It was when we began the more serious work on the mountain that we made the most interesting observations on acclimatisation, and proved both its rapidity (which was known before) and its persistence to great heights. Scientists of various schools had, before the start of the Expedition, predicted that acclimatisation would be impossible above the height of 20,000 feet. Why they had done so will always remain a mystery to me; but possibly they were misled by the fact that so many climbing expeditions in the past have failed somewhere in the region of 23,000 feet above sea-level. We were enabled, however, to prove conclusively that acclimatisation does go on to greater heights; in fact, I do not see a theoretical limit to it at any elevation below the top of Mount Everest. Our observations were largely subjective, but for that reason they are perhaps all the more to be appreciated by the general reader; and in view of their subjective nature I may perhaps be pardoned in substituting “feelings” for figures and putting information in the form of a personal experience. When Mallory and I arrived at Camp III and established it on the site chosen by the reconnaissance party, our first concern was the preparation of another camp at the North Col. I shall never forget our first ascent up that accursed slope of snow and ice, each step a hardship, every foot a fight; until at last we lay almost exhausted on the top. After a day or two at Camp III below, we went up again to the col, this time with Strutt and Morshead, and I think Norton. The ascent of the col this time was hard work, but not more than that; and after the col had been reached Morshead and I were sufficiently cheerful to explore the way leading up to Everest. A day or two later we again ascended the North Col, and never really noticed more discomfort than was occasioned by breathlessness. Though not possessing the scientific data which explained this change in our condition, yet in those few days of life at 21,000 feet we had become acclimatised to our altitude to a very remarkable degree; what had previously been a hard struggle had now become a comparatively easy job. By this rapid change in our constitution we had not only proved the predictions of scientists to be wrong, but had gained the physical power which took us without artificial oxygen supply to 27,000 feet, and we had determined that acclimatisation is not only possible but is also quite rapid at these high altitudes.