It is therefore evident that the climber must be equipped according to the zone in which he finds himself. In the first zone clothing somewhat warmer than that used in the Alps in the summer is practically sufficient. Owing to the marked intensity of the sun’s rays, however, it is advisable to cover the trunk with at least one layer of sunproof material, such as a sunproof shirt with spine pad, while a solar topee and suitable snow-glasses constitute the best form of headgear. Crookes’ glasses of smoke-blue colour proved superior to other varieties; they afford complete protection from glare and do not cause eye-strain and subsequent headache. As sunburn, even very superficial and involving only a small area, is invariably followed by conditions of feverishness which must impair one’s fitness, a veil should be worn over the face and gloves on the hands. Oxygen should be employed from the foot of the North Col slopes onwards, for no useful purpose can be served by tiring oneself through not using it, when, as we have seen elsewhere, full recovery from fatigue is no longer possible at 23,000 feet. The second zone (from 23,000 feet onwards), where a radical change in climatic conditions is manifest, demands more complicated preparation. Wind is seldom absent, and the degree of intensity of the cold is comparable with that met with at the Poles, and indeed probably often exceeds it. Also, owing to the rarefied state of the atmosphere, the cold is felt much more severely than would be the case at sea-level. A far greater volume of air is expelled from the climber’s lungs, and this air, at blood heat and under a low pressure approximating to one-third of an atmosphere, is saturated with moisture drawn from the body viâ the lungs. The result is a proportionately far greater loss of animal heat. Further, the partial pressure of oxygen contained in a normal atmosphere becomes so low at altitudes over 23,000 feet that, unless the climber has recourse to a supply of oxygen carried by himself, his climbing efficiency is enormously reduced. The climbing equipment of the mountaineer in this second zone of high altitude should therefore include, firstly, a supply of oxygen; secondly, warm and windproof clothing and foot-gear; thirdly, plenty of food and drink, as the use of oxygen has a most stimulating effect upon the appetite.