"So you have, but it was always after the four rings that followed the whistle. A little while ago I was near the tank, and I heard the whistle. The attendant sprang to the wheel, and when the four rings came he turned the wheel around twice. When the four next rings came (without the whistle, of course), he quickly turned it back again," said Alfred. "That is the submerging tank," said the captain. "I see you are rapidly learning how to handle a submarine," and he laughed at the eagerness of the boys trying to conquer the details of signaling. During that night there was hardly a half-hour but some movement or other was indicated by the bells. They submerged, halted, rose to the surface, steamed at full speed, and in one or two instances it was evident from the sudden stopping that the submarine had to reverse. This constantly kept them alert, and while engaged in conversation late in the morning, they were thrown forward on their seats with a motion that indicated a collision with something which was not very rigid, for there was no concussion such as usually accompanies the contact of the hull of a vessel with a hard object. The boys looked at the captain in astonishment. They could now feel the propeller pulling in the opposite direction, only to be brought back again with the same springy collision, as when it had gone forward and first struck the strange obstacle. The captain's face paled, and the boys plied him with questions as they saw his perturbed countenance. "What do you think it is?" asked Ralph, as he saw the anxious seamen, and the second officer rushing about shouting orders, while one of them seized the main valve wheel and turned it. "We are caught in one of the steel nets," said the captain quietly. The boys' faces grew deadly pale. They knew what such a calamity meant. Few, if any of the submarines caught in the nets, ever escaped. The boys, while they did not know this, were, in a measure, aware of the great danger to submarines from this source. They were alarmed particularly on account of the serious manner in which the captain acted the moment the first impact took place. The captain now arose, followed by the boys, and marched through the narrow passageway toward the lieutenant who was leaning over one of the air compressors. "Is there anything we can do to help you?" asked the captain. The lieutenant looked up and replied: "We can do nothing but change the trim of the ship. Everything portable in the stern must be moved forward. Your assistance will be appreciated," was the reply, an answer that was in marked contrast with his former demeanor.