The registration of targets from the air, however, is not the most important part of this work. For instance, a machine will be flying over enemy territory; the observer will see the flash of an enemy gun and will pin-point its position on his map, which is marked off into large and small lettered and numbered squares. This operation enables him to send by wireless what is known as a zone call, giving the exact location of the enemy battery to all of our batteries within range. The enemy battery then has to move suddenly, if it is ever to move at all. Barrages can also be controlled very efficiently from the air, so, considering the comparatively short time that aeroplanes have been used in this work and the wonderful results that have been obtained, it does not take much imagination to see the necessity for all future artillery officers to be trained as aviators. In the earlier stages of the war it was very difficult for Headquarters to keep in close touch with the infantry during a "push"; consequently, considerable loss of life might result from one portion of the line advancing out of contact with another. Probably the eagerness of raw troops to keep on advancing regardless of their objective has led to a considerable and unnecessary loss of life. The aeroplane can be used in these situations to great advantage, and after the development of what is known as "contact patrol" the aeroplane became the connecting link between Headquarters and the infantry. It was not until 1916 that the full powers of the aeroplane as an offensive weapon began to be realized. Bombing was done, but it was of a desultory nature, and although the number of machines engaged in this work steadily increased, and the work itself became more and more diversified and specialized, it was not until 1918 that the possibilities of the aeroplane as a purely offensive weapon were appreciated.