-15. THE FIRST MAN-MADE OBJECTS IN THE SKY
Long before men learned how to fly, they sent objects soaring through the air. The arrow dates from the Stone Age. The ancient Chinese flew kites. The early inhabitants of Australia invented the boomerang, the blades of which they carved in the shape of an airfoil. As early as the Middle Ages, men of scientific mind prophesied human flight. About 1250, Roger Bacon, an English friar, suggested the orthopter, a machine that flaps its wings like a bird. He also conceived the balloon, proposing "a hollow globe filled with ethereal air or liquid fire." Some 250 years later, the great Italian artist and scientist Leonardo da Vinci studied the flight of birds. About 1490, he drew sketches for flying machines, also of the orthopter type. Leonardo made drawings of a propeller and a helicopter. An Italian monk, Francesco de Lana, in 1670 proposed a vacuum balloon. Four spheres, from which air had been exhausted, were to support a car equipped with oars and a sail. He overlooked the phenomenon of atmospheric pressure, however, which would have crushed the spheres. Not until a hundred years later was the first balloon flown successfully in public. In 1783, J. Etienne and Joseph M. Montgolfier inflated a big paper balloon with hot air, which rose 6,000 feet.
According to the passage, over 500 years before the first balloon flight, ------ .
boomerangs were the only means of air travel for men
men didn't even know how to fire arrows
Leonardo da Vinci studied the flight of birds and drew flying machines
successful "orthopters" had already been transporting people by air
knowledgeable men believed successful flights would be achieved one day
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