It is imperative, therefore, to classify the flyer for the kind of work he is physically capable of performing. Some men are not able to fly at higher levels than a few thousand feet without suffering deleterious effects, while others may operate at five miles above the surface of the earth without physical harm. It is necessary to know a flyer’s limitations before his training is specialized, for the saving of time and money, and, indeed, the flyer himself. Just as the trainer of a varsity track team classifies his available material into sprinters, distance men, broad jumpers, high jumpers, and weight throwers, so the director of a flying-school must classify his material into men fitted for combat, observation, and bombing. It would be an obvious waste of time and effort to train a man for combat work at high altitudes and then discover that his physical limitations permitted only of his doing bombing work at comparatively low levels. In order to accomplish this work of classification, branch research medical laboratories were established at the various flying-fields, which, by means of certain standardized tests, especially the one on the “rebreather” machine, placed the flyers in their proper categories. The rebreathing tests were conducted in a room so designed, by the gradual expulsion of its oxygen, as to create the exact and various conditions that would exist at any known altitude. Physicians and physiological experts, themselves supplied with oxygen through tubes, remained in the room throughout the tests, closely observing the effect produced on the candidate by the gradual decrease of the oxygen supply. It was soon found that a man’s faculty to respond to sight, sound, and touch becomes more dormant as the air becomes more rarefied, and it was to offset this condition that the oxygen apparatus which I have described in preceding pages was designed. The effect of low oxygen upon the mental process varies greatly, however, according to the individual. He usually becomes mentally inefficient at an altitude at which there is as yet no serious failure of his vital bodily functions. By simple tests of mental alertness during these rebreathing experiments, such as directing the candidate to press designated buttons controlling electric lights of certain colors, controlling a volume of sound by operating a pedal with his feet, and the like, it was easy to determine that one flier would lose his mental alertness at 15,000 feet, while another would retain full control of his faculties at nearly double that height.