William D. (William Duane) Ennis

William Duane Ennis (January 6, 1877–October 14, 1947) was an American mechanical engineer and educator. He earned a mechanical engineering degree from Stevens Institute of Technology in 1897 and later received a Doctor of Engineering degree. His career included engineering work in industry, teaching at the Polytechnic Institute of Brooklyn and the U.S. Naval Academy, wartime service with the U.S. Army Ordnance Department, and senior academic and consulting roles.

Ennis wrote about engineering, industrial organization, and aviation. His best-known book, Flying Machines Today (1911), introduced readers to the science, design, dangers, and possibilities of early aircraft, while other works included Applied Thermodynamics for Engineers and Works Management. He later served as the Alexander Crombie Humphreys Professor of Economics of Engineering at Stevens Institute of Technology until his retirement in 1944.