One of the most serious problems, and one of late development, was that of enlisted men, the ground force needed to keep the planes and engines always in prime condition, repair minor breaks, tighten up wires, strengthen struts, and make sure that no airman went up in a faulty plane. This was work wholly new to American mechanics, and of a delicacy and carefulness to which they were quite unaccustomed. Moreover, mechanics of the skill required had largely been drained off by the draft, by enlistment, or by other war industries. Consequently, a whole series of schools was necessary. At first, in the fall small detachments of mechanics were sent to various factories—ignition, magneto, propeller, welding, instruments, sail-making, cabinet work, copper work, machine guns, and motors to secure as much experience as possible. While about 2,000 men were being graduated from 17 courses at 34 different schools of this type, more fully worked out courses were established at five northern flying fields closed for flying during the winter. With 2,500 graduated here, still more detailed courses were opened at four large mechanics’ schools, which added another 5,000 men. By the end of the year two large and complete Government schools were in operation at Kelly Field, Texas, and St. Paul, Minn., capable of graduating 5,000 men every three months. A noteworthy event of the year was the opening on May 15 of the first regular aerial mail service in the United States between New York, Philadelphia, and Washington. The Army furnished six planes and pilots, shortly doubled, for a daily round trip, carrying about 350 pounds of mail each way, and with a record of 50 minutes for the 90 miles between Philadelphia and New York, and 1 hour and 50 minutes for the 135 miles from Philadelphia to Washington. Ninety per cent of the trips were made successfully. Another vitally important phase of the Air Service is that of ballooning, which during the war has been developing into a system of ever-watchful sentries on guard all the way from the North Sea to Switzerland. Less spectacular, perhaps, than the heavier-than-air work, this branch of the service has a quite indispensable function. The observer, swinging in a captive balloon at an altitude of a mile, 2 to 5 miles from the enemy’s lines, and with a range of vision of 8 miles in all directions, can make a far more detailed, minute-by-minute analysis of the enemy’s movements than the wider visioned but transitory aviator, and can maintain such a flow of minute information to the staff below that no important movement can take place unobserved within his view.