All this activity brought about the development of more comfortable cabin planes and led to a demand for large and small private ships. The small, light plane field expanded with amazing speed once there was a demand. In the late twenties Aëronca, Taylor, and Piper began to bring out safe, comfortable, and inexpensive planes. By the middle thirties flying schools and private landing fields were a common sight throughout America. As the light planes became popular, the training of private pilots developed into a big business. Flying lessons became an important source of income to aviators who heretofore had operated their little airfields on the revenue derived from sightseeing hops and an occasional charter trip. Student pilots became logical prospects for [Illustration: PIPER _CUB_ TWO-PLACE PERSONAL LIGHT PLANE. LAMINATED BIRCHWOOD PROPELLER FOUR-CYLINDER AIR-COOLED ENGINE METAL COWLING DUAL CONTROLS LANDING GEAR METAL WHEEL “PANTS” LIFT STRUTS WELDED STEEL TUBE FUSELAGE FRAME FABRIC FUSELAGE COVERING TAIL WHEEL RUDDER ELEVATOR ALUMINUM RIBS FABRIC COVERING STABILIZER] light planes and the more successful flying schools became sales agencies for the aircraft manufacturers. Students became expert fliers and graduated to instructors’ jobs. A number of these young instructors in turn bought light planes and started flying schools of their own. Thus, light plane flying spread like wildfire over the country. The light planes of the late thirties were mainly high-wing monoplanes. They were powered with light, air-cooled engines and were so designed that they had a high factor of safety. They were sturdily built and easy to fly. The average student was able to solo after eight or ten lessons, though real flying ability came only through constant practice. Light planes cost from $1,500 to $2,000. Many of them were equipped with accessories such as heaters, radios, navigation lights, and flight instruments. All of them had comfortably upholstered, enclosed cabins. In the years just before World War II light plane flying for business and pleasure was an accepted mode of travel for boys and girls as well as men and women of all ages. SUPERCHARGERS AND SUPER-AIRLINERS High above the earth, 14,000 to 20,000 feet, lies a region of smooth air called the substratosphere. [Illustration: BOEING 307 _STRATOLINER_] Pioneer fliers had reached this region years before, but its thin, rare air made life and movement impossible. Men had long looked to this smooth-air region as the ideal flight path--a path without rough air, or fog, or storm to slow their progress. But both they and their engines needed plenty of air for operation.