MOST of the aircraft engines of the 1900 era were little more than adaptations of the existing reciprocating internal combustion engines used in motor cars, but progressive design improvements eventually led to the very efficient power plants of high horsepower which had by the 1940's enabled piston‐engined bomber aircraft to reach cruising speeds of 300 m.p.h. and piston‐engined fighter aircraft to attain level‐flight speeds in excess of 400 m.p.h. At this stage, piston engines had almost reached their peak performance and aircraft speeds could be significantly increased only by the adoption of some radically different type of engine. The development of the gas turbine resulted in this new power plant and aero gas turbines are now in the final stages of replacing piston engines as propulsive units for medium and large aircraft.