讲述意大利比萨斜塔
Why does the tower lean? In fact,it was already leaning while they were building it. As soon as three floors had been built,by 1178, it began to lean as the ground beneath it sank. The builders tried to make the tower straight again as they added more floors, but the lean only got worse as the tower grew. It continued to lean by about 1 millimetre every year until finally, on 7 January 1990,it was closed to the public. For ten years,the rescue of the tower was a serious problem. The top was leaning over by 5 metres to the south and if its movement was not stopped, the tower would one day fall over. Measurements taken in 1911 estimated that the tower would collapse by 2050,if not sooner. After 10 years of hard work,it was offivially reopened on 16 June 2001. However it was not opened to the public until the end of 2001 and even now, the number of visitors is restricted. It is now safe for the next 300 years because engineers have reduced the lean by 50 centimetres to give it the same lean that it had in 1838.