"To get the wind behind us," he explained, "we shall cross well to the south of Péronne. Next, we go to Boislens. After that we pass by Nimporte, over the Forêt de Charbon to Siègecourt; then up to Le Recul and back by Princebourg, St. Guillaume, and Toutprès. "As regards the observers, don't forget to use your field-glasses on the rolling stock; don't forget the precise direction of trains and motor transport; don't forget the railways and roads on every side; don't forget the canals; and for the Lord's and everybody else's sake, don't be surprised by Hun aircraft. As regards the pilots--keep in close formation when possible; don't straggle and don't climb above the proper height." The pilots ran their engines once more, and the observers exchanged information about items such as Hun aerodromes and the number of railway stations at each large town. An air reconnaissance is essentially the observer's show; its main object being to supply the "I" people at headquarters with private bulletins from the back of the German front. The collection of reconnaissance reports is work of a highly skilled nature, or ought to be. Spying out the land is much more than a search of railways, roads, and the terrain generally. The experienced observer must know the German area over which he works rather better than he knows Salisbury Plain. The approximate position of railway junctions and stations, aerodromes, factories, and depôts should be familiar to him, so that he can without difficulty spot any new feature. Also he must be something of a sleuth, particularly when using smoke as a clue. In the early morning a thin layer of smoke above a wood may mean a bivouac. If it be but a few miles behind the lines, it can evidence heavy artillery. A narrow stream of smoke near a railway will make an observer scan the line closely for a stationary train, as the Boche engine-drivers usually try to avoid detection by shutting off steam. The Hun has many other dodges to avoid publicity. When Allied aircraft appear, motor and horse transport remain immobile at the roadside or under trees. Artillery and infantry are packed under cover; though, for that matter, the enemy very rarely move troops in the daytime, preferring the night or early morning, when there are no troublesome eyes in the air.