Even the innocent-looking surface of a weed-grown field is not above suspicion. The naked eye is suddenly drawn to it by what seems at first almost inspiration, but one becomes conscious as one watches of an indeterminable movement taking place on its surface. Mark the place very carefully and bring the telescope to bear upon it. The sense of movement resolves itself into the periodic sprinkling of brown earth thrown up as by an industrious mole. These are spadefuls of earth, showing that a trench is being dug. Natural features themselves have a habit of changing their positions with the same disconcerting effect as that phenomenon had upon Macbeth. Of course, one is never lucky enough to catch them actually in motion, but a morning of surprises will often reveal the disappearance of a well-known hedge, or the sudden apparition of an orchard of full-grown trees in the middle of a ploughed field, or even a stately plantation of elms on what was formerly a _pavé_ road. The hedge was removed to provide something with a field of fire, or to allow somebody to see a particular part of our line; the game is now to discover the whereabouts and nature of that something or somebody. The orchard and the elm trees were required as cover, probably for guns; the surest plan is to shell them and await developments. It may be possible to drive the detachments out into the open, when every weapon that can be brought to bear will sing its own particular song of triumph. A certain redoubt was located by our aeroplanes, and its position indicated to us by the fact that it lay right in front of the seventh from the northern end of a row of trees such as occur at intervals along the side of most French _Routes Nationales_. For many days we used this mark, until it suddenly struck one of our observation officers that the trees looked somehow different to what they did when first he noticed them. Suspicion being thus aroused, further aeroplane reconnaissance was undertaken, when it was found that the third tree of the row now marked the position of the redoubt. The enemy, seeing that they had been "spotted" by the first aeroplane, had dug up the four trees at the northern end of the row and replanted them at the southern end, and must consequently have watched, with a delight not very difficult to imagine, our shells raising a little inferno of their own a couple of hundred yards away from them.