It was not until 1939, when Dr. Sanford Moss invented the turbo-supercharger, that high engine performance at altitudes above 30,000 feet became a matter of fact. The turbo-supercharger, a simple machine driven by the force of the engine exhausts, pumped air into the engines to give them sea-level pressure at high altitudes. This took care of the engines in the smooth-air substratosphere regions. Next came the human element. Human beings, like engines, cannot live without sufficient air. This brought about the development of the supercharged cabin for airplanes. In 1936 “Tommy” Tomlinson, a brilliant ex-Navy flier, started making experimental substratosphere flights for TWA in a specially designed plane. He found that the speed of a properly equipped airplane would increase some 36 per cent at 30,000 feet. At the same time Army engineers were experimenting with a Lockheed plane having a supercharged cabin. The Boeing Company, working in co-operation with Tomlinson, Transcontinental and Western Airways, and Pan American, developed the Boeing 307. The 307 was a big all-metal, low-wing monoplane with a pressurized, high-altitude cabin, which made possible flight at altitudes up to 20,000 feet. This was accomplished in a manner similar to that used in supercharging the engines. Engine-driven superchargers pumped air into the cabin-ventilating system and the atmosphere in the plane was kept at normal low-level pressure regardless of how high the plane flew. The Boeing 307 _Stratoliner_ was put into service by TWA and Pan American Airways in 1940 and marked a tremendous step forward in the speed and comfort of modern air travel. In 1941, fifteen years after the operation of the nation’s airlines had been turned over to private firms, air transport was approaching perfection. The first single-engined, two-passenger mail planes, cruising at 100 miles per hour, took thirty-three hours to make the coast-to-coast trip. Now giant luxury airliners were doing it in fifteen hours. In contrast to the frequent stops of the low-flying plane of the early days, the high-flying air transports of 1941 were making the journey with only three stops. Where the air traveler of the twenties rode in an uncomfortable seat in a cold, gas-smelling plane, and was lucky if he got a box lunch, the modern passenger rode in luxuriously upholstered chairs in a heated salon, and dined on hot fried chicken or steak with all the “trimmings.” Even more significant was the change in flight and safety aids. No longer did the pilot fly with his eyes on the railroad tracks and the family wash on the line below. Radio communication with the ground, continual weather information, and precision navigation and flight instruments changed all that and brought safety to air transport.