The Great Arc (13 page)

They were further encouraged by having theodolites and zenith sectors superior to those available to their predecessors in the mountains, and by having the option of barometers, which instruments had occasionally been tested by Lambton but never yet employed in the mountains. A barometer indicates atmospheric pressure; and two barometers, read simultaneously, reveal the difference in atmospheric pressure between their different locations. By factoring in the temperature at the time and place of each reading, the difference in altitude at which the readings have been taken can also be deduced, since pressure decreases with altitude. Obviously this was not much use for measuring the snowy peaks; you had to get your barometer to the top first, and no sane person yet even contemplated shinning up mountains twice as high as The Chur. But it did promise to solve the problem of not knowing the height of the place from which one was observing such peaks. If, for instance, barometrical readings were taken on top of The Chur and other readings were taken simultaneously
at sea-level, say in Calcutta, Hodgson could, and did, work out the height of his observation post on The Chur.

There were several other variables which had to be taken into account in such comparisons, but it was the barometers themselves which presented the greatest challenge. Before the 1843 invention of the aneroid (which measures the air pressure by its effect on the surface of a contained vacuum) the barometer relied on the volatile properties of mercury and closely resembled a giant thermometer. A tripod was needed to hold the thing upright, and the glass tube, three feet long, which contained the mercury posed an almost insurmountable carriage problem. Spare tubes were recommended but, as Hodgson found, those not already broken before they reached the mountains soon succumbed to the knocks and tumbles which were an inescapable hazard of Himalayan portage. The last of his tubes was broken on The Chur at the very beginning of his survey. Although some replacements did eventually reach him intact, it proved almost impossible to fill them. The mercury had to be boiled so as to exclude air bubbles, and boiling was as sure to shatter the glass as bashing it.

‘Those only,’ says an 1850s manual on Indian surveying, ‘who have had any practical experience with such delicate and expensive instruments as Mercurial Mountain Barometers can be fully aware of the disappointments met with in a country like this.’ Indeed, barometers were so prone to breakage that almost any alternative method was reckoned to be worth a try.

The manual in question recommended a simpler ‘thermometrical’ solution, which was precisely that to which Hodgson and Webb duly had recourse. All that was needed in this case was a pocket-size thermometer and a kettle. You boiled the kettle and when, as Hodgson put it, the water ‘reached full ebullition’, you took its temperature. Since the boiling point decreases as the altitude increases, reference to a simple conversion table would give your height. Accuracy depended on very careful reading of a well-calibrated thermometer, and
could not be counted on to within less than a few hundred feet. It could also be a problem getting your kettle to boil. Snow and ice, often the only form of water available at high altitudes, took time to melt, let alone ‘reach ebullition’. Meanwhile the high winds common to exposed locations toyed with the fire, blowing it either sideways, out, or sometimes away. Nevertheless it is curious that this comparatively simple test seems to have been very little used until Hodgson and Webb adopted it as standard practice.

In the summer of 1816 Hodgson worked north, triangulating where possible, perambulating where not, and planting potatoes as he went. (The potato had been rightly identified by the then Governor-General as having an important contribution to make to Himalayan diet and economy.) But, ‘having some doubts as to the precise latitude of my grand station on The Chur on which everything depends’, Hodgson returned there in October and again in 1818 after an adventurous foray to the most distant of the Ganges’ sources. Near the top of The Chur he had constructed a thirty-foot pyramid from which, as well as from Saharanpur in the plains and from Surkananda to the east of Mussoorie, he plotted the angles of some fourteen major peaks and many more minor ones. In fact The Chur for Hodgson, as later for Everest, became a second home. He wrote of it with feeling – although not
in situ
, since the ink invariably froze to the pen.

At a place like the summit of this vast mountain no one who has not resided in such a stormy region can have an idea of the violence of the wind, and the suffering of an observer by night from the cold … On the 10th of October, water instantly froze when poured out at 9 in the morning, tho’ the sun shone out, the thermometer being then at 31°. Judge then what it is by night, accompanied by a wind which peels the skin from the face, and blows with a violence which seems
to shake the very ground. I had a tent to protect the instrument [his theodolite] but at the time of observation the wind rushes in and shakes the instrument, and blows out the lights and creates confusion, and people holding the tent to prevent its being carried away are apt to touch or shake the stand [of the instrument] so that I found it impossible to keep the adjustments in order from night to night.

Nor was this the worst of Hodgson’s problems. The latitudes of his Saharanpur-Chur base as established by zenith observations simply could not be reconciled with those computed by triangulation from a third point. Trying other points from which to complete the triangle made no difference. And the discrepancy remained when his colleague and friend James Herbert carried out a check. It was said that if experienced observers, taking all possible precautions, found themselves confronting an anomaly for which they could not account, they were probably ‘on the verge of some important discovery’. Hodgson quoted this maxim with approval and was perfectly candid about the discrepancy, hoping that others might someday be able to identify what it was that he had discovered.

In fact, of course, they already had. But Webb, Hodgson and Herbert seem to have been totally unaware that mountain masses exercise an attraction over the plumb-line (so critical in zenith readings). Nor were they aware that, as per Lambton’s conjectures, the earth’s density might have a completely contrary effect. Moreover, they were in the worst possible place for this distortion. A geodetic paper on the subject would later confirm that the plumb-line deflection amounted to fifteen seconds of a degree at Saharanpur and at least thirty-six seconds at The Chur. The resultant discrepancy would be something like a third of a mile in the sixty-one miles of Hodgson’s base, a quite unacceptable ratio of error which,
according to the Survey’s historian, ‘rendered all Hodgson’s care and labour of no avail whatsoever’.

To resolve the problem, Hodgson did in the end revert to his original plan of a ground measurement. But, after long exposure on the hill tops, his health was no longer up to the task and it therefore fell to his assistant and successor. To James Herbert in 1819 belongs the honour of measuring the first Himalayan base-line.

Herbert was clearly a resourceful individual. Although he had no experience of base-line measurement, had never even witnessed it, and lacked much of the necessary equipment, he entered into the business with enthusiasm. His only complaint was that he had no assistant. Lambton reckoned that at least three Survey officers were essential to oversee such an operation and Everest, for his Calcutta line, would muster no fewer than ten officers. Herbert was alone except for his Indian staff, none of whom knew the procedures any better than he, and none of whom he could trust to take a reading. How many times he had to scuttle from one end of each measurement to the other he does not say, but it must have been many.

A site was found in the Dun which, though on a slope, was as near flat as anywhere in Garhwal and Kumaon, and the necessary vegetation clearance was soon underway. Meanwhile Herbert wrestled with the means of measurement. He had a chain but no coffers, no tripods and no elevating screws with which to level the tripods. Assembling and making these things would be the work of months. Reserving the chain for comparison purposes, therefore, he decided to use rods. A well-seasoned roof joist of Himalayan cedar was removed from a derelict house and sawn into four lengths, each of twenty-five feet. William Roy had tried rods on his Hounslow base, trussing them to reduce bowing and fitting the ends with ivory markers. Herbert used brass buttons for the critical contacts but, with his rods being only 1
³
/
₄
inches square and trussing being out of the question, he was much concerned how to
prevent them from bowing. In the end he devised a retractable stand on which the rods would rest, and ordered thirty-seven of these, one being required every six feet. As a visual check on bowing he also tensioned a thin brass wire which ran the hundred-foot length of his rods in a specially cut groove.

A bench incorporating a brass standard was designed for the daily check on the expansion or contraction of each rod, and various markers, pickets and flagstaffs were also improvised. All in all, and considering that everything except the rods themselves had to be made out of unseasoned pine, Herbert was not displeased. In the final measurement of about four miles he reckoned that the error could not exceed two feet, ‘an uncertainty which will only affect the distances of the remotest peaks by about sixty or seventy feet’.

Once the carpenters had finished, the whole operation took no more than a month. It then remained only to connect the base by triangulation to Hodgson’s problematic Saharanpur-Chur-Surkananda triangle. This done and the doubtful zenith readings of the triangle corrected, the observations previously conducted to all the snowy peaks could be adjusted and reduced to a table entitled ‘Latitudes, Longitudes and Elevations of principal Peaks and Stations in the Survey’. It was published in 1822, just as an apoplectic Everest was laying out his Sironj base-line on the Great Arc.

Of the forty-six Himalayan peaks in the Garhwal-Kumaon region which Hodgson and Herbert had located and listed, only fourteen were less than 20,000 feet above sea-level. Five were over 23,000 feet, and three of these giants, which Hodgson had numbered as ‘A1’, ‘A2’ and ‘A3’, were far enough east to have also come within Webb’s Garhwal survey, with which they provided a useful link. Webb too had remarked the height of this threesome and had produced very similar values. At Hodgson’s 25,749 feet, ‘A2’ proved the biggest of the three; and as Herbert discreetly put it in a note printed so small as
to be barely legible, ‘A2 is thus, so far as our knowledge extends, the highest mountain in the world.’

Reviewing the Garhwal and Kumaon surveys, Andrew Waugh, Everest’s successor as Surveyor-General, would find them ‘highly creditable to the scientific ability of the officers employed’, although he greatly regretted the ‘inartistical’ nature of their maps. Everest himself applauded Herbert’s efforts to determine a base-line and rated the Garhwal survey ‘quite sufficiently accurate for geographical purposes’. Where not duplicated by his own triangulations, it would be incorporated into the Great Trigonometrical Survey. He was less happy with Webb’s effort in Kumaon. Lacking a base-line, he thought it ‘little better than what is called a wheel and compass survey’.

Webb’s observations had, though, been good enough to win a rather oblique retraction from the
Quarterly Review.
Tucked away in an 1820 notice of a recent paper in French ‘On the Height of India’s Mountains’, the
Review
’s contributor cited findings lately received from Webb. Evidently Webb, incensed by what he had read at Kedarnath in the earlier
Review
, had quickly set up his barometer. The readings, forwarded to London, convinced the reviewer that Kedarnath stood at 12,000 feet above sea-level and that a nearby pass into Tibet was over 16,000 feet. Both places were snow-free at the time. According to Alexander von Humboldt, the distinguished author of the paper under review, who had made extensive observations in both South America and Europe, such altitudes should be well above the level of perpetual snow and must therefore be exaggerated. Earlier the
Review
had also used this argument. Now it conceded that snow-lines were not necessarily consistent and that its conclusions had been ‘as erroneous as those of the Baron de Humboldt’. Moreover, if the heights of such observation points were correct, then so probably were those of the peaks observed from them.

Webb’s sightings, back in 1808, of Dhaulagiri from south
of the Nepal border were still suspect. The distance had been too great, the angle too low, refraction too uncertain, and his instrument too primitive. Even Webb now agreed. Crucially, the distance between the places from which he observed the mountain and calculated its position was also suspect, in that it had been deduced from zenith observations just like Hodgson’s at Saharanpur and The Chur. Until the Central Himalayas in Nepal could be approached as closely as the Kumaon and Garhwal Himalayas, or until such time as they could be observed from points in the plains whose locations were more precisely known, such long-range observations were reckoned a waste of time.

But the heights given in Herbert’s 1822 listing for Garhwal looked good. They had been grounded on his base-line, however crude, and they had been carefully observed at comparatively close quarters from triangulated positions whose altitudes had sometimes been verified barometrically. ‘A2’ therefore went unchallenged as the world’s highest peak. Curiously, it also went unnamed. Herbert’s list included a Mount Moira (after Governor-General Lord Moira) and Mounts St Patrick and St George, all clustered around Gangotri. They had been so christened by Hodgson, either in an onset of patriotic fervour or in a gesture of Anglo-Christian bravado. Contemporaries disapproved and the names have since been expunged from the map.