In the calculation of what will be required at various stages in order to reach the summit of Mount Everest it is necessary to begin at the highest; and the climber imagines in the first place where he would like to have his camps. He may imagine that on the final day he might rise 2,000 feet to the summit; if he is to give himself the best chance of success he will not wish to start much lower than 27,000 feet, and in any case he cannot camp much higher, for he is very unlikely to find a place on the ridge above the North-east shoulder (27,400 feet) or on the steep rocks within 200 feet of it. We may therefore fix 27,000 feet approximately as the desirable height for the last camp. And we have another camping ground fixed for us by circumstances, approximately at 23,000 feet, the broad shelf lying in the shelter of the ice-cliffs on the North Col—there is no convenient place for a comparatively large camp for a considerable distance either above or below it. But to carry up a camp 4,000 feet at these altitudes would be to ask altogether too much of the porters. We must therefore establish an intermediary camp between these two, say at 25,000 feet if a place can be found. Now what will be required at these three camps? We must ask first with what number of climbers the assault is to be made. A party of two appears insufficient, for if one man should become exhausted the other will probably want help in bringing him down. This difficulty is met by having three climbers. But since an exhausted man cannot be left alone, certainly not without the shelter of a tent, nor should one man go on alone, a party of three must turn back so soon as one man is unable to go further. Four men would give a better chance of success in this case, for then two might go on and still leave one to look after the sick man. Granted, then, that the best hope is for four men to start from a camp at 27,000 feet, we have firstly to provide them with tents. Two tents are better than one, for it may be difficult to find a place for four men to lie side by side, and the greater weight of two smaller tents above one larger is inconsiderable; and they must have sleeping-bags, provisions for two days, fuel, and cooking-pots. All these necessities have been previously carried up to the camp below at 25,000 feet; but other things besides are required there. We may assume that this camp is to be used as a stage on the way up only and not on the way down. Even so, six porters at least will have to sleep there before carrying up the highest camp, and their requirements will be the same as we have laid down for the four climbers; we must add another day’s provisions and fuel for the climbers themselves.