Stirling was then told to hold one end of the second and third sections. The united length of tubing was now nearly nine feet. One end Denbigh wedged into the opening in the ceiling for the electric fan. The other he held in his hand in readiness. At length, after a tedious wait, Denbigh saw the tip of the drill emerging from the bulkhead. Marking the spot he instantly switched off the light. A dull thud announced that the boring tool had made a complete perforation and that the handle had struck home against the steelwork. The drill was withdrawn. In its place a small metal tube was inserted. Deftly and noiselessly Denbigh slipped the lower end of the flexible piping over the projecting nozzle. Then he waited. He could hear the Irishman breathing heavily. The portion of the tube that he was holding quivered in his excitable grasp. Stirling, cool and collected, gave no sign of the potential alertness that possessed him. A gentle hissing sound, repeated at short intervals, announced that the Germans were injecting the stupefying fumes by means of a bellows. A faint, sickly odour assailed Denbigh's nostrils. He had to fight hard to refrain from gasping. Grimly he stood by until the hissing noise ceased. His plan had been successful. Save for a slight leakage the fumes had travelled through the pipe and had been carried through the louvres of the ventilator, while the hot air of the cabin was sufficient to create an up-draught to disperse the noxious vapour. Denbigh removed his end of the tube. As he did so he heard a voice exclaim: "It is enough. More will kill them. You had better enter the cabin, Herr Doktor, and see that they are still breathing." The sub drew the piping from his companions' grasp. "Turn in and pretend you're insensible," he whispered, fearful lest the sound should be heard through the newly-made hole in the bulkhead. It was less than five minutes later when the door was unlocked and a dim figure cautiously entered. "Not half so bad as I expected," said a guttural voice. The smell of the anæsthetic had almost dispersed. "Where is the switch?" "Here, Herr Doktor," replied a petty officer. The next instant the cabin was bathed in brilliant light. In spite of their efforts to the contrary the three supposed sleepers twitched their eyelids. The ship's surgeon bent over O'Hara. A short scrutiny confirmed his suspicions. He turned to the bunk on which Stirling was lying, and, lifting the sub's eyelid, placed the tip of his forefinger upon the eyeball. "Ach, is it so?" ejaculated the German, for Stirling had been compelled to contract his eyelids. A similar test bore the same result in Denbigh's case; then, without another word, the doctor hurried from the cabin. "The old pillbox has tumbled to it," muttered Denbigh. "Now what will their little game be?"