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. Hydrogen is a useful gas for balloons. It is lighter than air. Helium 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 bemuse 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 engine. 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.
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。