The flashes of countless guns and the lurid flares of abandoned ammunition-dumps and petrol-stores illuminated the misty sky, while the sodden earth trembled under the thunder of artillery-fire. At frequent intervals Hun bombing-'planes, soaring at great heights, fearful lest their careers might be cut short by the British machines, dropped bombs indiscriminately, the loud clatter of which was distinctly audible above the roar of the howitzers and heavies. It was an inferno into which men, who a few years ago never thought to handle a rifle and bayonet, plunged bravely and resolutely to give their lives for their country. Realizing that Flanders and Northern France were Britain's bulwarks, and that should the Channel ports be lost the thorny problem of Ostend and Zeebrugge would be magnified a thousand-fold, every foot of ground was obstinately contested by the hard-pressed troops. Isolated battalions deliberately sacrificed themselves on this account, thus obtaining a temporary respite for their undaunted comrades, while in countless numbers fresh hordes of field-greys hurled themselves by day and night against the dauntless khaki lines. Derek soon found the reason for his hasty flight to France. With hundreds of other airmen he had been sent across to assist in stemming the tide of Huns. Success or failure in the present struggle depended mainly upon superiority in the air. Not only did aerial combination mean that the enemy's concentration could be clearly observed--mists and fogs alone preventing--but his lines of communication could be constantly interrupted, while a new factor, low-altitude machine-gunning, was "putting the wind up" the German infantry in no half-hearted fashion. The young pilot was told off to start at dawn. Provided with a series of aerial photographs of the enemy's positions, and also a map ruled off in squares and numbered and lettered, he was able to obtain a clear idea of the sub-sector over which he was to operate. So elaborate were the preparations that there was hardly a square yard of ground captured by the enemy that was not mapped out for particular attention by the R.A.F. By bomb and machine-gun fire the Huns were to be unmercifully galled--but at a cost. With the first blush of dawn, when rosy tints glowing beyond the flame-tinged clouds of smoke betokened another wet day, GV 7, in company with others of her kind, was brought from the camouflaged hangar. During the night her crew had snatched a few hours' sleep, the work of replenishing fuel and ammunition being entrusted to the air-mechanics and ground men. With her cylinders shedding enough castor oil to dose a battalion at full strength, and every part of her construction carefully tested, she stood ready to start upon her errand of death and destruction.