Without stopping at the enemy's first-line organisations, the first attacking wave overwhelms the position, annihilates all defenders encountered, and only comes to a halt at the assigned objective. The following waves support the first one, and deal with points of resistance. The trench-cleaners or moppers-up "clean out" the position of enemy survivors with bayonet, knife and grenade, in indescribable death grapples. Progress is slow along the communicating trenches, and in the underground shelters, tunnels, cellars and ruins, where the defenders have taken refuge. From time to time hidden machine-guns are unmasked and have to be captured. _After the attack._--As soon as the "cleaning out" is finished, any prisoners are sent to the rear, being often forced to cross their own barrage-fire. Meanwhile the other defenders will have withdrawn to their positions of support. [Illustration: GERMAN PRISONERS HURRYING TO THE ALLIES' LINES.] Having reached their objective, the assaulting troops must hold their ground. Sentries are posted, while the rest of the men consolidate the position in view of the inevitable counter-attack, which is generally not long in coming. Under bombardment, the levelled trenches have to be remade, the shell-holes organised and flanked with machine-guns, and communications with the rear ensured for the bringing up of stores and, if necessary, reinforcements. The assaulting troops may thus reach their objectives without excessive losses or nervous strain, and may be kept in line for a second and third similar effort, after a few days' rest, during which the artillery will have destroyed the next enemy positions. The Flying Corps _Before the battle._--Metaphorically speaking, the Flying Corps (aeroplanes and observation balloons) is the "eye" of the High Command, which largely depends on it for precise information regarding the enemy's movements and positions. It likewise regulates the artillery fire, and furnishes that arm with photographs, showing exactly the progress made by the destruction bombardments. Another equally important duty is to "blind the enemy" by destroying their aeroplanes and observation balloons. [Illustration: OBSERVATION BALLOON.] _During the battle._--Flying low, sometimes within a few hundred feet of the ground, the airmen furnish invaluable information, and often photographs, showing the progress of the attack, the _terrain_ being marked out with panels and Bengal lights. They also often attack the enemy with their machine-guns. [Illustration: BRITISH TANKS MAKE THEIR DÉBUT.] _After the battle._--The massing of enemy troops for counter-attacks is signalled to the artillery, which regulates its barrages accordingly, then, working in _liaison_, the two services "prepare" the ground for the next attack. These tactics were, gradually perfected on the Somme battlefields, where the Germans learned by costly experience to improve their defences.