The first hot air balloon was made by two French brothers, Joseph and Etienne de Montgolfier. Both brothers worked in the family

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问题         The first hot air balloon was made by two French brothers, Joseph and Etienne de Montgolfier. Both brothers worked in the family business, which was paper manufacturing. The study of flight interested them, and they soon started experiments in this area.
        Joseph was the first to work on hot air balloons, in 1771. He noticed that, when certain gases were burned, it caused materials to rise. Later, Etienne worked with him. At last, they practised with paper bags. They found that hydrogen gas particularly good for this purpose.
        On 4 June, 1783, they produced a linen and paper balloon in the shape of a sphere about 100 meters in diameter. The balloon had an opening to receive the heated air rising from a fire of burning wool and wet straw on the ground. There were some fireproof materials inside the bag to prevent it catching fire. The balloon rose into the air to a height of 2000 meters, and stayed there for about 10 minutes. The only problem with this experiment was terrible smell from the fire of wool and straw!
        Encouraged by this success, the de Montgolfier brothers made a new balloon for passengers. The brothers put a sheep, a duck and a cockerel in it to test it for safety. On 19 September, 1783, the balloon took off from the King of France’ s palace at Versailles(凡尔赛宫). The flight was a success. The animal travelled over three kilometers before landing in a forest. On 21 November of the same year, the brothers launches another balloon. This time, there were passengers on board, a scientist, Jean Pilate de Rozier, and a nobleman, the Marquis d’ Arlandes. The hot air balloon flew for 25minutes over the city of Paris before landing safely. A new means of transport was bom!
Why was the brothers’ invention called a "hot air" balloon?______________________

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答案Because it can fly rely on the heated air.

解析 由文章第二段第二句话“when certain gases were burned,it caused materials to rise”可知,当一定的气体燃烧(加热)时,可以引起某些材料的上升。而“热气球”就是利用加热气体的原理,使之飞行的,因此叫作“hot air”balloon。
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