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How round is the earth? Until the 19th century, that was a serious question. For one, the earth is not a perfect sphere. And its imperfections would have a bearing on maps, and maps brought knowledge, and knowledge meant power. The British had a particular, and predictable, interest in that sort of thing. And so they decided to determine the curvature of the earth by measuring the length of India by way of a sample arc. It became, as British historian John Keay describes in his book, one of the greatest scientific experiments the world has ever known—the mapping and measuring of the Indian subcontinent.It began 200 years ago at the Great Trigonometrical Survey of India when the first measurements were begun by Col William Lambton, surveyor general of India. By the time George Everest took over the survey after Lambton's death in 1822, the calculations were four years in arrears and many of them were going nowhere. Much of that changed in the mid-1830s with the hiring of Radhanath Sikdhar, a young mathematical genius from Calcutta's Hindu College....