"No message has been received at the Semaphore Tower, sir," they replied. The captain gave a deprecating shrug and descended the ladder. "They’re coming back, by Jove!" exclaimed Lieutenant Egmont, after another ten minutes had elapsed. "That proves that the message was a hoax." "They may have seen something," suggested the sub, unwilling to have his opinions shattered. "Not they. Do you mean to tell me that if they had spotted anything suspicious they would not follow it up. I was----" The navigating officer’s words were interrupted by a heavy detonation, like the report of a fourteen-inch gun fired with a full charge. Beyond the houses of Old Portsmouth, and at an altitude of about five hundred feet, a cloud of yellow smoke hung almost motionless in the still air. The aero-hydroplanes, overtaken by a wave of disturbed atmosphere, lurched violently, although fully a mile from the actual place of the explosion. It required all the efforts at the command of their crew to save the aerial vessels from destruction, but recovering their equilibrium by superb manoeuvring of the planes, the aero-hydroplanes turned and headed towards that portion of Spithead over which they had so lately been reconnoitring. "By Jove! There’s pluck for you!" ejaculated Egmont. "That was Restronguet’s signal. If it had been to time those fellows would have been done for; and now they’re trying to spot the submarine. You were right after all, Hythe. That paragraph was not a hoax." Captain Tarfag was in the middle of lunch when the detonation was heard. He rushed on deck, and realizing that it was a case where waiting for orders would be detrimental to success, he ordered the moorings to be slipped. Within the harbour all was commotion. Nearly a dozen destroyers, two scouts, and three tugs were making for Spithead, while five more aero-hydroplanes and the naval airship "Beresford" were ploughing their way against a stiff south-easterly breeze towards the scene of Captain Restronguet’s latest demonstration. One noticeable result of the explosion was that within a quarter of an hour the weather, hitherto perfectly calm, became changed. Clouds were rapidly banking up, with every appearance of a heavy thunderstorm, while the placid waters of Spithead were now white with foam-crested waves. For two hours the "Investigator" and her consorts cruised up and down, betwixt the Nab Lightship to the eastward and Cowes to the west. Aloft the aircraft kept anxious watch and ward, till it seemed impossible that any craft could lie at the bottom of that comparatively shallow roadstead without being discovered. "Nothing to report," came the wireless message from the aircraft with monotonous regularity. Captain Restronguet had outwitted the eyes and ears of the British Fleet.