PROF. YANDELL HENDERSON'S article on “The Last 1,000 ft. on Everest”, published in NATURE of June 3, p. 921, gives expression to the physiologist's doubts about the possibility of climbing to 29,000 ft. without the use of oxygen. Prof. Henderson attempts the difficult task of analysing the fall in climbing speed with increase in altitude and suggests that it reaches zero at 29,000 ft. If this conclusion is correct, it is folly to attempt the ascent of Mount Everest without oxygen, but mountaineers may answer that information about climbing speeds at great altitudes is too scanty to justify any precise estimate of limiting altitudes and may point out that, so far, climbers have done as well without oxygen as with it. The most compelling argument in favour of the use of oxygen would be the production of a really effective apparatus. On this point Prof. Henderson is emphatic that the so-called open apparatus is valueless and that a closed unit, involving absorption of carbon dioxide, is essential. But in spite of the possible theoretical advantage of the latter type, it has never been successfully used for climbing at high altitudes. The open apparatus, however, as is pointed out in an article on p. 961 of this issue, by a member of last year's Everest expedition, has twice been used with some success. For the future, it is clear that there is ample room for improvement in methods of oxygen administration in climbing. Increase in efficiency of utilization is desirable provided that it does not sacrifice other essential qualities, but another and perhaps easier line of attack is the reduction of weight by mechanical improvement of valves and cylinders.