[Footnote a: Concurrently with the Wrights' first engine work, Manly was developing the engine for the Langley Aerodrome, and a comparison of the Wrights' engine development with that of Manly is immediately suggested, but no meaningful comparison of the two efforts can be drawn. Beyond the objective of producing a power unit to accomplish human flight and the fact that all three individuals were superb mechanics, the two efforts had nothing in common. The Wrights' goal was an operable and reasonably lightweight unit to be obtained quickly and cheaply. Manly's task was to obtain what was for the time an inordinately light engine and, although the originally specified power was considerably greater than that of the Wrights, it was still reasonable even though Manly himself apparently increased it on the assumption that Langley would need more power than he thought. The cost and time required were very much greater than the Wrights expended. He ended up with an engine of extraordinary performance for its time, containing many features utilized in much later important service engines. His weight per horsepower was not improved upon for many years. The Wrights' engine proved its practicability in actual service. The Manly engine never had this opportunity but its successful ground tests indicated an equal potential in this respect. A description of the Langley-Manly engine and the history of its development is contained in _Smithsonian Annals of Flight_ number 6, "Langley's Aero Engine of 1903," by Robert B. Meyer (xi+193 pages, 44 figures; Smithsonian Institution Press, 1971)] It is not possible to state the exact quantities of each engine that the Wrights produced up to the time that their factory ceased operation in 1915. Chenoweth gives an estimate, based on the recollection of their test foreman, of 100 vertical 4s and 50 6s. My estimate (see page 2) places the total of all engines at close to 200. Original Wright-built engines of all four of these basic designs are in existence, although they are rather widely scattered. The Smithsonian's National Air and Space Museum has examples of them all, including, of course, the unique first-flight engine. Their condition varies, but many are operable, or could easily be made so. Among the best are the first-flight engine and the last vertical 6, at the Smithsonian, the first vertical 6, at the United States Air Force Museum, and the vertical 4, at the Carillon Park Museum.