"Those tubes," answered the professor, "are the boat's means of propulsion. You see," he explained, "being built of aethereum, the boat is extremely light, and draws so little water that a screw propeller would be quite useless to her. So I have substituted those tubes instead. One pair, you will observe, points toward the stern, and one pair toward the bow. The boat's engine is a powerful three-cylinder pump, and it sucks the water strongly in through the tubes which point forward, discharging it as powerfully out through those which point astern; thus drawing and driving the boat along at a speed of about twelve knots per hour, which is as fast, I fancy, as we shall ever want her to go. If you want to go astern the movement of a single lever reverses the whole process. There is a similar boat on the other side." The boat having been returned to her hiding-place, the professor next led his friends to the structure which occupied the centre of the deck. It was a perfectly plain erection, with curved sides meeting in a kind of stem and stern-post at its forward and after ends, with a curved dome-like roof, several small circular windows all round its sides, and no apparent means of entry. "Why, how is this, professor? You have actually built your pilot- house--for such I suppose it is--without a door," exclaimed the baronet with returning good-humour as he perceived that, even in the event of the _Flying Fish_ failing to fly, he would still have a very wonderful ship for his money. "As you have rightly supposed, this _is_ the pilot-house," answered the professor, with one hand pressing lightly against the gleaming wall of the structure. "But as to its being without a door, you are mistaken, for there it is." And as he spoke a door, hitherto unnoticed in the side of the building, flew open. "Why, you are a veritable magician, professor! How on earth did you manage that?" exclaimed the colonel. "Easily enough," answered the professor. "Just look here, all of you. This is a secret door which it is necessary you should all know how to open. Now, there are four of us, are there not? Very well; find the fourth rivet from the bottom in the fourth row from the after end of the building--here it is--push it to your left--_not_ press it; pressing is no good--and open flies the door. Push the rivet to the _right_ when the door is open, and you shut it--so," suiting the action to the word. "Now, Sir Reginald, let me see if you can open that door." The baronet opened and closed the door without difficulty; and then the other two essayed the attempt with similarly successful results. "That is all right," commented the professor. "Now step inside, please; and close the door--so: when you want to open it from the inside you simply turn this handle--so, and open it comes."