"It's very simple," said the lieutenant, "and at the same time I'll tell you something else. The submarine is unaffected by tempests, and for this reason also is more deadly than a battleship. The submarine can dive down into the depths where there is no movement of the waves, and it can remain under water for fourteen hours continuously. This is accomplished by tanks which can be filled with water and, overcoming what is known as the 'margin of buoyancy,' submerge the vessel. The air is replenished by special purifying devices and by tanks of oxygen. When the vessel wants to rise, it simply pumps out the water from the tanks." "It certainly is a wonderful invention," said Frank, when the lieutenant had concluded his explanation. "Indeed it is," agreed Jack. "You should be aboard when we are in action," smiled the lieutenant. "I am sure you would be greatly interested." "I don't doubt it," said Jack, "although from what you have told us regarding the deadliness of submarines, I believe that I should rather witness action on a British submarine." "Nevertheless," said the lieutenant, "you are likely to see action aboard the X-9, for I do not believe Captain Von Cromp will return to port until he has at least tried the effect of his torpedoes, on a ship or so of your countrymen." "May he go to defeat if he tries it!" said Jack fervently. "In which case," said the lieutenant with good natured tolerance, "you would undoubtedly go with us." "Even so," replied Jack, "I still could not wish to see you get away." The lieutenant glanced at him admiringly. "I believe you mean it," he said. "You are a brave lad. But come, we had all better turn in now." "I guess you are right," said Frank; "and thanks for the trouble you have taken to explain all this to us." "It was a pleasure, I am sure," was the lieutenant's reply, and they all made their way to the officer's cabin, where they prepared to retire for the night. CHAPTER V UNDER THE SEA But there was to be no sleep for any aboard the German submarine X-9 that night. As the boys were just about to tumble into their bunks, there was the sound of a sudden commotion on the vessel. Lieutenant Stein sprang to his feet, hastily donned what few clothes he had removed, and dashed from the cabin. With all possible haste, the boys followed suit. Men were rushing to and fro and no one heeded the boys' presence, although they were rudely thrust aside by hurrying members of the crew several times. "Wonder what's up?" said Jack. "Don't know," replied Frank, "unless they have sighted one of our ships." "By Jove! Let us hope not," breathed Jack. But this was indeed the cause of the excitement aboard the submarine. A British battleship had been sighted in the distance, and Captain Von Cromp was preparing to attack the unsuspecting vessel, which had failed to sight her enemy, although the latter was fully exposed to view.