The Great Arc by John Keay Read Free Book Online Page B

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 fromwhich the height of the mountains might be computed and which gave, after due allowance for refraction, the elevation of conspicuous peaks.
    This sounded most promising. It looked as if Crawford hadmade the first serious attempt at measuring the Himalayas. Sadly expectations, raised to the snowline in one paragraph, were promptly dashed to the plains in the next.
    But the drawings and journal of this survey have been unfortunately lost.
    The loss might have been recouped by another writer who happened to have cited Crawford’s original findings, but he had done so only in a tantalising telegraphese: ‘Double altitudes observed by sextant – allowances for refraction – bearing – computed distance – height by trigonometry – additional height for curvature of the earth – Result, 11,000–20,000 feet above stations of