Our next subject for study was "liaison," which means the science of maintaining communication between the several branches of an army. During an attack upon the enemy’s position each arm of the service has its own part to play. The artillery has to prepare the way for the infantry, and at a given signal the infantry must be ready to rush forward to the attack. As the infantry carry the positions before them and move forward the artillery-fire must be correspondingly lengthened; the supply-trains have to keep the necessary amount of ammunition and shells and other material supplied to the infantry and artillery; while the cavalry must be ready to charge the moment a favorable opening presents itself. For all this co-ordination there are various "agents de liaison." There are messengers on foot, and despatch-bearers on horseback, and motor-cycles; there are visual signals, such as the signal-flags, the semaphore, and the colored fires and star shells; and there are the telephone and the telegraph. None of these are depended upon by the army headquarters as much as the aviation corps, the "agent de liaison par excellence." It is one of the most important rôles that aviators are called upon to play at the front, and we were being prepared for this work by very special instruction. Under the subject of "war aviation" we studied the designs of the various enemy aircraft, and the pitfalls which are encountered at the front. Then followed a course in bomb-dropping. This was a practical course, and our method of learning was as peculiar as it was ingenious. A complete bomb-dropping apparatus was mounted on stakes about twenty-five feet above the ground. Under this there was a miniature landscape painted to scale on canvas. It was a regular piece of theatrical scenery mounted on rollers so that it could be revolved to represent the passage of the earth under your machine. We would climb into the seat on the stilts and consider ourselves flying at some arbitrary height. Through our range-finder we would gaze down at the "land," and as a town appeared we would make allowances through the system of mirrors arranged by the range-finder for our speed and height and for an imaginary wind. At the calculated moment the property bombs would be loosed. When I came to Châteauroux I thought that I knew something about aviation because I had obtained my "brevet." I soon realized how very little of the ground I had actually covered. In fact, after four weeks of this advanced work I felt as if I would never acquire all the knowledge required for work at the front. Just then about twenty of us were selected to go into the reserve near the front, to fill vacancies caused by casualties. At last! We were off for the front!