Four times this was repeated, then once more came an ominous and prolonged silence. While the attention of all hands was directed towards the harbour the grey, porpoise-like hull of the submarine floundered to the surface less than a cable's length from the port bow of the _Olive Branch_. Thus, having announced her safe return, the little craft disappeared beneath the waves, and, guided by consummate skill and judgment, she glided underneath the hull of her parent ship. Half an hour later her crew came on deck of the _Olive Branch_. "All correct, sir," reported Lieutenant Palmer. "Every mine exploded; six in all." "Very good," replied Captain Brookes. There were volumes of commendation in those two words, and every man who took part in the dangerous enterprise realised the fact. "We must wait another hour till there's enough water on the bar, so our pilot informs me," continued Captain Brookes. "Mr. Sinclair, I'm going below; please report if anything unusual occurs." "Well, old chap," exclaimed Stockton as the two friends gained the seclusion of their cabin "how did you get on?" "Splendidly," replied Gerald, enthusiastically. "The submarine is a masterpiece. This was the mysterious object at which the captain hinted during our inspection of the wireless room. I think I've grasped the principle of it. The submarine is barely 30ft. in length and 8ft. in beam, and is housed in a cavity underneath the ship corresponding with the upper half of the submersible. Apparently its natural buoyancy keeps it adhered to the hull of the _Olive Branch_, but to prevent undue friction when running at full speed there are several steel doors, which, when closed, complete the normal curvature of the ship's bottom. You saw how we entered the submarine?" "Yes, by means of a watertight door in the orlop-deck, and a corresponding one in the submarine's conning-tower." "Well, directly I descended into the little craft I noticed that she was propelled by electric motors, of about 35 horse-power I should imagine. Forward there is a bow tube of less than 4ins. in diameter, consequently she can carry twenty small torpedoes to our submarines' four. I noticed that the vessel was submerged by means of water ballast instead of relying upon horizontal rudders, although the latter are utilised when running beneath the surface. "Directly all hands were aboard Palmer opened a valve, and I saw the pale green light filter through the glass of the conning-tower; we had sunk clear of the _Olive Branch's_ hull. We descended quite 80ft., in fact, almost to the bottom of the sea, for I could see the water discoloured by the muddy sediment caused by the rush of the still ebbing tide. Then the motors were started, and, once well away from the cruiser, Palmer hoisted the periscope, which hitherto had been housed within the submarine.