Airship, lighter-than-air craft equipped with a bag containing a gas to lift the ship, a means of propulsion, means for adjusting buoyancy, and one or more gondolas for the crew, passengers, and power units. The bag invariably contains helium, although hydrogen was formerly used. The bag is elongated or streamlined to enable easy passage through the air. The means of propulsion usually comprises one or more engines and propellers. A means for releasing ballast, usually sand or water, may be used to increase buoyancy; and a means for releasing gas may be used to reduce buoyancy. An alternative is to inflate or deflate air bags called ballonets set inside the main gas bag; this alters the overall density of the airship. To steer the airship, the pilot uses one or more vertically hinged rudders; to control climb or descent, the pilot employs one or more horizontally hinged lifts

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