"You may have noticed the almost total absence of a bow wave," continued the captain. "This is owing to the vessel's remarkable flare--for which the designers must take the credit. By means of an ingenious contrivance the displaced water is led close alongside, yet the 'skin friction' of the hull is not increased. The effect of this diversion is to give the propellers a better grip. As a matter of fact, the 'slip' of the propellers amounts to less than 5 per cent. Now we will make our way for'ard to the conning-tower." "But what is the motive power?" asked Gerald. "Petrol, paraffin, or, in fact, any inflammable oil capable of passing through the vaporisers. The motors, which can be attended to by a staff of ten engineers only, actuate five propellers. Thus we are not under the obligation of having to carry stokers. As a matter of fact, the _personnel_ of the _Olive Branch_, thanks to mechanical appliances of the most modern type, amounts to 105 officers and men." The conning-tower consisted of a circular armoured structure 25ft. in diameter, and barely 5ft. above the upper deck. Around the walls were electrical indicators and a maze of pipes painted in distinctive colours similar to those on board a British man-of-war. But the apparatus that riveted Gerald's attention was a board composed of copper and zinc squares resembling a draught board, with a pair of pointers at two adjacent corners. This device travelled on steel lines that formed an almost complete circle, a gap being left in the direction of the after end of the ship. "Now, Mr. Tregarthen, what do you suppose this arrangement is for?" "It appears to me to be a sort of position-finding instrument." "It is more than that. But first I must request you to maintain a strict secrecy regarding what you see in the conning-tower, at least until I withdraw this restriction. Beyond that you are at perfect liberty to discuss with your companion any details concerning the ship," replied Captain Brookes. "Before you left Poole Harbour did you see a report in the papers concerning the German cruiser _Zietan_?" Like a flash the truth swept across Tregarthen's mind. The ex-Brazilian vessel, _Almirante Constant_, the mysterious agency that had temporarily incapacitated the _Zietan_, and the ship on which he stood were one and the same. So astonishing was the revelation that even he--an iron-nerved naval officer--gasped with amazement. The captain, who was watching the effect of his question, kept silent, awaiting Gerald's reply. "I did," he assented after a prolonged interval. "But how did you do it? And how were you aware?"