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1、第一次世界大战中AIRSHIP的背景知识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 an
2、d 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 g
3、as called balloonets. In the non-rigid, the envelopes shape is maintained by the pressure of the gas that fills it; there is no framework. The rigids 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
4、somelater 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, w
5、as 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 , knownasZeppelins. Zeppelin is a pr
6、oprietary, 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
7、apparent belief that it is generic. 3It has becomecustomary to refer to all Germanrigid 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 aluminu
8、m alloy used in Zeppelins. 4The vast amount of literature devoted to Germanairships 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
9、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
10、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.飞艇按其结构可分为软式、半硬式、硬式。(1) 软式飞艇艇体由主气囊和前后副气囊组成。气囊不仅要求密封，还要有相当 强度能承受一定的压强。 气囊上装有安全活门， 压强超过规定值时能自动放气保 证
11、气囊不被胀破。 主气囊内充浮升气体， 副气囊内充空气。 副气囊的作用是在不 排放主气囊内气体的条件下， 保持主气囊内外压强差为定值。 当飞艇爬高， 外界 大气压强降低时， 副气囊放气使主气囊增大容积， 从而保持主气囊原来的内外压 强差。当外界大气压强增大时， 向副气囊内充气使它膨胀， 从而压缩主气囊的容 积，使主气囊的压强仍能保持略高于外界大气压强。 设置前后副气囊还可调节浮 力中心的位置。仅向后副气囊充气时，重心后移，飞艇产生抬头力矩；反之，产 生低头力矩。(2) 半硬式飞艇气囊构造与软式飞艇相似，但在气囊下部增加刚性的龙骨架，组 成半硬式飞艇的艇体。(3) 硬式飞艇艇体由刚性骨架外罩蒙布或
12、薄铝皮构成整个艇体不密封，主要起维 持流线型和连接各部分的作用。 艇体内部由隔框分割成许多小气室， 每个小气室 内放有密封的小气囊， 内充比空气轻的气体。 在地面时， 小气囊没有完全胀满气 室。随着飞行高度增加，外界大气压强降低，囊内气体随之膨胀，在达到规定高 度时，气囊恰好胀满气室。 众多小气囊可提高飞艇的抗损性和安全性。 部分小气 囊受损，整个飞艇的浮力不会完全丧失。第一艘飞艇是法国的H 吉法尔于1852年制成的。1900年德国的齐柏林公司开始制造大型硬式飞艇，在第一次世界大战期间，曾多次用飞艇进行远程轰炸托福听力考试背景知识综合辅导 (三十五 )雪花 对于一片六角形雪片来说， 由于它表面
13、曲率不等 （有凸面、平面和凹面 ） ，各面上 的饱和水汽压力也不同， 因此产生了相互间的水汽密度梯度， 使水汽发生定向转 移。水汽转移的方向是凸面一平面一凹面， 也就是从曲率大的表面，移向曲率小 的表面。 六角形雪片六个棱角上的曲率最大， 边棱部分的平面次之， 中央部分曲 率最小。 这样， 就使六角形雪片一直处在定向的水汽迁移过程中。 由于棱角上水 汽向边棱及中央输送， 棱角附近的水汽饱和程度下降， 因而产生升华现象。 中央 部分由于获得源源不断的水汽而达到冰面饱和， 产生凝华作用。 这种凝华结晶的 过程不断进行，六角形雪片逐渐演变成为六棱柱状雪晶。这是假定外部不输送水汽的理想状况。 事实上，
14、事物与周围环境保持着密切的联 系，空气里总是或多或少存在着水汽的。 如果周围空气输人水汽较少， 少到不够 雪片的棱角向中央输送水汽的数量，那么雪片向柱状雪晶的发展过程继续进行。 在温度很低水汽很少的高纬和极地地区，便因为这个原因经常降落柱状雪晶。空气里水汽饱和程度较高的时候， 出现另外一种情况。 这时周围空气不断地向雪 片输送水汽， 使雪片快速地发生凝华作用。 凝华降低了雪片周围空气层中的水汽 密度，反过来又促进外层水汽向内部输送。这样，雪片便很快地生长起来。当水 汽快速向雪片输送的时候， 六个顶角首当其冲， 水汽密度梯度最大。 来不及向雪 片内部输送的水汽， 便在顶角上凝华结晶； 这时， 顶
15、角上会出现一些突出物和枝 杈。这些枝叉增长到一定程度，又会分叉。次级分叉与母枝均保持 60 的角度， 这样，就形成了一朵六角星形的雪花。在高山或极地的晴朗天气里， 还可见到一种冰针， 象宝石一样闪烁着瑰丽的光彩， 人们把它叫做钻石尘。冰针的生长有二种情况：一种是在严寒下（-30 C以下）湿度很小时水汽自发结晶的结果，另一种是在温度较高（-5 C左右）湿度较大时沿着 雪片某一条辅轴所在的顶角特别迅速生长的产物，是雪花的畸形发展托福听力考试背景知识综合辅导 （ 三十六 ）KEYSTONE SPECIESA keystone is the stone at the top of an arch th
16、at supports the other stones and keeps the whole arch from falling. A keystone species is a species on which the persistence of a large number of other species in the ecosystem depends.If a keystone species is extirpated from a system, the species it supported also will disappear, as will the other dependent species. Keystone species maybe top carnivores that keep prey in check, large herbivores that shape the habitat in which other
