>Diagram of a modern gas balloon. (1) Landing-run line. (2) Valve cord. (3) Rip cord. (4) Equator. (5) Rip panel. (6) Safety device. (7) Valve. (8 and 9) Net. (10) Rain deflector. (11) Appendix. (12) Load ring. (13) Bask ropes. (14) Anchor cable.
A balloon is an unpowered, nonrigid, lighter-than-air craft comprising a bulbous envelope containing the lifting medium and a payload-carrying basket or gondola suspended below. Balloons may be captive (secured to the ground by a cable, as in the barrage balloons used during World War II to protect key installations and cities from low-level bombing) or free-flying (blown along and steered at the mercy of the wind). Lift may be provided either by a gas (usually hydrogen and noninflammable helium) or by heating the air in the envelope. A balloon rises or descends through the air until it reaches a level at which it is in equilibrium in accordance with Archimedes' principle. In this situation the total weight of the balloon and payload is equal to that of the volume of air which it is displacing.
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.