"Full speed astern," was the next order, but beyond churning up the brownish black ooze the propellers failed to do the slightest good. "Lay out an anchor, Mr. Devoran," exclaimed his superior officer. "Be sharp, for the tide is falling." Hastily the boat was lowered, the heaviest anchor slung underneath, and laid out a hundred yards astern. The stout hempen hawser was led to the motor capstan, but as the powerful machinery made it revolve it simply "walked home" with the anchor. The soft mud offered little or no resistance to its broad palm. The "Aphrodite" seemed doomed to remain hard and fast for another ten hours. Night with all its terrors would envelop her ere the next tide was at its height, and to work in the germ-laden mists was to court the deadly fever. "Can the ballast tank be got at from the inside, sir?" asked Hythe, struck by an inspiration. "Not without flooding one of the sub-divisions of Number Three Platform," replied Captain Restronguet. "But why do you ask?" "I thought perhaps I might put on a diving-dress, and enter the tank and examine the valves." "It’s feasible," observed Captain Restronguet. "If you attempt it and succeed I shall be still more indebted to you." "Then I have your permission?" "Certainly." "Look here, Hythe," exclaimed Kenwyn. "If you tackle the job I’m with you." "Thanks," replied the sub. "The sooner we start the better." Donning their diving-suits and equipping themselves with brushes, spanners, rubber-sheeting for the valves, and an electric lamp each, the two men entered No. 4a compartment, as the subdivision was designated. As soon as the water-tight doors were shut they began unbolting the man-hole cover that gave access to the faulty ballast-tank, guided in their self-imposed task solely by the rays of the lamps. Presently the water began to ooze between the partially removed cover and side of the tank, increasing in volume till the whole of the compartment was flooded with a liquid resembling thick pea-soup. It was only by the concentrated rays of the two lamps and by keeping his helmeted head within six inches of the metal lid that Hythe was able to complete the work of opening a means of communication with the ballast-tank. As soon as this was done the sub squeezed through the manhole. His feet touched a thick layer of stiff mud on the floor of the tank. Bringing his lamp down he carefully examined the outlet valve. The secret of the defect was his: when the "Aphrodite" had been submerged by the side of the wharf at Marromea the inrush of water had brought with it a quantity of sediment sufficiently plastic to form a layer over the outlet valve and thus prevent its action.