From the stern the professor took the boys to the conning tower, where there were several wheels and levers, that placed most of the important machines and engines in the boat under the direct control of the steersman. A lever turned one way would send the ship ahead. Turned in the opposite direction it would reverse the course. A wheel like that on an automobile served to direct the rudder and so guided the _Monarch's_ course. Other levers controlled the speed of the engines, and the supply of gas that filled the silk bag. "Here is where we shall carry our supplies of condensed food," the professor went on, leading the way back into the middle room. "We will take along capsules that will supply us in a small space with meat, vegetables, soups, tea and coffee, besides milk. "The water we will get as we speed along, dropping down to earth whenever it is necessary. As for clothing, I have an abundant supply." He opened a locker and disclosed a pile of fur garments. There were big coats, caps and boots, everything made with a furry surface within as well as without. "Any one would think you were going into some cold country, professor," said Jack, looking at the warm garments. "So we are! We are going to find the north pole!" exclaimed the old inventor. "The north pole?" cried Mark. "That's what I said. Do you boys want to go along in the _Monarch_ to a place where never mortal man has been?" At that instant there came a loud knock at the door. CHAPTER V A PLAN TO SEEK THE NORTH POLE "Hark! What was that?" exclaimed Professor Henderson in a hoarse whisper. "Sounded like some one at the door," replied Mark. "Quick, Washington! Put out the lights! You boys creep back and hide under the bed. My secret must not be discovered now when everything is ready for the trial!" The boys started back toward the living room, Washington began putting out the lights and then, with the professor, joined the boys. The shed containing the airship was in total darkness, and the negro, turning down the lamp in the cabin, shrouded that in gloom also. Once more the knock was repeated. It was a peculiar one; first two raps, then a silence, then three blows, followed at intervals by six single raps. "Who is there?" asked the professor, going close to the door. "A friend," was the reply. "Give the countersign." "The North Pole, and long may it stand!" was the queer answer. It was the same the colored man had given when he sought admission after his second trip to the wreck that afternoon. Slowly the inventor unfastened the door. As he cautiously opened it a roughly dressed man slipped in. "What's the need of all this foolishness?" he demanded. "Why have you made it so dark? It's like a pocket. Is any one here?" The two boys had crawled under the bed before the door was opened, in accordance with the instructions from the old man. The inventor and Washington were the only ones visible in the cabin.