William D. (William Duane) Ennis

Born in 1877, William Duane Ennis was an American mechanical engineer and technical author whose books reached across several fast-growing fields of his time. Records for Applied Thermodynamics for Engineers identify him as a professor of mechanical engineering at the Polytechnic Institute of Brooklyn and a member of the American Society of Mechanical Engineers.

Ennis wrote clear, instructive works including Applied Thermodynamics for Engineers, Thermodynamics, Abridged, Works Management, and Flying Machines Today. That range suggests a writer interested not only in engines and heat, but also in industrial efficiency and the new possibilities of flight.

His work now survives mainly through library and public-domain collections, where it still offers a window into how engineers and students learned about power, manufacturing, and technology in the early 1900s. A reliable portrait image could not be confirmed from the sources reviewed here.