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This engine, built by the Glenn H. Curtiss Manufacturing Company of Hammondsport, New York, about 1907 is believed to be a Model C-4 or D-4 engine. The Model C-4 was designed specifically as a power plant for a U.S. military dirigible. It was built as a light aeronautical engine and was fitted with mechanical valves and auxiliary ports. A propeller was connected directly to its drive shaft. The Model D-4 was identical with the Model C-4 excepting that it was water cooled. The D-4 was designed as a lightweight engine recommended by Curtiss for marine applications where the added weight of about 34 kg (75 lb) for the water-cooling system was not objectionable. Both engine models were rated at 19 kW (25 shp). This motor, which has both air- (cylinder heads) and water-cooled (cylinder barrels), is believed to have powered a 1910 aircraft owned by Ralph M. Kinderman. Artist Curtiss Aeroplane and Motor Company. (Photo by Heritage Art/Heritage Images via Getty Images)