Man has long wanted to fly. He saw birds, envied them and tried to imitate them. Over the

ages, countless attempts were made: men constructed wings, fastened them to their arms and legs and jumped off towers and hilltops. These "bird-men" flapped (拍动) their wings for a short space of time and then fell to the ground.

【B1】What was not realized in those early years was that birds have muscles very much stronger, in proportion to their size, than men. Human limbs cannot provide sufficient strength to lift the body off the ground.【B2】The secret of flight did not lie in the making of wings, but in discovering the right kind of power, and how to use it.

In the 18th century, the invention of the hot-air balloon by the Montgolfier brothers of France was seen as a great step forward,【B3】But balloons and the cigar-shaped airship, which was invented slightly later, did not solve the problem of flying because they had no means of power or control. So the difficulty remained: a true flying machine which was heavier than air and capable of carrying people was still to be invented.【B4】Experiments were carried out in many countries, sometimes with models driven by steam engines, but these were too heavy to be used in an airplane with a pilot. The answer finally came at the beginning of this century with the invention of the internal combustion engine (内燃机)--the kind used in motor cars. Here at last was a powerful, yet comparatively light engine, driven by petrol and capable of being fitted into an airplane.

In 1903, two Americans, the brothers Wilbur and Orville Wright, flew a powered airplane for the first time. Their success encouraged designers everywhere.【B5】Although there were now newer, different problems, mainly to do with safety and the training of pilots, progress was rapid. These were exciting days and interest was intense. The age of airplane arrived.

What was not realized in those early years was that birds have muscles very much stronger, in proportion to their size, than men.