engineering, a slaq, is a, barrier to constrain. ox keep back a body of water.: Dams vary in struciure as widely as the physical character of: their, sites, thepurpOses for which' they are etuvtrected and the materials used. Every dam is, la special adaptation,. and may only be classified by referring it to some more °v ices., varying type. . , The Art 0 dam building is Coeval with civil- , taation when the stream frOm the mountains, itself in the arid ,plain, was diverted for irrigation.:,'The gardens , and , the water-. wheels •Of.Damascus are back,pf history. The Nile the Euphrates-Tigris plain, the Oxus and ,Jaxartes,.:and much of India, devel, oPed civiliaatiop the application of the waters to the land, and,-this will be..the history, of their It was a time of mighty en dgagpr engineering,, little uader-, stood in humid lands, demanding a: high atid,,a•feloSeie knit state. Cyrus tented the .42eleratitsi from Babylon:and an em-' pit*, fell regulated the floods of the Nile An the days of Joseph, and its oilewation is credtted•with the seven fat and the, seven leap yew,* by some antiquarians; and,: Bahr Jussuf (meaning, the water ,carrier) still; leads, to basin. Modern ,Egypt has built :the , barrage across : the Rosetta midi I)quietta,mouths of the Nile for the continuous, irrigation of the delta, and dair•at Assouan puts, under: service. more than 6,000,000 acres of new lands, nearly :doubling agricultural. Egypt. - r The 'old Hindu anicut turned ,the flow of ) formidable rivers into,,irrigation ,eanals• and as a-rockfill with long paved slope and.an down. stream, it serves, the engineers :o , India The ' ancient 4,anks5 of India, .are'. natural: dosed by a bank of earth, sortletimes of grettt.height and! miles in length,, formed probably by basket, carriage and con-, solid:iced by tramping. The: Veranum Tank still: in use has a .water area of 35 square miles held by a• darn of .clayey earth 12 miles long.
The protection of lands against.:flond byi embankments,, levees or belongs: to pnitnitirre!: timea. ., Along the . Yellow Rivet
("China's Sorrows) the Chinese built levees in double line with cross-banks, so that the failure of the front line at any point by undercutting or other cause would only inundate the check immediately behind. The Po and Adige of northern Italy were embanked in remote times and the history of these works is of great interest.
The basket-work dam of moderate height on alluvial streams was long since devised, and from this has evolved the modem bank pro tection of braided willow mattress in use on Western rivers. The primitive neighborhood grinding-mill led the head-race up-stream to a dam of simple type.
The utilities widen in approaching the mod em era. The stream flow of the growing season no longer suffices for the needs of irri gation and the surplus is impounded for larger areas and the time of need. The uncertain navigation of natural streams no longer satisfies commerce, and canal systems follow with exten sive storage and feed-water developments, and this again, with steam navigation, gives way to the direct improvement and the canalization of rivers by means of fixed or movable dams. Growing cities need water supplies, a demand now universal, and often extraordinary struc tures are required. Industries develop with the water power, the turbine makes higher heads available and changes the problem of dam con struction for power purposes; and finally comes the electric generator and distant transmission of power to satisfy further demands.
The United States contains water power capable of enormous development, in fact, more than tenfold all the power now in use. Next to the land itself the running streams are the most valuable economic asset. The future may well see an era of dam and reservoir building, in the humid as well as in the arid regions, for the better equalization of the flow of streams, in the interest of navigation, the reclamation of alluvial lands, the development of water power, and the creation of fisheries. The acre of water may be made even more valuable than the acre of land.