The Great Arc: The Dramatic Tale of How India was Mapped and Everest was Named - John Keay

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When in use, the chain was drawn out to its full hundred feet and then supported and tensioned inside five wooden coffers, each twenty feet long, which slotted cleverly onto tripods fitted with elevating screws for levelling. Each coffer he now equipped with a thermometer which had to be read and recorded at the time of each measurement. By comparison with the other chain, which was kept in a cool vault, a scale of adjustment was worked out for the heat-induced expansion.

      But April and May are hot months in Tamil Nadu. The temperature seesawed between 80 and 120 degrees Fahrenheit. Although Lambton says nothing of the inconvenience of working in such heat, he was worried sick by the variations. After endless experiments he came to the conclusion that a one-degree change of temperature made a difference of 0.00742 of an inch in the hundred-foot length of the chain. But were the locally purchased thermometers sufficiently accurate? And might the temperature not have changed in the interval between marking the measurement and reading the thermometer? Lambton was deeply concerned; measurements and readings were to be taken only at dawn or in the early afternoon when the temperature was as near stable as it got; the thermometers were checked and rechecked, both chains measured and remeasured against a standard bar. Nothing gives a better idea of his passion for shaving tolerances to an infinitesimal minimum than this pursuit of a variable amounting to just seven thousandths of an inch.

      To complete the full seven and a half miles of the base-line required four hundred individual measurements with the chain. For each of these measurements the coffers and tripods as well as the chain itself had to be moved forward. It was a slow business even after Lambton’s men had been drilled to do it by numbers. The whole measurement took fifty-seven days, and that did not include the time needed for the construction of end-markers. These were meant to be permanent and so had to combine the durability of a blockhouse with the hairline precision required for registering in the ground the actual mark over which the theodolite would be aligned for triangulation.

      And still the all-important theodolite had not arrived. In fact report now had it that the ship in which it was stowed had been captured by the French. This turned out to be true. The ship had been conducted into Port Louis in Mauritius and the great theodolite had there been landed and unpacked. Happily the French authorities, when they realised what it was, rose nobly to the occasion. Repacked and unharmed, it was gallantly forwarded to India and arrived in September ‘along with a complimentary letter to the government of Madras’.

      Lambton could at last begin his triangulation. In late September he took angles from his base-line to pre-selected points to the south and west. The short southern series of triangles down the coast was to determine the length of a degree; it took about a year. Then in October 1804 he turned his back on the coast. Heading west and inland, he would carry his triangles right across the peninsula and then begin the north – south series known as the Great Arc.

      Over the next twenty years sightings of Lambton in Madras would be of rare occurrence. As in Canada, he seemed again to have disappeared into a continental void; perhaps after six years on the public stage, he was happy enough to slip back into the wings of obscurity. But the government insisted on progress reports and the scientific world awaited his findings. Lambton’s personal papers would disappear with him. Until the young Everest joined him in late 1818 there are few firsthand accounts of his conduct or his establishment. But his reports found their way into the Survey’s files and his scholarly monographs into learned journals. Additionally one of his assistants would pen some recollections; and there is the unexpected evidence of two Lambton children, both born while he was working on the Great Arc. As he later admitted, the years spent in India pursuing his obsession would be the happiest of his life.

       THREE Tall Tales from the Hills

      When measuring a base-line it was important to discover, as well as its precise length, its height above sea-level. Other heights ascertained in the course of triangulation could then be expressed in terms of this universal standard rather than in terms of individual base-lines. To establish what would in effect be the vertical base of his whole survey Lambton had therefore chosen a site for his base-line which was only three or four miles from the Madras coast and looked, given the lie of the land, to be only a few feet above it. But working out exactly how many was still a matter of some delicacy.

      First, on the sands to the south of Madras’ famous Marina Beach, the highest tides had been carefully observed and their maximum reach marked with a flagpole. (In 1802 ‘sea-level’ was construed as high water, although later in the century a mean between high tide and low tide would be adopted as the standard and all altitudes adjusted accordingly.) From this flagpole on the beach the horizontal distance to the grandstand of the Madras racecourse, still today hard by St Thomas’s Mount, was carefully measured by chain; it came to 19,208 feet. Next, from the railings at the top of the grandstand the angle of depression to the flag on the beach was observed by theodolite. Then the process was reversed with the angle of elevation from the beach to the stand being observed.

      The repetition was necessary because Lambton was keen to measure the effect of a phenomenon known as refraction, whereby sight-lines become vertically distorted, or bowed, by the earth’s atmosphere. Here was another of those subtle variables which bedevilled geodetic surveying. In particular, refraction would play havoc with long-range observations to distant mountain peaks, although, as George Everest would discover, it also had its advantages.

      Having deduced a factor for this refraction, Lambton adjusted his measured angles accordingly. Now, conceiving the sight-line between the flagpole on the beach and the grandstand of the racecourse as the hypotenuse of a rectangular triangle (the right angle being deep beneath the grandstand where a vertical from its railings would intersect with a horizontal from the beach), Lambton had measurements for two of the angles and for one side (the 19,208 feet). Elementary geometry then revealed the length of the other two sides, one of which was the desired elevation of the grandstand above sea-level.

      It was important to factor in the height of the flagpole, since its flag, not its base at ground-level, had been observed from the grandstand. Likewise the height of the theodolite’s telescope above the ground. And finally, to get the height of the base-line, it was still necessary to deduct the height of the grandstand above it.

      This last was done by measuring the stand itself and then ‘levelling down’ towards the base-line, a comparatively simple process in which the incline was broken into ‘steps’ whose fall was measured by calibrated staves between which horizontal sightings were taken with a telescope equipped with a spirit level. The base-line itself was not perfectly level and had also involved some of this ‘stepping’. So had the original estimate for the distance from the flagpole to the grandstand. All having finally been ‘conducted with as much correctness as the nature of any mechanical process will admit of … I may venture,’ wrote Lambton, ‘to consider it as as perfect a thing of the kind as has yet been executed.’ He then proudly announced that ‘we have 15.753 feet for the perpendicular height of the south extremity of the [base-]line above the level of the sea.’

      Not much attention was paid to this calculation at the time. It had taken several days and much careful planning, but a rise of fifteen feet was no great revelation, and the account of its measurement was buried deep in more technical data about the base-line itself. This in turn was buried deep in a large leather-bound volume whose 1805 publication happened to coincide with news of rather more dramatic elevations elsewhere.

      Twelve hundred miles away, beyond the northern borders of British Bengal, a surveyor named Charles Crawford had entered the Kingdom of Nepal in the heart of the Himalayas just as Lambton was laying out his base-line. From around Kathmandu Crawford had got a good look at the Himalayas and, according to an 1805 report of his journey, he had become ‘convinced that these mountains are of vast height’.

      … bearings were taken of every remarkable peak of the snowy range, which could be seen from more than one station; and consequently the distances of those peaks from the places of observation were … determined by the intersection of the bearings and by calculation. Colonel Crawford also took altitudes from which the height of the mountains might be computed and which gave,

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