The submarine had evidently been compelled to rise to recharge batteries, the heavy sea notwithstanding. Her hydrophones had given indication of the presence of the convoy, and the latter's recent change of course had set the vessels slightly abeam and at gradually reducing distance. The kapitan-leutnant of the U-boat, quick to grasp the situation, had waited until the escorting destroyers on the convoy's port hand had passed, and was now manoeuvring to fire a torpedo at the rearmost tramp--which also happened to be the largest. Owing to the darkness it was almost impracticable to make use of the periscope, so the German submarine remained awash in order to take a direct bearing on her intended victim. In the shortest possible time the gun's crew of the for'ard 3-inch quick-firer were ready. At a bare two hundred yards the target was one that could not be easily missed and the gun-layer knew his job thoroughly. Too late the astounded and terrified Huns sought to submerge. Before the last Teuton gained the quick-action watertight hatchway the _Bolero's_ gun barked viciously. Fairly through the conning-tower at a height of a couple of feet above the tapering armoured deck the high-velocity shell passed. Exploding, it blew the top of the conning-tower to pieces, killing the kapitan-leutnant, the quartermaster, and two of the crew. The doomed U-boat began to sink, clouds of oil-laden vapour issuing from the jagged base of the conning-tower; but even that was not enough. It is the practice of the U-boat hunters to make doubly sure. At increased speed, and with slight port helm, the _Bolero_ scraped past the up-tilted stern of her victim. Resisting the temptation to ram her with the destroyer's knife-like bows, Seton held on his course, while right aft a couple of petty officers were busily engaged in allowing a wire to run out. Attached to the wire was a powerful depth-charge--one of two ready for instant use. Fifty--sixty--seventy--eighty fathoms, the P.O. brought his hovering finger down smartly upon the firing-key of the battery. He performed the act without emotion, although it meant sealing the death-warrant of a score or more of human beings. To him it was merely the performance of duty: frequency of opportunity had made it matter of routine. With a stupendous roar a column of water, showing greyish-white through the darkness, was hurled a couple of hundred feet into the air. The _Bolero_, as the tremendous wash created by the explosion met and overrode the crested waves, shook violently from stern to stem, while fragments of metal, hurled upwards to an immense height, fell all around her. For some minutes it seemed as if the fury of the wind was subdued by the blast of displaced air, while astern the waves subsided in a rapidly-increasing circle under the influence of tons of heavy oil liberated from the shattered wreck of the modern pirate. "Hard a-starboard, quartermaster!"