The contention, therefore, that the disadvantages of its weight would more than counterbalance the advantages of an artificial supply of oxygen, may be dismissed as groundless, and the assumption may be made that on any further attempt upon Everest oxygen will form a most important part of the climber’s equipment. The question next arises as to the exact stage in the proceedings at which recourse should be made to the assistance of oxygen. The strongest members of the Expedition felt fit and well, and recuperated readily from fatigue, at Camp III, 21,000 feet above sea-level, but at the North Col this was no longer the case. Thus it would seem that the upper level of true acclimatisation lies somewhere between 21,000 and 23,000 feet. I would therefore advocate commencing to use oxygen somewhere between these two levels, preferably at the foot of the steep slopes leading up to the North Col. The use of small quantities would allow the climber to reach the Col without unduly tiring himself. From the North Col to a high camp situated at an altitude of about 26,500 feet, a slightly increased quantity of oxygen would suffice to enable the climber to progress almost as rapidly as he would in the much lower levels of the Alps. We know from experience that a camp at the above-mentioned altitude can be readily established, and in all except the worst of weather conditions a party can make its way down again. Between the camp and the summit there would be a vertical height of only 2,500 feet, and it is conceivable that with a full supply of oxygen this distance could be covered in as little as four hours. I am strongly of the opinion that only one camp should be used between the North Col and the summit. No matter what precautions are taken, man’s strength is rapidly sapped during the stay at these great altitudes, and the plan of campaign most likely to ensure success would appear to be leisurely and comfortable progress as far as the North Col, the establishment of a high camp at 26,500 feet, and a final dash to the summit. This last part of the programme, however, would not be feasible unless a small dump of oxygen were made at a height of about 27,500 feet. To do this it would be necessary for a specially detailed party to spend one night at the high camp, and on the following day employ their strength in making a dump somewhere above the shoulder. This done, they would then be able to return to the North Col with the satisfaction of knowing that they had made it possible for the actual climbing party to win through.