英语科普文选教程第一册第5课
People have always wanted to fly like birds. Birds can fly easily because they are light; but men’s bodies are heavier.
People 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. Helium is also lighter than air, but it costs a lot of money. So balloons were, and are, usually filled with hydrogen.
Helium was first discovered in the sun. After that it was also found on the earth.
Hot air is also light and some early balloons used hot air. You have seen a hot smoke rising over a fire. Bits of paper do the same. The hot air takes them up. Some balloons carried fires under them; then they took people up into the sky. It was not a safe form of travel.
Hydrogen burns easily in oxygen or in the air. So if the hydrogen has escaped, it might burn. But this does not happen often. There is no fire in the usual form of balloon.
Balloons have to fly with the wind. They have no engine to drive them against the wind. Later, people made air ships. 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. The hydrogen escaped and the engines heated it; then the air ship was completely burnt in a few seconds.
Aircrafts 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 motorcars, but they are more powerful.
There is another kind of engine which we call a jet engine. Sir Frank Whittle 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 German was 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. But some big aircrafts have six. Anyone in the airplane feels the power of the engine; jet planes travel at about 1000 kilometers an hour. Some jets travel faster than that; they travel faster than sound. Sound travels at about 330 meters a second; that is about 1220 kilometers an hour. Then we shall not hear them until they have gone; they will leave their noise behind them.
