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  得听力者得托福，备考托福听力考试中，除了基本的听力练习之外，还需要多些参考一些背景知识，这有助于各位考生们能取得高分数。下面，上海新航道托福培训 小编给大家分享一篇关于学科听力背景知识-第一次世界大战的飞艇，希望对各位备考托福听力考试有所帮助。

  By?the?start?of?the?Great?War?the?airship?had?been?developed?into?two?main?types?-?rigid?and?non-rigid?(technically,?the?latter?is?more?accurately?described?as?pressure-rigid).?Cylindrical?in?cross-section,?both?were?given?buoyancy?by?gas?and?motion?by?engine-driven?propellers?and?were?controlled?by?vertical?rudders?and?horizontal?elevators.

  ?In?the?rigid?type,?a?solid?framework,?which?might?be?likened?to?a?skeleton,?supports?an?external?covering?of?fabric?called?the?envelope?(a?very?few?experimental?types?had?a?metallic?covering).?Within?the?framework?are?contained?bags?of?gas?called?balloonets.?In?the?non-rigid,the?envelope's?shape?is?maintained?by?the?pressure?of?the?gas?that?fills?it;?there?is?no?framework.?The?rigid's?control?car?and?engines?are?suspended?from?the?framework;?in?the?non-rigid?they?are?attached?directly?to?or?suspended?directly?from?the?envelope.?(In?some?later?rigids?the?engines?were?mounted?internally,?driving?the?propellers?by?transmission?belts.)

  ?There?was?an?intermediate?type,?now?long?vanished,?called?the?semi-rigid.?It?had?a?pressure-rigid?envelope?but?a?solid?keel.

  ?The?rigid?airship,?employed?so?extensively?by?Germany?during?the?Great?War,?was?perfected?soon?after?the?turn?of?the?century?by?a?former?Wurttenberg?army?cavalry?officer,?Ferdinand?Adolf?August?Heinrich?Graf?von?Zeppelin,?who?had?been?inspired?by?a?balloon?ascent?he?had?made?in?the?United?States?on?19-Aug-1863.?2?His?craft?were,?naturally?,?known?as? Zeppelins.?"Zeppelin"?is?a?proprietary,?or?trade,?name?(such?as?Kodak,?Ferris?Wheel?and?Autogiro),?and?is?applied?properly?only?to?craft?constructed?by?Luftshiffbau?Zeppelin?G.m.b.H.?or?firms?licensed?to?use?its?patents.?It?should?thus?be?spelled?with?a?capital?Z;?modern?writers?who?spell?it?in?lower?case?are?mistaken?in?their?apparent?belief?that?it?is?generic.?3

  ?It?has?become?customary?to?refer?to?all?German?rigid?airships?of?the?Great?War?as?Zeppelins,?but?in?fact?not?all?were.?Some?were?constructed?by?a?rival?firm,?Luftschiffbau?Schutte-Lanz?G.m.b.H.,?which?employed?a?framework?of?laminated?plywood?instead?of?the?aluminum?alloy?used?in?Zeppelins.?4

  ?The?vast?amount?of?literature?devoted?to?German?airships?can?easily,?and?has,?led?to?the?belief?that?these?were?the?dominant?LTA?craft?of?the?Great?War.?Actually,?the?British?navy?was?the?greatest?exponent?of?the?airship,?receiving?more?than?200?non-rigids?during?1915-1918?for?anti-submarine?patrolling.?This?type?was?also?used,?although?in?far?fewer?numbers,?by?the?U.S.?Navy?and?the?French?and?Italian?armies?and?navies.?The?German?army?and?navy?also?had?a?handful.

  ?The?rigid?airship?no?longer?exists;?none?has?been?built?since?the?1930s.?5?The?demise?is?usually?blamed?on?the?Hindenburg?disaster?6?although?that?had?been?preceded?by?the?loss?of?a?number?of?American,?British,?French?and?Italian?rigids.?All?airships?flying?today?are?non-rigid?types,?popularly?known?as?blimps.

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