The lieutenant's cabin, well forward, was fitted up luxuriously. There were several bunks in the little room, and the lieutenant motioned to them. "You will sleep there," he said quietly. "Make yourselves perfectly at home. I guess there is no danger of your attempting to escape. However, you must remain below and not ascend to the bridge under any circumstances." He bowed, and left them. "I don't know as I am particularly fond of this kind of travel," Frank confided to Jack. "It's all right as long as we remain on the surface, but I'll bet it would feel queer to be moving along under the water." "Right you are," replied Jack. "However, we are here and we shall have to make the best of a bad situation. Then, too, perhaps we can learn something that may prove of use to us later on." The lads dined that night at the officers' mess and became quite well acquainted with all of them. They found Captain von Cromp not half so gruff as he had been when they first came aboard. They were questioned about the service they had seen, and their story greatly surprised all the officers. Upon Lieutenant Stein's request, the commander granted the lads permission to look over the vessel. The lieutenant showed them how the vessel was submerged, by allowing one of the tanks to fill with water; how it rose again by forcing the water from the compartment by means of compressed air; how the air was purified when a lengthy submersion was necessary, and how the vessel was handled in times of action. He showed them the periscope, and allowed them to peer through, although there was no need to use this, as the vessel was above water. "When the submarine is submerged," explained Lieutenant Stein, "the periscope is the eye of the vessel. Peering over the waves, it reflects what it sees into the watching human eye in the conning tower. Destroy it, and the submarine is a blind thing, plunging to destruction." "Then the periscope is the one weak spot in a submarine?" asked Frank. "Exactly," was the reply. "Of course, if it were destroyed, the vessel might rise immediately to the surface and so gain its bearings. But in the midst of battle it would probably mean certain destruction; for when it rose the submarine would naturally be so close to the enemy that a single big shell would put it out of business." The boys looked long at this strange mechanical eye. Shaped like a small pipe, it ran up from the conning tower and protruded above the vessel. A large lens at the top turned off as does an elbow in a stove pipe. This portion, when necessary, moved in all directions. When raised to its maximum height everything within a radius of ten miles is reflected in it. "The shaft can be lowered to within a few inches of the top of the water," the lieutenant explained, "thus guarding against the danger of being hit. The officer in the conning tower peers into the binoculars and sees just what the periscope sees."