The Cirrus and Hermes or Cirrus-Hermes are a series of British aero engines manufactured, under various changes of ownership, from the 1920s until the 1950s. The engines were all air-cooled, four-cylinder inline types, with earlier ones upright and later designs inverted.

Major Frank Bernard Halford CBE FRAeS was an English aircraft engine designer. He is best known for the series of de Havilland Gipsy engines, widely used by light aircraft in the 1920s and 30s.