William D. (William Duane) Ennis

William D. Ennis, also listed as William Duane Ennis, was born in 1877 and wrote a substantial body of engineering books in the early 1900s. Library and archive records credit him with works including Applied Thermodynamics for Engineers, Flying Machines Today, Works Management, Linseed Oil and Other Seed Oils, and later Thermodynamics, Abridged.

The surviving title pages of his books give a useful picture of his career. They identify him as a member of the American Society of Mechanical Engineers, a professor of mechanical engineering at the Polytechnic Institute of Brooklyn, and later as a consulting engineer. One later book also describes him as formerly a professor of mechanical and marine engineering in the postgraduate department of the United States Naval Academy.

What stands out about his writing is its range. Ennis wrote for engineers, students, and practical readers at a time when industry and flight were changing quickly, and his books suggest a strong interest in making technical ideas usable rather than abstract. I couldn't confirm more personal biographical details from the sources I found, so this overview stays focused on his documented publications and professional roles.