In considering the development of military aeronautics it must be borne in mind that the maximum altitudes attained by airplanes increased enormously during the war. In 1914 the record for altitude was 26,246 feet, or slightly less than five miles. By January, 1919, the record had been raised to 30,500 feet, an increase of more than four-fifths of a mile. In 1915 the Western Front pilots worked at 7,000 feet without fear of attack from the ground, and few machines flew at heights of more than 10,000 feet. In fact, the “ceiling” with the early equipment was about 12,000 feet. In the closing months of the war, however, as a result of the development of the antiaircraft artillery, it became necessary for aviators to climb to 15,000 feet over the enemy lines, and tactics of the air made that machine safest which could fly highest. Now it may not have occurred to you that the higher you ascend the greater becomes the decrease in atmospheric pressure. At 19,000 feet the pressure of the atmosphere is one-half the pressure at sea-level. That means that a given amount of air in the lungs of an aviator flying at that height gives only half the oxygen that it would were he on the ground. It is, then, the lack of oxygen, and not, as many suppose, the low pressure itself, which makes men weak and slow of action at high altitudes. Though these facts have been determined by medical research, it is a curious phase of the flyer’s psychology that most aviators laugh at the idea. Yet any one who has crossed the Rockies or ascended one of the Alpine peaks by funicular has noticed that as the altitude increases the breathing becomes quicker and deeper, the heart beats faster and faster. But though the pilot may, as he asserts, continue to feel perfectly fit and well, he is not as efficient as when near the ground. His reactions become slower, he is less prompt to judge distances, to aim his guns, to fire, to manœuvre his plane—and this despite the fact that he is usually quite unconscious of any impairment of his faculties. He will feel dizzy but perfectly happy—autointoxication, I believe the doctors call it—whereas, as a matter of fact, he has lost his judgment; and if he attempts to stay at these altitudes he will gradually pass into a condition of partial and sometimes total unconsciousness, lose control of his machine, and come crashing to the earth.