Gas balloon

A lighter-than-air aircraft lifted by a gas less dense than the surrounding air. Unlike a hot-air balloon, it relies on gas buoyancy rather than heating its lifting gas.

Buoyancy: The upward force on an object immersed in a fluid, equal to the weight of fluid displaced. A gas balloon rises when its displaced air weighs more than the balloon and its contents.
Montgolfier brothers: Joseph-Michel and Jacques-Étienne Montgolfier, French inventors associated with the first publicly demonstrated hot-air balloons. Their 1783 flights preceded hydrogen ballooning and helped launch the era of balloon flight.
Scientific ballooning: The use of balloons to carry instruments or researchers for atmospheric and other scientific investigations. Their ability to reach high altitudes makes them useful for measurements above ordinary aircraft ceilings.
Hot-air balloon: A balloon lifted by heating the air inside its envelope until it is less dense than surrounding air. It produces lift by temperature difference, whereas a gas balloon uses a separate low-density gas.
Siege of Paris (1870–1871): The Prussian siege of the French capital during the Franco-Prussian War, when balloons carried people and mail beyond the encirclement. Gas balloons became a practical means of escape and communication during the siege.
Lifting gas: A low-density gas used to provide lift in an aircraft or other buoyant system. Hydrogen and helium lift the envelope because they are less dense than air.
Jacques Charles: French scientist and balloonist who made the first successful hydrogen balloon flight in 1783. His flight demonstrated that hydrogen could lift a balloon far beyond a hot-air envelope.
Weather balloon: An uncrewed balloon carrying instruments to measure atmospheric conditions, often released by meteorological services. Weather balloons are a widespread modern use of gas-filled balloon technology.
Airship: A powered, steerable lighter-than-air aircraft with a structural or non-rigid gas envelope. Unlike a free balloon, an airship has propulsion and directional control.
Balloon Corps: The Union Army organization that operated observation balloons during the American Civil War. Its work showed how gas balloons could provide military reconnaissance from above.

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BlimpCompared with: A gas balloon shares a blimp’s source of buoyancy but lacks its powered, steerable flight.