Men have always wanted to fly like birds. Birds can fly easily because they are light， but men’s bodies are heavier.
Men first went up into the air in balloons. These are big bags， and they are filled with gas. Hydrogen① is a useful gas for balloons. It is lighter than air. Helium② is also lighter than air， but it costs a lot of money. So balloons were (and are) usually filled with hydrogen.
Balloons have to fly with the wind as they have no engines to drive them against it. Later， men made airships. These were balloons with engines， but they were not round. They were long， and the engines were at the back. They were also filled with hydrogen and some of them caught fire because the hydrogen escaped and the engines heated it. Then the airship was completely burnt in a few seconds.
Aircraft with wings now take people across the world. Powerful engines drive these machines across the sky. Some of the engines are like the engines of cars， but they are more powerful.
There is another kind of engine which we call the jet engine③. An English engineer invented the jet engine. In May 1941 his new engine was fixed in an aircraft， and the aircraft flew quite well. At the same time the Germans were also building a jet engine， but neither country told the other， of course.
Jet engines are very powerful. Usually two， three or four are enough for an aeroplane； but some big aircraft have six. Anyone in a moving jet plane can feel the power of the engines. Jet planes can travel faster than sound (Sound travels at about 1100 feet a second. That is about 760 miles an hour). As a flying jet plane leaves its noise behind it， we do not hear it until it has gone.
① hydrogenn. ［化］氢[U]
② heliumn. ［化］氦
③ jet enginen. 喷射引擎
飞行
人类总想像鸟儿一样地飞行。鸟儿能够轻易地飞行是因为它们的身体轻，但是人类的躯体可就重多了。
人类起初是通过气球进入空中的，这些气球是充满气体的大袋子。氢气对于气球来说是一种有用的气体，它比空气轻多了。氦也是一种比空气轻的气体，但是它太昂贵，因此气球通常都是装满氢气的。
由于气球没有发动机来作动力，所以气球不得不顺风飞行。后来人类创造了飞艇，它们是具有发动机的气球，但不是圆的，而是长的，发动机在后面。它们也是被充满氢气；其中一些不幸失火，是由于氢气泄漏后被发动机加热而引燃，几秒钟之后，飞艇就整个燃烧起来。
现在有冀的飞机可以带着人们穿世界，强有力的发动机载着机器横过天空，一些发动机就像小汽车的一样，但比它们的功率大得多。
还有一种我们称之为喷气式装置的发动机。一位英国工程师发明了这种喷气式发动机。1941年5月，他发明的新发动机安装在一 个飞机里，这个飞机飞行得很好。与此同时，德国人也建造了一个喷气式飞机发动机，当然两个国家都没告诉时方。
喷气式发动机马力很大。通常在一个飞机里装有两个、三个或四个已经足够了，但一些大的飞机要装六个。在一个正在运动着的喷气式飞机里，任何一个人都可以感觉到这些发动机的力量。喷气式飞机的速度要比声音的传播速度快(声音一秒钟传播约1100英 尺，也就是1小时传播760英里)。一个飞行着的喷气式飞机只有它飞过后我们才可听到噪声。
