THE MOUNT EVEREST FLIGHTS

L. V. STEWART BLACKER

THE advance of our scientific knowledge of Mount Everest has always possessed an exceptional personal interest for me because my ancestor, Colonel Valentine Blacker, C.B., who had been Quarter- master-General after the Maratha Wars, became Surveyor General in 1823, ^at the same time that George Everest was carrying northward through India his triangulation of the Great Arc of the Meridian. It was from the eastern extension of this arc, along the plains of northern India, that the first scientific observations were taken to the summit of the peak that was entered in the records as XV. The computations of 1852 showed it to be the highest mountain known. It was not for several years, after attempts to find a native name had failed, that Sir Andrew Waugh, then Surveyor General of India, called the peak after his distinguished predecessor in the Great Trigonometrical Survey. This triangulation may be likened to the first long range bombardment of a citadel. The next advance was made in the 'seventies and 'eighties, when Hindu explorers, trained by officers of the Survey, penetrated Nepal and Tibet in disguise, their compasses secreted in Buddhist prayer-wheels. The gallant work of two, in particular, Hari Ram and Gandarson Singh, added much to our knowledge of the approaches, but little to that of the peak itself. It was not till after the Tibet Mission had completed its main task in 1902 that a party of surveyors under Ryder approached within about 40 miles of the summit.

In 1913 Captain Noel carried out a reconnaissance of the approaches and brought the problem from the strategical sphere to the tactical. Opportunity came after the War to organize the close reconnaissance of 1921 and the first assault of 1922. The history of this magnificent exploit is so well known that it needs no description here. A second assault took place in 1924, and, as the world knows, the gallant Mallory and Irvine lost their lives not far from the summit.

By that year several people had conceived the project of a flight to the mountain, but such a flight was impracticable at the time, since there was no engine capable of carrying a pilot and his observer, fully equipped with cameras and oxygen, to such an altitude. Our expedition owed its origin to our appreciation of the fact that the development of the Pegasus engine early in 1932 had advanced sufficiently to make such a flight possible. It was probably the only engine in the world adequate to the task.

As an old pupil of the Bristol Aviation Company's school, as long ago as 1911, I had kept in close touch with their activities. Early in February 1932 it became clear to me that if the new model Pegasus developed in the splendid manner of its predecessor, the Jupiter, it would be able to carry the instruments of science and their operators over the summit of Mount Everest. Having assured myself of its potentialities, the next step was to place a considered project before the Council of the Royal Geographical Society. That body conveyed its support to the Secretary of State for India and to the Air Ministry. To reach Mount Everest it was necessary to fly over the independent kingdom of Nepal; such a flight could only be attempted with the sanction of the Indian authorities and of the Maharaja of Nepal; before the latter could give his consent he had to be assured by the highest possible authority that our plans were sound and that no avoidable contretemps were likely to occur through insufficient organization. To the Chief of the Air Staff and to the Secretary of State for Air, whose sympathy, vision, and imagination were a great encouragement to us, we owe a deep sense of gratitude for their support at this juncture.

From March onwards we were in the throes of organization and technical planning. The selection of the actual engines with their wonderful superchargers was by no means the most difficult matter. The development of the engine had to be watched, while the Bristol Company pushed forward their own attempt on the height record, which was captured by their test pilot, Captain Uwins. Such a high altitude flight calls for the solution of many problems. Fuel is liable to freeze at low temperatures, especially in the jets of the carburettor; we found, therefore, that we could not employ a great admixture of benzol, which freezes at —60⁰ C., a point actually reached on the test flights. This problem was solved by the experts of the Shell Company, who developed a blend which functioned admirably. Lubricating oil might have been expected to congeal and thicken at the same very low temperature; events proved otherwise, and our tests showed that in the rarified air of high altitudes, so few molecules impinge on the surface of the oil radiators that an equally large cooling surface was necessary when flying high as when flying near sea-level. The lubrication of all the working parts of the machine and the compensation for the very great contraction of control wires, due to intense cold, were matters that had to be studied most carefully. The fitting of the flying instruments bristled with difficulties; many allowances and compensations had to be made before the altimeters and air-speed indicators could be depended upon to give true readings; the supercharger of the engine involved a complete set of controls and it was necessary to study the 'boost⁵ and its gauges with care, so that the engine could not be overrun at any time during the climb.

A vital matter was the supply of oxygen. We consulted the leading expert in the country, whose advice was generously given with very happy results. We found it necessary to take oxygen from the ground upwards, using only a small trickle at first, progressively increasing the supply, until the full amount available, 10 litres per man per minute, was taken at about 30,000 feet. Meticulous care was necessary in the provision of the oxygen, for the slightest moisture would probably have frozen and blocked the tiny jet in the regulating valve through which it passed. To guard against this possibility the oxygen was kept heated on its passage from the Vibrac steel storage cylinders to the airman's mask. Furthermore, not only was a reliable and adequate supply of oxygen necessary, but the physiological fitness of the flying party themselves was important. Each man was put through a most stringent test in a steel rarefaction chamber. In this the airmen performed the remarkable feat of going up to 11,500 metres without leaving the ground! They were packed into this great mysterious cylinder and a steel door bolted behind them. They fitted on their oxygen masks, and, under the keen supervision of two physiological experts of the Royal Air Force, the air was steadily exhausted. Mysterious lights indicated the operation of the pumps controlling the oxygen supply. When the air was exhausted there were some strange phenomena: when one of us shook some coins in the hollow of his hand the chink was inaudible, for there was no air to carry the sound. A sheet of paper, instead of fluttering lightly in the manner of its kind, dropped like a stone to the ground. All the flyers successfully passed the tests, applauded by the medical officers, who regarded this as testimony of a virtuous past.

The amount of oxygen and fuel to be carried had to be calculated with the greatest precision and co-ordinated with the length of film to be expended. This was done with the aid of numerous graphs. If we planned to carry out the photography for a long distance at a great height we could save on the weight of the film, but on the other hand we should use more oxygen and burn more fuel; if we flew low, we should use more film but less oxygen. Even that was not all; for the amount of fuel depended not only on the climb, but also on the force of the wind which we might encounter. This consideration involved an investigation of available meteorological data. A strong side wind would necessitate the machines flying with their centre lines oblique to the direct course to the mountain, with a consequent increased fuel consumption. We were advised by experts that 40 miles an hour was the greatest wind velocity in which it would be safe to make the attempt. What happened when we were actually confronted with the mountain will be seen later. It is enough to say here that it was necessary to fly over 320 miles there and back, and to climb to over 30,000 feet in the process.

The oxygen apparatus and supply, as we have seen, was electrically warmed; so were the aviators themselves and the cameras. Electricity was to be provided by very fine thousand-watt generators directly driven by the engines and, as such large dynamos had not been previously installed in this way, a whole crop of electrical problems presented themselves. Everything connected with the personnel had to be warmed—boots, gloves, suits, and even the glass of the goggles.

The aeroplanes themselves demand a few words. We were fortunate in finding two machines which had the necessary design, namely, accommodation for the big air-cooled radial engines, sufficient ground clearance for the specially large-sized propellors, and a roomy fuselage in which the observer could perform his all- important tasks with efficiency. These qualities we found in the Westland machines, together with an admirable • airworthiness, a robustness of construction, and an honesty of workmanship that inspired us with the greatest confidence from the start.

When, at the end of nine long and strenuous months of planning, all the political obstacles and most of the technical ones had been overcome, victory was almost snatched from our grasp by financial troubles. As the world economic crisis deepened in intensity, those who had originally undertaken to support the enterprise were reluctantly compelled to withdraw, until the prospect looked dismal in the extreme. As time wore on the Indian Monsoon approached. It was essential for our purpose that we should have clear weather and no cloud over the mountain or to the south of it within about 40 miles. We had hoped to attempt the flight in November 1932, that is, before the minor rains of late December and early January, but monetary troubles prevented us from being ready in time.¹³ Fortunately the munificent patroness who has done so much to further the interests of British aviation, Lady Houston, came nobly to our rescue. Meanwhile a committee of influence was being formed, with Lord Peel, twice Secretary of State for India, as chairman, and Lord Lytton, Lord Clydesdale, the Master of Sempill, Col. John Buchan, Lord Burnham, His Highness the Maharaja of Nawanagar, and Wing-Commander Orlebar, the former chief of the famous Schneider Trophy Flight, as members.

On the 11th November we were joined by Air Commodore P. F. M. Fellowes, d.s.o., in the capacity of chief executive officer. His presence was of the utmost value, for not only did he shoulder a great share of the burden of co-ordinating the plans and conducting the tests, but his splendid personality welded the whole expedition together and made it a band of brothers working to a common end. Fellowes is an outstanding figure in British aviation, and one who has risen through his exploits in the hard school of war, not the least of his claims to fame being his single-handed bombing of the lock gates at Zeebrugge. Under his guidance, in the months of December and January, we were soon hard at work putting both the aeroplanes through their full rehearsal tests at the maker's aerodrome at Yeovil, taking them up to nearly 12,000 metres, and accustoming the crews to the strange environment while performing their multifarious tasks.

 

Our object calls for some words of explanation. It was not simply to carry out an air survey of that portion of Nepal south of the Everest range, for the less elevated country and the valleys have been very well surveyed within recent years by efficient men trained by the Survey of India. Nor was it merely to take oblique photographs, either still or moving, of the great mountain itself, but a combination of both these as well as strips with cameras mounted with their axes vertically. From the very beginning of our project we determined to take with these cameras strips of photographs of the untrodden ground, the inaccessible glaciers, and the high ranges to the south of the Everest massif. We knew that the ground surveyors had, in 1926, reached the remote monastery of Dingboche, but this is still some 9 miles in a direct line from the mountain. We could be certain that these 9 miles were not mapped in any modern sense of the word, and we could quite reasonably suspect that much of the very difficult mountainous country, immediately south of the line drawn eastwards through Dingboche, had been comparatively roughly sketched during the rapid survey from 1924 to 1927. When it is realized that this region includes mountains over 24,000 feet in height with several extensive and inaccessible glaciers, it must be conceded that no party of ground surveyors could have made an accurate map in the short time available. Nepal has an area of 55,000 square miles and the neighbourhood of the Great Himalaya is difficult country throughout. It was our object to take photographic strips running northwards, from a point which could be identified on the ground at least 15 miles south of the mountain to a triangulated point on the massif of Mount Everest itself, and thereby demonstrate the power of air survey to map the most inaccessible country.

It was essential that these strips should be continuous and that each exposure should have a very large overlap with its neighbour. This overlap was necessary in order that common points should appear on three adjacent photographs; the experts on whose advice we were working foresaw that the ground depicted on each negative would vary very much in height and that at the edges of each the country shown would be very much distorted. Again, each strip would have to include two 'ground controls', points identifiable on the photographs with points on the ground, whose exact positions in latitude and longitude were known by triangulation. The triangulation net, of course, forms as important a framework for topographical air survey as for topographical ground work.

This briefly was our plan of campaign as regards the making of air photographic strips. The Expedition is under a debt of gratitude to Mr. A. R. Hinks, the well-known Secretary of the Royal Geographical Society, and to Colonel M. N. Macleod, Chief of the Geographical Section of the General Staff at the War Office, for their advice and guidance in these important points.

Apart from the survey 'strips', we intended also to take oblique photographs of the great declivities near the summit. The difficulty of speeding up the interval between photographs taken with the 'vertical' cameras, which would be necessary in the proximity of the summit, the rapid alteration in the vertical scale of the mountain slopes as shown on the photographs, the necessity of banking the machines, were all matters of concern. For the oblique photographs we relied on both 'still' and 'motion picture' cameras, in the hope that we should be able to combine selected pairs in the stereoscope, whereby we should be able to map, or at least sketch, some of the topography that could not be surveyed by the 'vertical' cameras.

We therefore had to select many cameras, five in each of the two aeroplanes; all our plans and the equipment of ourselves and of the machines centred round them. Firstly, each machine carried a big electrically-driven Williams Eagle automatic survey camera in a gimbal mounting, to point vertically downwards when the plane was flying on a level keel. This took a film about 5 inches wide and over 40 feet long, and for it we carried two spare films, each of which weighed 4 ½ lb. In addition, the leading aeroplane carried two handheld cameras for taking oblique pictures on glass plates; in one the plates were 5 X 4 in., in the other3 ½ X 2 ½. For each of these cameras twenty-four plates in dark slides were carried. Finally, the leading plane carried one, and the supporting plane two cine cameras, each loaded with 200 feet of standard professional film, 35 mm. wide. For these there were spare films in specially warmed compartments. The leading plane also carried a Baby Kodak. This completed the outfit of cameras, apart from the special instruments used for infrared photography, which was undertaken on subsidiary flights.

The cameras, however, could by no manner of means be used as they stood for this very special work, which involved climbing into the extremes of cold at very high altitudes. In the first place, very careful provision had to be made to warm both cameras and films. Into the survey cameras were fitted electric heating elements, a task that called for some care owing to the danger of placing a highly inflammable nitrated film in close proximity to a red-hot electric filament. Considerable heat was necessary inside the cameras, for the film becomes brittle with extreme cold and flies to pieces if bent. Not only had the cameras to be fitted with internal heaters, but the lens cone, camera body, and the film magazine had to be provided with textile jackets, into which additional heating elements were inserted. The design called for a great deal of experiment and research. Time was limited, but we were much helped by the cooperation of a leading firm of refrigerating engineers, Messrs. J. & E. Hall of Dartford, who improvised a special chamber for us and kept it chilled at a temperature of —6o° C., and in this our final tests were conducted immediately before taking the cameras into the air for their trial flights. The wiring and leads for the cameras and films were so intricate that the task of handling them in the air was no light one. The research work and experiments for warming the cameras were very ably carried out by Mr. C. W. Williamson.

Another detail of importance was the calibration of the camera lens systems which was carried out by the courtesy of the National Physical Laboratory. The survey cameras also contained altimeters and watches, so that the height and time of each exposure could be photographed on to the negative; special high-level altimeters had to be provided. The cine cameras were to be driven by clockwork; they carried only 200 feet of film. It was therefore necessary to reload them in the air. To do this, the spare spool boxes had to be taken out of their electrical jackets, various small controls actuated, and the new film threaded through the 'gate' as quickly as possible. There were sixteen leads, cables, and conductors for electricity and oxygen inter-connecting the machine and the observer with his cameras and various items of his outfit. There were four more leads to his gloves and boots—twenty in all. Anyone who has tried to change a cine film against time in a comfortable room at sea-level will appreciate a small part of the difficulty of doing it in a temperature of —40⁰ or —50⁰ C., amidst the tangle of all these pipes and leads.

The light-filters for the cameras were a source of much thought and anxiety. No one could possibly forecast the intensity of the light over the mountain. Both intensity and quality might be subject to factors encountered in ordinary high flying, to others which affect photography in sub-tropical latitudes, or to still others which are met with in making exposures among high snow mountains. Some technical authorities foresaw a possibility that ultra-violet rays would affect the question; others predicted 'cosmic rays'. The absence of cloud was postulated, but we could not foresee a minor factor which obtruded itself in practice, an amazingly high dust haze. So little did we, or for that matter any one else, know about the question, that the solution was positively easy! We took a large variety of filters and tried them at every possible opportunity in every condition of altitude, latitude, and atmosphere, up to the day of the great flight. It was the only sound solution, and we are not ashamed! I should perhaps mention one point of importance. During the course of the trial flights at high altitudes, over 8,000 metres, the dark slides of the plate cameras showed signs of being penetrated by light. The same slides at low altitudes were perfectly efficient; this seems to indicate that the light at high altitudes may have peculiar properties. Eventually we found it necessary to wrap all the slides in two thicknesses of black paper, to pack them in a specially designed box with a spring lid, and to place them immediately after exposure in another special box whose lid closed down automatically.

The infra-red camera was far too large to be manipulated from the cockpit, or even to be lifted into the 'slip-stream' by the observer. It was a formidable instrument with a splendid Cooke lens of 25 in. focal length, mounted outside the fuselage. This prevented it from being used during the main flights over the mountain, owing to the effect of its air resistance on the performance of the machine. Special flights were therefore made, with the infra-red camera mounted in the undercarriage, where there had originally been space for a torpedo. It lay underneath the fuselage, packed with pads of horsehair to kill vibration, pointing straight ahead but with its axis depressed six degrees to compensate for the angle of climb. Exposures were made by the concerted action of the pilot, who aligned the plane on the object to be photographed, and the observer, who directed the pilot, loaded the camera, set the shutter, and released it at the exact moment.

Important accessories to the survey cameras on the main flights were the drift sights, fulfilling two functions. In the first place they enabled the observer to move his camera in azimuth to the correct amount, by measuring the angle of drift and applying this angle to the camera in the opposite direction. In the second place a knowledge of the drift was necessary to enable the pilot to navigate correctly. It was most important that the machine should fly not only with exactitude to a point: over the summit of the mountain, but also that it should fly there in as straight a line as possible, over the ground. A curved strip of photographs taken by the 'vertical' cameras is less satisfactory than a straight one for plotting the map afterwards. We evolved therefore a novel procedure. As the pilot steered towards the summit on his original compass course we would make allowance for the estimated drift due to the wind velocity, forecasted for us by the meteorologists, and so cause the plane to head at an angle to its course. The observer would then measure the actual drift and communicate it to the pilot, who would note a point on the under surface of the upper wing, which would be the necessary number of degrees to the right or left of the centre line of the plane. This point he kept aligned on the summit.

Other instruments carried were thermometers for recording air temperatures, reading to —6o° C., alcohol cockpit thermometers, and, of course, mercury-in-steel thermometers for the oil temperatures of the engine. Each machine carried either a recording barograph or a meteorgraph giving a permanent record of air pressures, temperatures, and humidity encountered throughout the flights and at the different altitudes. Very important for aerodynamic research was the accelerometer which registered, in terms of the acceleration due to gravity, the force of the 'bumps' encountered in the air. And, on subsidiary flights, we took up a liquid dip-needle with which to make investigations concerning the earth's magnetic field at high altitudes and comparatively close to the mountains.

With the exception only of the heated gloves and boots of the crew, and a bare half-dozen instruments, every one of the innumerable 'gadgets' in the two aeroplanes was either novel, experimental, or specially modified. Even the new 'Vibrac' oxygen cylinders called for a special representation to the Home Office authorities, to relax restrictions that had been applied to earlier types. We felt that the battle was won when everything worked on the test flight.

At length all the researches, tests, and experiments were over, and the two big Westland machines with their valuable freight of instruments reached Karachi in the holds of the P. & O. s.s. Dalgoma. A fortnight later, after being assembled by the efficient staff of the R.A.F. Depot, they were tested and, on the 22nd March, the Expedition flew across India, encountering cordial hospitality from H.E. the Viceroy downwards. We came at last, on the 24th, to our improvised but good and level aerodrome at Lalbalu, nine miles to the east of Purnea, to the very spot where the uncle of our chairman, Captain Peel of the Royal Navy, had attacked and defeated with his bluejackets the mutinous cavalrymen of a Bengal regiment during the rising of 1857.

A few days were necessary for final arrangements, but nothing had been left to chance and soon all was ready for the great trial except the weather, which was exceptional for the time of year. Every few days disturbances would come up from the west, gradually filling the sky with cloud and the mountain valleys with impenetrable mist, fatal to photography. In perhaps eight or ten days came a climax, showers of rain, dust storms, and high winds. The skies cleared but the wind velocities remained so high that the extra fuel required made the flight impracticable. Then came a clear spell while the winds lessened, before the fresh over-clouding of the sky should arrive with the next disturbance.

There was no exact meteorological knowledge of the behaviour of the winds over 8,000 metres, so we were provided, by the courtesy of the Government of India Weather Bureau, with an Upper Air Sounding-station. This was under the capable care of Mr. S. N. Gupta, who sent up sounding balloons twice daily and gave us information of wonderful and surprising accuracy.

The wind in the upper levels of the air was totally different from that at a lesser height. It came invariably from the west, and its velocity was disquietingly great. It was not till the morning of the 3rd April that the wind velocity at 25,000 feet dropped to 58 miles an hour. We were torn by anxiety. If we were to take the risk and make the attempt, we might finish in disaster from burning too much fuel. If we waited, the wind might increase again, instead of lessening in intensity, clouds might reform, and the opportunity be lost indefinitely. We took the risk, and after final preparations took off at last for our long journey over the mountains.

In the leading machine were Lord Clydesdale and myself; in the supporting machine, Flight-Lieu tenant Mclntyre and Mr. S. R. Bonnet, our expert professional cinema photographer. Clydesdale and myself were to concentrate our energies on oblique photography of Mount Everest itself, exposing cine film as a subsidiary task, whilst Mclntyre and Bonnet were to film the leading machine passing over the mountain, oblique photography being subsidiary to the film work. The survey cameras of both machines were to be in full operation.

We climbed first to about 13,000 feet, the chequered plough-lands of the flat plains of Bihar fading away into misty olive-green monochrome. Behind us on the far horizon a faintly perceptible white line marked the course of the immense sandy bed of the Ganges. Beneath us little specks almost at regular intervals marked the villages and townlets of the plain. An almost impenetrable haze, toned to the wonderful deep purple of the grape of Burgundy, masked the details. After half an hour perhaps we could just descry the black line of the low Mahabharat range of Nepal, running from east to west, athwart our course, with the gorge broken by the Kosi through it, a little before our port wing. We climbed over the lower mountains, and as we reached a height of nearly 19,000 feet, there swam up into our view three entrancing white peaks. Away to starboard, over seventy miles distant, rose Kangchenjunga, a fairy-like tiny white triangle perched alone on the top of the upper surface of the wine-dark haze. An instant later two more infinitesimally small summits, almost incandescent in their whiteness, appeared exactly where we expected them. 'Exactly', because Mount Everest, the left of the two, lay precisely 18 degrees to starboard of the axis of our machine; and this angle was exactly the angle of obliquity which our calculations for drift, due to wind velocity at that height, had foreshadowed. The wonderful peak was marked out from all the other summits by its remarkable plume of wind-driven particles of ice. A fraction of a hand's breadth to the right lay the unmistakable peak of Makalu. As early as this we knew that our plan of navigation was sound and we were exactly on our course.

There is no need here to describe in detail the incidents of the flight. It is enough to say that we flew on a direct course of 342⁰ and arrived exactly over the summit of Mount Everest. We spent from 15 to 20 minutes circling over the peak and then turned southward to get back again before exhausting our fuel. It was more difficult to fly a true course on the homeward than on the outward journey, because all landmarks were totally obscured by haze. However, our trusty Husun compasses and drift-sights stood us in good stead and punctually, almost to the minute, our landing-ground loomed out of the mist, but a mile or two on our port-hand.

We had cause to be more than gratified with the oblique photographs taken on the flight, thirty-five in number, and, as it proved, with the cine films. The former included at least two good pictures taken looking steeply down on to the actual summit from close range, and of a quality good enough to justify enlargement to more than 4 feet by 3, from 5 inches by 4. The survey strips were a disappointment, and a sad one. It is enough to say that the thick dust haze was the main reason for the trouble. We therefore felt in duty bound to make a further attempt on the 19th, when at last the weather became sufficiently favourable.

On the 4th April, in the meanwhile, Air-Commodore Fellowes and Flying-Officer Ellison flew over Kangchenjunga and secured some fine film shots and obliques. When properly studied, these will, we hope, add to our knowledge of the mountain.

To return to Mount Everest. On the 19th the wind velocity was reckoned at over 80 miles an hour at 25,000 feet, increasing as usual with height. This forced us to fly first to the westward for a hundred miles, in a zero wind at 3,000 feet, and then to turn roughly NNE. to get the wind behind us during the second 'leg⁵ of the course. This procedure was successful and this time we flew over the main southern ridge of Mount Everest, connecting the summit with Lhotse, its southern peak. According to plan, one machine kept rather to the south of the actual massif in order to obtain obliques of the western face and of the south-western ridge, more comprehensively than had been possible on the first flight. It was a piece of ill luck that, instead of the dust haze of the 3rd April, a blanket of cloud covered the lower mountains up to 18,000 feet. Fortunately, the ground we wished to survey lay almost entirely above this level; but the cloud blanket prevented the observers from using their drift-sights, so that the courses flown were neither so accurate nor so straight as they had been on the 3rd. We found a wind velocity of nearly no miles an hour at 30,000 feet, and since our air speed at that altitude was just over 120, drift became a matter of great importance.

The high wind prevented us from lingering long. On our return there was only a small margin of fuel in the tanks. To our immense joy both the survey films, when developed, came out magnificently, showing wonderful definition, and, under the circumstances, remarkable freedom from tilt and distortion. Furthermore, the continuity of the strips was unbroken, and there was very full, in fact almost excessive, overlap.

This flight marked the conclusion of the survey flights in India. The plotting of the photographs was carried out, under the general direction of Colonel Macleod at the War Office, by Lieutenant J. S. A. Salt, r.e., who has described the very great difficulties of the process in a most able paper read by him before the Royal Geographical Society.¹ He surmounted these difficulties in a masterly manner. Some of them arose from the very great differences in the relative height of the ground, not only between one photograph and the next but between different parts of the same exposure; others were due to the blanket of cloud. For these Nature was responsible. They were aggravated by the fact that we had to fly in April; we believe that, as we had intended, October or November would have been more suitable.

Another difficulty was caused by political considerations. We were forbidden to fly over Tibetan territory, and though the sanction of the Nepal Government was generous, we were only allowed to make one flight, afterwards extended to two. This forced us to arrange for very large overlaps and compelled the observers to tackle the excessive tasks of handling five cameras, when encumbered with oxygen masks, during a single busy quarter of an hour. Had we been able to do so, we should have approached the mountain from the west, down-wind from Darbhanga, or even from Raxaul or Khatmandu, flown over it with the wind behind us, and landed in the neighbourhood of Siliguri, thus covering a much longer stretch of virgin ground. Had the pilot been able to circle over Tibet, the observer could have changed his magazine in comparative leisure and resumed his work at a pre-arranged point with the new magazine in action. Each plane could then have exposed twice as much survey film as was possible on our actual flight; or when we found the lower valleys filled with cloud, we could have turned back and awaited a finer day. As it was, when once we had crossed the Nepal frontier, we were committed to the flight and had to see it through for better or for worse. Thus it may be affirmed that most of the difficult problems that were set Lieutenant Salt were not 'operational', nor inherent in the air survey of these high mountains. Yet even so, he surmounted them in a manner which compels every one's admiration, and we of the expedition are convinced that, with the experience gained, a complete air survey of the higher glaciated regions of Nepal is now a straightforward problem.

 

We hope that we have in some degree furthered the cause of air survey, and thus shown the aeroplane to be a great instrument of the progress of civilization, and one that can be stopped by no physical obstacle on this planet.

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