The most opposite statements and opinions have been advanced concerning this matter. The extremes range from saying that fatal results may occur, and have occurred, from some obscure cause, at comparatively moderate elevations, down to that no effects whatever have been experienced at the greatest heights which have been attained. Allegations of the latter class may be set aside for the present, for the evidence is overwhelming that, from 14 000 feet above the level of the sea and upwards, serious inconveniences have occurred; that prostration (amounting in the more extreme cases to incapacitation) has been experienced; and that, in some instances, perhaps, even death has resulted through some cause which operates at great elevations.