As the "Grigsby" neared the spot speed was considerably reduced. Overhead hovered the "blimp," ready for instant signalling of one word. The command had already been passed to the men stationed by the depth bomb to let go as soon as the messenger gave the word from the operator. As Darrin glanced upward he saw the "blimp" nearly overhead. Suddenly the messenger's startled voice roared out the message passed by the radio operator: "_Full speed astern!_" In the same instant Lieutenant Fernald repeated the order over the engine-room telegraph. There was a jolting jar as the "Grigsby" shivered, then glided back in her own wake. "Jove! That was a narrow squeak!" came down from the sky. "That hornet laid an egg in your path. It came within an ace of bumping your keel." "Never did speed pay a prompter profit, then," uttered Darrin, his cheeks paling slightly. For the Englishman's laconic message meant that the submarine had just proved herself to be of the mine-laying variety. Further, the Hun craft, hearing the destroyer's propellers almost overhead, had judged the moment at which to let loose a mine, which, rising to its proper level under water, would have struck the hull of the advancing destroyer. Had that happened, the career of the "Grigsby" would have been over, and several officers' and seamen's names would have been added to the war's list of dead. "Going to try again, sir?" asked Lieutenant Fernald, quietly, as Dave himself changed the full-speed-astern order. "It's out of our line, I guess," Darrin confessed, with a smile. "Signal yonder mine-sweeper to close in on the job." As a result of the message, and aided by the "blimp" overhead, the snub-nosed mine-sweeper steamed into position. First, her wire sweeper picked up the mine that had been sprung for the "Grigsby's" undoing, and backed away. Then, under Dave's further order, after the mine had been hoisted on board, the snub-nosed craft moved in with a different type of sweeper. To different wires of this implement were attached small but powerful contact bombs. Jauntily the snub-nosed craft moved over the lurking place of the submarine, and passed on ahead. From the depths came muffled sounds, followed by a big and growing spread of oil on the water. "Enemy done for!" signalled the "blimp." "Thank you, sir. We know it," the "Grigsby" wirelessed back. The mine-sweeper, having passed on ahead, now circled back, her crew grinning at sight of the mass of floating oil. The contact bombs dangling from the sweep wires had struck against the submarine's hull and exploded, letting in the water at several points. The Hun seamen were even now drowning, caught without a show for their lives, just as they had probably sent many souls to graves in the ocean. For some minutes more the dirigible moved back and forth through the air, her observers watching for the presence of hidden enemy craft. Then, without warning, came the message: