Читать книгу The Great Arc: The Dramatic Tale of How India was Mapped and Everest was Named - John Keay - Страница 11

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, after due allowance for refraction, the elevation of conspicuous peaks.

This sounded most promising. It looked as if Crawford had made 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 observation.’

The method of operation remained unclear. How, for instance, had the distance of the peaks from Crawford’s points of observation been ‘computed’? Clearly not in the manner of Lambton constructing his triangle between the beach and the grandstand; but if by horizontal triangulation, this required a base of precisely known length between two points of observation at least twenty miles apart. Crawford’s base was rumoured to have been less than a quarter of a mile, and of doubtful accuracy.

Moreover, ‘heights above stations of observation’ were useless without knowing how high such stations of observation were above sea-level. This information was not given, and an inferred height of about 4,500 feet was mere conjecture. Sea-level deep in the mountains would remain conjectural for the next fifty years, another of the many imponderables which dogged Himalayan observations.

Nevertheless the report put paid to one common misconception. The Himalayas were not a line of active volcanoes. The plumes of smoke which appeared to stream from their summits were simply windblown snow. Additionally, Crawford’s attempted measurements represented an important advance on the guesswork which had preceded them. During the next two decades, while Lambton laboured at the triangles of his Great Arc far away in the tropical south, Crawford’s Himalayan claims would trigger a wave of both curiosity and controversy in respect of the snowy mountains which, swagged below the Tibetan plateau, defiantly described a great arc of their own along India’s northern frontier.

The existence of the Himalayas had been known to the ancients. Ptolemy, the first-century astronomer and geographer, had called them the ‘Imaus’ and ‘Emodi’, both words presumably derived from the Sanskrit (H)ima-alaya, or ‘Abode of Snow’. He showed them as a continuation of the Caucasus mountains running east from the Caspian Sea. Subsequent travellers, like Marco Polo in the thirteenth century, usually trod some version of the ancient Silk Route which, though skirting the north of the western Himalayas, left Tibet and the central Himalayas well to the south. But Tibet had been regularly penetrated in the seventeenth century by Jesuit missionaries from India, and the first convincing account of the mountains comes from one of their eighteenth-century successors. This was the Italian Ippolito Desideri who in 1715 departed Kashmir for Lhasa and was horrified to find, even in late May, the snow deep on the trail and the mountains ‘the very picture of desolation, horror and death itself’. ‘They are piled one on top of another,’ he wrote, ‘and so close as scarcely to leave room for the torrents which course from their heights and crash with such deafening noise against the rocks as to appal the stoutest traveller.’

Fifty years later the eruption of British arms into Bengal which presaged the beginnings of the Raj brought more sober appraisals. In the 1760s Lord Clive had commissioned Major James Rennel to survey the territories which, as Colonel Robert Clive, he had so unexpectedly seized. Rennel, the father of the Bengal Survey and its first Surveyor-General, travelled north to the frontier with Bhutan and thence noted several peaks which were snow-covered throughout the year. One in particular stood out; it may have been Chomo Lhari. Although he made no attempt to measure it and considered the hills as outside his field of operations, Rennel did alert the world to the possibility that the Himalayas were ‘among the highest mountains of the old hemisphere’.

Curiously, their main rival as Eurasia’s highest summit was thought to be not Turkey’s Mount Ararat (16,946 feet) nor France’s Mont Blanc (15,781 feet), but ‘the peak of Tenerife’ (12,195 feet). While other quite prominent heights remained uncertain, mainly because they lay so far from the sea and could not therefore be assessed against sea-level, that on the island in the Canaries conveniently rose straight from the Atlantic and lay on a busy sea-route round Africa. Mariners usually possessed sextants, and so the Tenerife peak had been much observed. But at the then accepted height of 15,000 feet, it was still overvalued by almost a quarter. Such was the difficulty of measuring even convenient altitudes.

Rennel had made comparison only with ‘the highest mountains of the old hemisphere’. The new hemisphere, or New World, was a different matter altogether. Already the Andes in particular were known to be exceptionally high. Courtesy of that French expedition to measure a degree of latitude on the equator, the peak of Chimborazo in Ecuador had been correctly measured to within a few feet of its 20,700 above sea-level, and so was reckoned the world’s highest. That Bhutan’s Chomo Lhari was in fact over three thousand feet higher than Ecuador’s Chimborazo would have surprised Rennel.

One of Rennel’s most distinguished contemporaries was less reticent and actually knew the name Chomo Lhari, or ‘Chumalary’. Sir William Jones, a judge in the Calcutta High Court, was unquestionably the greatest scholar England ever sent to India. Dr Johnson had hailed him as ‘the most enlightened of men’, Edward Gibbon as ‘a genius’. Linguist, poet, historian, philologist and naturalist, Jones founded the Asiatic Society of Bengal, whose publications would include Lambton’s occasional reports, and he led the field in almost every branch of Oriental studies. It was thanks to Jones that the height of the Himalayas had been added to the agenda of Orientalist research.

‘Just after sun-set on the 5th of October 1784,’ writes Jones, ‘I had a distinct view from Bhagilpoor [Bhagalpur on the Ganges in Bihar] of Chumalary peak … From the most accurate calculations that I could make, the horizontal distance at which it was distinctly visible must be at least 244 British miles.’ This extraordinary sighting argued strongly for an immense elevation; but Jones also had the advantage of having corresponded with two men who had actually crossed the mountains. They had been sent on separate trade missions to Tibet and had followed an existing and not especially challenging route through Bhutan. But from their reports of latitudes observed and distances gauged, Jones correctly surmised that the mountain wall was many miles thick as well as high. The highest peaks lay well back from the immediate horizon ‘on the second or third ridge’. And despite Rennel’s caution, after careful study of these and other reports Jones was prepared to chance his arm. He was in fact the first to declare that there was now ‘abundant reason to think that we saw from Bhagilpoor the highest mountains in the world, without excepting the Andes’.