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            新概念英語聽力mp3下載第三冊lesson 47

            Source: 恒星英語學習網  Onion  2007-03-21  我要投稿   論壇   Favorite  
            Satellites orbiting round the earth have
            provided scientists with a vast amount of
            inf.mp3ation about conditions in outer
            space. By comparison, relatively little is
            known about the internal structure of the
            earth. It has proved easier to go up than
            to go down. The deepest hole ever to be
            bored on land went down 25,340 feet--
            considerably less than the height of
            Mount Everest. Drilling a hole under the
            sea has proved to be even more difficult.
            The deepest hole bored under sea has
            been about 20,000 feet. Until recently,
            scientists have been unable to devise a
            drill which would be capable of cutting
            through hard rock at great depths.
            This problem has now been solved. Scientists have developed a method which
            sounds surprisingly simple. A new drill which is being tested at Leona Valley
            Ranch in Texas is driven by a turbine engine which is propelled by liquid mud
            pumped into it from the surface. As the diamond tip of the drill revolves, it is
            lubricated by mud. Scientists have been amazed to find that it can cut through
            the hardest rock with great ease. The drill has been designed to bore through
            the earth to a depth of 35,000 feet. It will enable scientists to obtain samples of
            the mysterious layer which lies immediately below the earth's crust. This layer
            is known as the Mohorovicic Discontinuity, but is commonly referred to as
            'the Moho'.
            Before it is possible to drill this deep hole, scientists will have to overcome a
            number of problems. Geological tests will be carried out to find the point at
            which the earth's crust is thinnest. The three possible sites which are being
            considered are all at sea: two in the Atlantic Ocean and one in the Pacific. Once
            they have det.mp3inded on a site, they will have to erect a drilling vessel which will
            not be swept away by ocean currents. The vessel will consist of an immense
            platf.mp3 which will rise to 70 feet above the water. It will be supported by six
            hollow columns which will descend to a depth of 60 feet below the ocean surface
            where they will be fixed to a huge float. A tall steel tower rising to a height of
            nearly 200 feet will rest on the platf.mp3. The drill will be stored in the tower
            and will have to be lowered through about 15,000 feet of water before operations
            can begin. Within the tower, there will be a laboratory, living accommodation
            and a helicopter landing station. Keeping the platf.mp3 in position at sea will
            give rise to further problems. To do this, scientists will have to devise methods
            using radar and underwater television. If, during the operations the drill has to
            be withdrawn, it must be possible to re-insert it. Great care will therefore have
            to be taken to keep the platf.mp3 steady and make it strong enough to withstand
            hurricanes. If the project is successful, scientists will not only learn a great deal
            about the earth, but possibly about the nature of the universe itself.


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