Until men invented ways of staying underwater for more than a few minutes, the wonders of the world below the surface of the sea were almost unknown. The main problem, of course, lies in air. How could air be provided to swimmers below the surface’ of the sea Pictures made about 2,900 years ago in Asia show men swimming under the surface with air bags tied to their bodies. A pipe from the bag carried air into the swimmer’s mouth. But little progress was achieved in the invention of diving devices until about 1490, when the famous Italian painter, Leonardo da Vinci, designed a complete diving suit.
In 1680, an Italian professor invented a large air bag with a glass window to be worn over the diver’s head. To "clean" the air a breathing pipe went from. the air bag, through another bag to remove moisture, and then again to the large air bag. The plan did not work, but it gave later inventors the idea of moving air around in diving devices.
A. for nearly a hundred years
B. for almost a thousand years
C. for over a century
D. for a century
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