The simpler phenomena of acclimatisation have frequently been referred to in connection with Mount Everest. But still it may be asked why improvement should be expected during a sojourn at 15,000 feet. It is expected because as a matter of experience it happens: though why the red corpuscles in the blood whose function is to absorb and give up oxygen should multiply in the ratio of 8:5, I leave it to physiologists to explain. Whatever explanation they may give I shall not cease to regard this amazing change as the best of miracles. And this change in the hæmoglobin content of the blood evidently proceeds a long way above 15,000 feet. Nevertheless the advantage thereby obtained by no means altogether compensates at very high altitudes the effects of reduced atmospheric pressure. It enables a man to live in very thin air (11½ inches barometric pressure, at 27,000 feet), but not to exert himself with anything like his normal power at sea-level. His pace suffers. If at 23,000 feet he were able to exercise no less power than at 10,000 feet after a few well-spent days in the Alps, he would probably be able to ascend the remaining 6,000 feet to the summit in a single day. But if you cut off the supply of fuel you cannot expect your engine to maintain its pace of working; the power exercised by the climber in the more rarefied atmosphere at these high altitudes must be less; a rise of 6,000 feet in a day will be beyond his capacity. Therefore he must have camps higher on the mountain, and ultimately he must have one so high that in nine or ten hours even his snail’s pace will bring him to the summit. We must remember too that not only will his pace have suffered, his mind will be in a deplorable state. The experiments conducted in pressure chambers have a bearing on this point. I treasure the story of Prof. Haldane who, while in such a chamber, wanted to observe the colour of his lips and for some minutes gazed into his mirror before discovering that he held the back towards his face. Mountaineers have often observed a lack of clarity in their mental state at high altitudes; it is difficult for the stupid mind to observe how stupid it is, but it is by no means improbable that the climbers of Mount Everest will try to drink their food or proceed crabwise, or do some quite ridiculous thing. And not only is it difficult to think straight in thin air, it is difficult to retain the desire to do anything at all. Perhaps of all that tells against him the mere weakness of a man’s will when he is starved of oxygen is beyond everything likely to prevent his success.