Designed by Roy Fedden of Cosmos Engineering and built at Bristol by Brazil-Straker under the direction of Roy Fedden, and first run in July 1917. The Cosmos Mercury fourteen-cylinder twin-row air-cooled radial Mercury featured an unusual crankshaft and connecting rod arrangement that dispensed with the more normal design of a single master rod linking to individual rods for each cylinder. It produced 347 horsepower (259 kW). It was said to run well without vibration and set an unofficial time to climb record while fitted to a Bristol 21A Scout F.1, the aircraft achieving 10,000 ft (3,000 m) in 5.4 minutes and 20,000 ft (6,000 m) in 16.25 minutes.
The name was re-used by Fedden for the later nine-cylinder Bristol Mercury radial engine.