"One, six, four, seven, nine, three, two," he said, reciting the combination of numerals in German with the certainty of familiarity. "One, six, four, seven, nine, three, two, it is," replied the observer, in a similar, ill-spoken tongue. "That's 'proceed,'" he added, referring to his book. Forthwith the signaller produced a pocket telephone connected with the conning-tower by a long insulated "flex," and spoke over it. A moment later the throb of engines made itself felt, and, in response, the spume broke on the vessel's cut-water, and left a frothing wake astern. The vessel passed the mist-hooded granite headlands. It left them behind, and itself became engulfed in the grey threat lying between the overshadowing heights towering upwards nearly five hundred feet towards the leaden sky. The two men on deck gave no heed to their immediate surroundings. They were men of the sea, hard and unimaginative. They were concerned only with the safety of the vessel under them. They would drive her into the very gates of Hell, if such were their orders. But they would avoid, with all their skill, the pitfalls by the way. They knew that the secrets of this gloomy abode were many, as many perhaps as those of the very Hades they would have been willing enough to face. They knew, too, that those secrets, just as the secrets of the other place, were calculated to destroy them if they diverged one iota from the laws which governed the place. So they worked exactly, and took no chances. The channel quickly began to narrow. The vast cliffs drew in upon them in their overpowering might. The barren shores were visible to the naked eye, and the white line of heavy surf boomed and boomed again in its incessant attack upon the grim walls. Higher up small patches of pine trees clung desperately to insecure root-holds, like the intrepid Alpini seeking to scale impossible heights. A few minutes passed and a boat, a small petrol-driven vessel, like some cockle-shell amidst its tremendous surroundings, shot out from the shore and raced towards them. It had a high, protected prow, and its great speed threw up a pair of huge white wings of water till it had something of the appearance of an enraged swan charging to the attack of an enemy. Again the signaller spoke over his telephone, and the vessel slowed down, and finally hove-to. The patrol boat drew alongside. Two men, amidships, in oilskins, held a brief conversation with those on board the intruder. Then their vessel passed ahead, and the bigger craft was left to amble leisurely along in its wake. The cliffs had closed down till less than half a mile of water divided them. The narrow strip of leaden sky above looked pinched between them. For a mile and more ahead there was no change. The narrow passage, with its racing tide, was full of hidden dangers, not the least amongst which being a crowded mine-field which lined either side of the channel.