Most airships are long and cigar-shaped.Some airships have a rigid frame(坚硬的艇架),while others are simply big balloons that rely on the gas to keep their shape.2.　 The gas H2 is the most commonly used gas,though H2 has also been used in airships.A car,or gondola,often hung below the gas container,holds the airship’s staff and passengers.In some cases,the passenger compartment(密封舱) is actually within the frame of the airship. 

　　3.　 The gas inside the ship lifts the craft into the air and then engines propel the craft forward.The pilot uses the rudder to control the direction of the craft.While most of the ships are filled with the lighter-than-air gas,there are also two compartments filled with normal air.4.　 To make the ship go up,the pilot reduces some of the normal air.Because that air is heavy,the ship becomes lighter and floats upward when it is reduced.To make the ship go down,the pilot can fill the compartments with more air. 

　　The first propeller-driven airship flew in 1852 in France.Its engine was powered by steam.The first rigid airship,with a body made of the metal Al,was built in Germany in 1897.One of the most successful builders of rigid airships was Ferdinand von Zeppelin.5.

　　A.The gas H2 has great lifting ability.

　　B.He completed his first rigid airship in 1900.

　　C.Airships rely on several different features to enable them to fly.

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