"Wish I had a gas-mask," exclaimed Alec chokingly. "Same here," agreed the pilot. "Here," he added, pointing to a pile of what appeared to be short sticks, "take a few, they'll come in handy." The acquired articles were bombs of German manufacture, and had evidently been placed there as a reserve stock. "Know how they work?" inquired Smith. Alec shook his head. He understood the mechanism of the Mills' grenade, but he had never before had an opportunity of handling a German bomb. "Simply pass this thing round your wrist and chuck the thing as hard as you can. The cord is tied to the safety-pin, and the jerk releases the pin. Quite easy." "And easy to blow yourself up," added Seton. "All right, carry on! We'll do our best with the things." Proceeding cautiously, for they could hear Huns talking in the tunnel, the two men worked their way along the tunnel for nearly a hundred yards. Then they paused abruptly and flattened themselves against the wall. A few feet farther along, the corridor terminated in a flight of steps, seven in number, leading upwards to a fairly spacious casemate. From where Alec and his comrade stood the legs of several Germans could be seen, the rest of their bodies being hidden by the curvature of the roof of the tunnel. The men were formed up round a quick-firing gun of 15 centimetres, or approximately 4.1 inches--a weapon of great hitting power and rapidity of action. Evidently they were waiting to train the weapon upon some moving objective that had not yet entered the arc of fire. The two officers glanced at each other. Their teeth gleamed in the dull light, as they exchanged grins of delight. They were no longer prisoners of the tyrannical Hun, but strong men armed. Providence had delivered the enemy into their hands, but it would not be a one-sided contest. The surprise of the attack would compensate for the inequality of numbers, and there were the survivors and possibly crews of guns in another casemate to be reckoned with. Simultaneously both officers took a step forward, and launched their deadly missiles. The two reports sounded as one, outvoicing in the confined space the din of the conflict without. Amid the rattle of metallic splinters could be heard the thud of bodies falling and the startled squeals of wounded men who find themselves unexpectedly hit. The rapid crack of an automatic pistol and the splaying of bullets against the stonework gave Seton and his companion warning that their work had not been thorough. Through the pall of smoke a Hun--perhaps more than one--was "letting rip". Four bombs in quick succession gave the unseen foe his quietus. Silence reigned in the casemate. The roar of battle without was increasing in violence. [Illustration: THE PILOT THREW A BOMB FULL IN THE FACE OF A PRUSSIAN UNTER-LEUTNANT]