The system of training had not only to produce men for work in the air. It had also to train large numbers for a great variety of work necessary to sustain and coöperate with the flying fighters and observers. In addition to unskilled labor, fifty-two trades and occupations are essential to the aviation service and men had to be either wholly or partially trained in each of them. At first, in order to secure skilled men with the utmost speed, mechanics were sent in detachments to a great number of factories where special training was given them and afterward, as experience began to disclose what would be needed, carefully worked out courses of training were established in nearly a dozen different schools. Government schools giving thorough training, in operation at the end of the first year of war, were graduating 5,000 mechanics every three months. Aerial photography had developed during the war to an exact science, but when we entered the conflict very little was known about it in the United States. Instruction in it was of a threefold character, for observers had to learn how to operate cameras in an airplane, intelligence officers on the ground had to be instructed in the interpretation of the results and enlisted men to be taught to do the developing, printing, and enlarging and to keep the equipment in condition. Schools for training in all these things soon produced the necessary instructors for the flying fields where training in aerial photography was given. It was a complicated and difficult problem that the United States faced when it undertook to work out a system of air training while it was training the men for air service. But within a year and a half it had evolved an efficient system that set higher standards than did other nations and also better safe-guarded the lives of the men in training, and while doing this it had sent overseas 4,776 trained flying officers, had as many more at home fields, and had in training at home more than 5,000 cadets, of whom nearly half were in advanced stages of the work. In the final test of service at the front the men who had been trained by that system received for their ability, skill and deeds the heartiest and highest praise. CHAPTER XVIII THE BALLOON CORPS