To foil these attempts at concealment is the business of the observers who gather information for Army Headquarters and G. H.Q. For observers on corps work the detective problems are somewhat different. This department deals with hidden saps and battery positions, and draws and photographs conclusions from clues such as a muzzle-blast, fresh tracks, or an artificial cluster of trees. All reconnaissance observers must carry out a simultaneous search of the earth for movement and the sky for foes, and in addition keep their guns ready for instant use. And should anything happen to their machines, and a forced landing seem likely, they must sit tight and carry on so long as there is the slightest hope of a safe return. A nos moutons. I made a long list in my note-book of the places where something useful was likely to be observed, and tried my gun by firing a few shots into the ground. We hung around, impatient at the long delay. "Get into your machines," called the Squadron Commander at last, when a telephone message had reported that the weather conditions toward the east were no longer unfavourable. We took to the air and set off. V. led his covey beyond Albert and well south of the Somme before he turned to the left. Then, with the strong wind behind us, we raced north-east and crossed the strip of trenches. The pilot of the emergency machine, which had come thus far to join the party if one of the other four dropped out, waved his hand in farewell and left for home. Archie barked at us immediately, but he caused small trouble, as most of his attention was already claimed by a party of French machines half a mile ahead. Anyhow we should have shaken him off quickly, for at this stage of the journey, with a forty-mile wind reinforcing our usual air speed of about ninety-five miles an hour, our ground speed was sufficient to avoid lingering in any region made unhealthy by A.-A. guns. The water-marked ribbon of trenches seemed altogether puny and absurd during the few seconds when it was within sight. The winding Somme was dull and dirty as the desolation of its surrounding basin. Some four thousand feet above the ground a few clouds moved restlessly at the bidding of the wind. Passing a few small woods, we arrived without interruption over the railway junction of Boislens. With arms free of the machine to avoid unnecessary vibration, the observers trained their glasses on the station and estimated the amount of rolling stock. A close search of the railway arteries only revealed one train. I grabbed pencil and note-book and wrote: "Boislens, 3.5 P.M. 6 R.S., 1 train going S.W."