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Young World
Lambton’s great arc
RADHA H.S.
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The arc took 50 years to measure and was 2,500 km long.
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William Lambton’s primary interest was geodesy — the study of the earth’s shape. He joined the British Army and travelled to south India. In 1799, the East India Company wanted to map the newly acquired Mysore state. Col. Lambton was given the job. He proposed measuring an arc to provide an accurate base for systematic topographic and revenue surveys. This would also contribute to measuring the earth, which was a great challenge for the scientific communi
ty. The project began in 1802. The arc was eventually 2,500 km long and took 50 years to measure. It ran from Kanyakumari to the foothills of the Himalayas.
Hard task
So, how was the Arc measured? A baseline was drawn in Chennai and measured using 100-foot chains. This meant two end points and a distance between them was known. A high observation tower was used from end points to spot a third visible point. By measuring the inner angles using a theodolite and applying trigonometric laws the position of this third point was determined. So now they had three sides of a triangle. Using the measured sides of this triangle, other points were marked out using the same method. A series of such triangles connected various points was collectively termed an arc.
When the Arc was measured southwards from Chennai, a baseline was marked in Bangalore and even today this can be seen at a park near Mekhri circle, next to one of the four towers put up by Kempegowda to mark the limits of Bangalore.
William Lambton was eventually succeeded by George Everest his assistant. After Everest’s retirement, an anonymous Peak XV in the Himalayan range was measured as the highest peak in the world almost at the end of the exercise. Everest’s successor moved a proposal to name this in honour of Everest, which is how we know Peak XV: as Mt Everest.
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