"At that!" he ordered, addressing the men standing by the valve of the ballast chamber. "Turn horizontal rudders to ten degrees down. Half speed ahead." A gasp of relief echoed through the confined space. The men gave a cheer. They understood: one danger had been successfully combated. The steady thud of the throttled-down motors alone imparted the sense of motion as the submarine, fifty feet beneath the surface, resumed her blind grope up The Narrows. Twice only during the negotiation of that intricate passage did the Lieutenant-Commander show the top of the periscope above water. The first observation revealed the feet that Nagara was well on the starboard hand, and that an alteration of helm was necessary to avoid piling the submerged craft upon shoals that, owing to the irregularity of coast-line, were directly ahead. The second convinced Huxtable that the course was now practically a straight one for the Sea of Marmora as far as direction went; but he knew that in that stretch of water which lay between him and the inland sea were at least six rows of anchored electro-contact mines. "A vessel on our port bow, sir," declared Dick, who before the submarine could submerge her periscope had glanced into the object-bowl. It was a picture of darkness which met his eye, but across that expanse of dull leaden colour, that indicated a blend of sea and sky on a rainy night, he had detected a still darker object travelling slowly across the "field" of the periscope, the "iris" of which was opened to its fullest extent in order to admit the maximum amount of light, which was very small. "A torpedo-boat," decided the Lieutenant-Commander after a brief survey. "She would make an easy target, but we'll let her go. It would mean betraying our presence, and that would never do. One thing she's taught us, that is that the passage through the next mine-field is hard against the Asiatic side. We'll let her pass, and then pick up her wake." The Turkish torpedo-boat was evidently bound from the neighbourhood of Nagara for the Bosphorus. She was steaming at fifteen knots and, judging by the clouds of black smoke tinged with dark-red and orange flames, was under forced draught. Unsuspectingly the boat held on her course, little thinking that within two cables' length of her a British submarine was following the phosphorescent swirl that marked her track. Either the men of her watch on deck were lax in their duties or else they devoted their attention to keeping a look-out ahead and abeam, for the cascade of foam that marked the swiftly-moving periscope passed unnoticed. Ready at the first alarm to tilt the horizontal diving-rudders, Huxtable conned the submarine. He was in high spirits, for the thought that an enemy torpedo-boat was acting as a pilot amused and elated him. Although prepared to take the risk of diving under the mine-field, he fully admitted his preference to be conducted in safety through the danger-zone.