National Geographic : 1931 May
MOVABLE DAMS AID NAVIGATION ALONG THE OHIO RIVER Between Pittsburgh and Cairo 46 such dams have been constructed by the Corps of Engineers, United States Army. They consist ofarow of heavy oak wickets faced at the top with steel and firmly hinged to a concrete base. These are raised orlowered byaderrick boat toaccommodate oa the stage of water. ILLINOIS LEADS THE UNION IN PRODUCTION OF FARM IMPLEMENTS The value of the State's annual production of agricultural machinery exceeds $1oo,ooo,ooo. A storage yard fulloftractors awaiting shipment near Chicago.