advances in engineering practices. Ejector exhausts The Merlin consumed an enormous volume of air at full power (equivalent to the volume of a single-decker bus per minute), and with the exhaust gases exiting at it was realised that useful thrust could be gained simply by angling the gases backwards instead of venting sideways. During tests, 70 pounds-force (310 N; 32 kgf) thrust at , or roughly was obtained which increased the level maximum speed of the Spitfire by . The first versions of the ejector exhausts featured round outlets, while subsequent versions of the system used "fishtail" style outlets which marginally increased thrust and reduced exhaust glare for night flying. In September 1937 the Spitfire prototype, K5054, was fitted with ejector type exhausts. Later marks of the Spitfire used a variation of this exhaust system fitted with forward-facing intake ducts to distribute hot air out to the wing-mounted guns to prevent freezing and stoppages at high altitudes, replacing an earlier system that used heated air from the engine coolant radiator. The latter system had become ineffective due to improvements to the Merlin itself which allowed higher operating altitudes where air temperatures are lower. Ejector exhausts were also fitted to other Merlin-powered aircraft. Supercharger Central to the success of the Merlin was the supercharger. A.C. Lovesey, an engineer who was a key figure in the design of the Merlin, delivered a lecture on the development of the Merlin in 1946; in this extract he explained the importance of the supercharger: As the Merlin evolved so too did the supercharger; the latter fitting into three broad categories: Single-stage, single-speed gearbox: Merlin I to III, XII, 30, 40, and 50 series (1937–1942). Single-stage, two-speed gearbox: experimental Merlin X (1938), production Merlin XX (1940–1945). Two-stage, two-speed gearbox with intercooler: mainly Merlin 60, 70, and 80 series (1942–1946). The Merlin supercharger was originally designed to allow the engine to generate maximum power at an altitude of about . In 1938 Stanley Hooker, an Oxford graduate in applied mathematics, explained "... I soon became very familiar with the construction of the Merlin supercharger and carburettor ... Since the supercharger was at the rear of the engine it had come in for pretty severe design treatment, and the air intake duct to the impeller looked very squashed ..." Tests conducted by Hooker showed the original intake design was inefficient, limiting the performance of the supercharger. Hooker subsequently