While this preparation and development were going on ground schools and flying fields for the training of the personnel of the Air Service were being planned and built. For the study of airplane engines and of the elements of aviation and for military training arrangements were made with universities and technical institutions in various parts of the country and within a few weeks after the declaration of war young men were at work in “ground schools” at eight of these institutions. This first step in the training required eight weeks and when the first students to be graduated from it were ready for primary instruction in flying the land for some of the flying fields had been acquired and tents set up. Here, under primitive conditions, they began their work, and kept it up while the fields were being developed underneath the wings of their planes. Construction proceeded rapidly and in a few months every one had its comfortable barracks for the cadets and men of the squadrons, shower baths, lecture buildings, mess halls, officers’ quarters, long rows of hangars for the housing of the planes, and all the usual structures of a large cantonment planned and built according to the principles of sanitary engineering and provided with telephone, electric lighting, water, sewage and garbage disposal systems. One of these fields, representative of them all, although they varied in size, with its hangars, machine shops, machine-gun ranges, landing fields, fire department and its many buildings, covered five square miles--more than 3,000 acres. As the system of training was evolved the fields were specialized and each one was developed according to the purposes for which it was used. In all, thirty-six flying fields were built in the United States, while in France several great air instruction centers, one of them the largest in the world, comprising in all seventeen fields, were prepared and in use when the war came to its sudden end.