The few dozen airmen who accompanied the contemptible little army on the retreat from Mons had no precedents from other campaigns to guide them, and the somewhat vague dictum that their function was to gather information had to be interpreted by pioneer methods. These were satisfactory under the then conditions of warfare, inasmuch as valuable information certainly was gathered during the retreat, when a blind move would have meant disaster,--how valuable only the chiefs of the hard-pressed force can say. This involved more than the average difficulties, for as the battle swayed back towards Paris new landing-grounds had to be sought, and temporary aerodromes improvised every few days. The small collection of serviceable aeroplanes again justified themselves at the decisive stand in the Marne and Ourcq basin, where immediate reports of enemy concentrations were essential to victory. Again, after the Hun had been swept across the Aisne and was stretching north-eastward tentacles to clutch as much of the coast as was consonant with an unbroken line, the aerial spying out of the succeeding phases of retirement was of great service. Indeed, tentative though it was, the work of the British, French, and German machines before the advent of trench warfare proved how greatly air reconnaissance would alter the whole perspective of an open country campaign. After the long barrier of trenches deadlocked the chances of extended movement and opened the dreary months of more or less stationary warfare, the R.F.C. organisation in France had time and space for self-development. Aerodromes were selected and erected, the older and less satisfactory types of machine were replaced by the stable B.E2.C., the active service squadrons were reconstructed and multiplied. To the observation of what happened behind the actual front was added the mapping of the enemy's intricate trench-mosaic. For a month or two this was accomplished by the methodical sketches of a few observers. It was an exceedingly difficult task to trace every trench and sap and to pattern the network from a height of about 2000 feet, but the infantry found small ground for dissatisfaction as regards the accuracy or completeness of the observers' drawings. Then came the introduction of aerial photography on a large scale, and with it a complete bird's-eye plan of all enemy defence works, pieced together from a series of overhead snapshots that reproduced the complete trench-line, even to such details as barbed wire. By the infallible revelations of the camera, untricked by camouflage, concealed gun positions were spotted for the benefit of our artillery, and highly useful information about likely objectives was provided for the bombing craft.