These breakwaters, and indeed all the fortifications on shore, were armed with the latest type of air-craft repelling armament; a three-inch automatic gun, capable of firing one shell per second. The bursting charge of each shell was proved to have an effective radius of a hundred yards, while the creation of air-waves and "pockets" resulting from the detonation, would seriously imperil the stability of every aeroplane within three hundred yards. At night each of these guns was supplied by an ingeniously constructed searchlight that, projecting a narrow ray of light almost parallel with the axis of the gun-barrel, rendered a "miss" an impossibility unless the range was greatly miscalculated. As the sights of the weapon were altered the beam of the searchlight was automatically adjusted. All the gunlayer had to do was to train the searchlight upon the hostile aircraft and fire. Yet in spite of all these elaborate means of defence the main portion of the British Navy, seemingly anchored in perfect security at Spithead, had received a most unpleasant moral blow. Who and what is this mysterious Captain Restronguet? CHAPTER II. SUB-LIEUTENANT HYTHE DISCOVERS THE SUBMARINE. "Pipe away the diving-party!" H.M.S. "Ramillies," the flagship of the First Battle Squadron of the Home Fleet, had just anchored in almost the identical position that she had occupied barely a quarter of an hour previously. With mathematical precision the other battleships of the squadron had also returned to their late anchorage and were preparing to investigate the mysterious occurrence in the shape of a complimentary message from the still more mysterious Captain Restronguet. Up from below tumbled the diving-party. Air-pipes, life-lines, pumps, dresses, and helmets were produced from some remote yet properly apportioned part of the ship and were thrown down in a seemingly chaotic manner upon the steel deck. Actuated by electric power several sections of the armoured shields between the upper deck and the eaves of the V-shaped shell-proof roof were lowered till they lay flat upon the deck, and steel ladders for the divers’ use were rapidly placed in position. "Do you wish me to go down, sir?" asked a sub-lieutenant of the Number One. "Certainly, Mr. Hythe," replied the first lieutenant. "Make a careful examination for a radius of say fifty yards from the shot-rope. You will doubtless be able to see the place where the flukes of our anchor held before. Ascertain if there are any traces of independent work; such as footprints in the ooze, tracks of the underbody of a submarine settling on the bottom, for example." "Very good, sir," replied the sub, who, saluting, went off to be assisted into his diving-dress.