THE three shoals selected by Darrin extended over a length of about thirty miles along the coast. It was the center one of these shoals on which he had had previous experience. Further, it was arranged that Dalzell should, in general, cruise along the lower fifteen miles of this stretch, while the "Grigsby" should cover the upper half. From time to time the two destroyers would meet. After sending three mine-sweepers and the three patrol boats to the shoals, two craft to each shoal, Darrin saw to it that the other six were assigned to duty in the deeper waters off shore. Then, with a hearty signal to the "Reed," the "Grigsby" started northward. She steamed by the southernmost shoal, and was passing the second when Darrin was called to the bridge by Ensign Ormsby. "That patrol boat in there signals that she has made a find, sir, so I have changed the course and am heading in." Dave's eyes gleamed as he made out the next signal from the patrol, which was: "Soundings show her to be a big craft. Shall we rig the small bombs on the sweep wires?" "Wait until we arrive," was the answering signal from the "Grigsby." In a few minutes the destroyer was within hailing distance of the patrol boat, which was lying to in the neighborhood of the find. "The enemy submarine appears to be at least 275 feet long, sir," reported the patrol boat commander. "Then a depth bomb should do the business better," Dave shouted back through the megaphone. "Sail over the craft with your sweep, and I'll follow. Signal when you judge us to be squarely over her." Under bare headway the "Grigsby" fell in behind the now slow-moving patrol boat. Almost at once the wire sweeps discovered the hull of the hiding monster. Ahead steamed the patrol boat, the destroyer following. Aft two men stood by the depth bomb apparatus. Down came the white flag of the British signalman on the smaller craft. Dave's hand rested on the telegraph lever to the engine-room. He signalled for full speed ahead, then at the proper moment he shouted: "Let her go!" An instant later the bomb splashed into the water. Immediately following the splash there came a sullen, rending roar under water. A great column of water leaped up from the sea, a heavy volume of it landing on the after deck of the destroyer, all but washing overboard one of the lookouts. The pressure of water fairly lifted the stern of the "Grigsby" until her bow dipped far in. Ensign Ormsby was thrown flat, almost rolling from the bridge. Dave, fortunately, had taken a grip that saved him from falling. It seemed as though the destroyer herself had been blown up, but she quickly settled and scooted ahead at a furious rate. "Half speed ahead," Darrin signalled, as soon as he could let go his grip, and the "Grigsby" slowed down. At the same time she swung around. Even at that distance the huge spread of oil on the surface could be seen. A wild Yankee cheer rose, which was promptly echoed by the British tars of the patrol boat.