If the pilot of a gas balloon wishes to ascend, he throws ballast (usually sand) over the side, thus reducing the overall density of the craft; to descend he releases some of the lifting gas through a small valve in the envelope. The altitude of a hot-air balloon is controlled using the propane burner which heats the air; increased heat causes the craft to rise; turning off the burner gives a period of level flight followed by a slow descent as the trapped air cools.
The Montgolfier brothers' hot-air balloon became the first manned aircraft in 1783, and in the same year the first gas balloon was flown by Jacques Charles. In 1785 Jean Blanchard piloted a balloon across the English Channel. In due time the powered balloon, or airship, was developed, though free balloons have remained popular for sporting, military, and scientific purposes. The upper atmosphere has been explored using unmanned gas balloons, and radiosonde balloons are in regular meteorological use. In 1931 Auguste Piccard pioneered high-altitude manned flights.
Related entry history of flight
Related category AERODYNAMICS AND AERONAUTICS
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