"Pressure is calculated on the square inch of surface; for every twenty-eight inches the pressure is equal to one pound. If, therefore, 200 is multiplied by 12 and then divided by 28, the quotient will represent the number of pounds on each square inch," answered the captain. "Why multiply 200 by twelve?" asked Ralph. "Because there are twelve inches in a foot," said the captain. "Oh, yes; I didn't happen to think of it; well, 200 by 12,--that's 2,400, and divided by 28, is----" "Eighty-five," interrupted Alfred. "Well, that's not very much." "Quite true," rejoined the captain; "but how many square inches are there in a square foot?" "One hundred and forty-four," replied Alfred. "Then, eighty-five times one hundred and forty-four makes quite a sum," continued the captain. "Whew,----" said Ralph with a half whistle in his tone, "why, if I have made it out right, it's over 12,000 pounds. No wonder it isn't safe to stay down very long, if at all, at that depth." "I have often wondered how it is that the submarine could rest on the bottom or come up at will," said Alfred. "All submarines are lighter than the water in which they float," answered the captain. "They are provided with tanks holding compressed air. Now, in order to submerge, the only thing necessary is to permit enough water to flow into special tanks within the submarine, until the combined weight of the water, hull and mechanism, is the same as the amount of water that the ship displaces. If an added quantity of water is now added, it will go down, and remain under water until the air in the compressed tanks is used to force out a quantity of water from the special tanks." "But is that the only way they can go down?" asked Ralph. "Oh, no; a submarine can submerge without doing that, but in such a case power must be used," answered the captain. "What! push it down by power?" asked Alfred. "Exactly; these vessels have fins, the same as fish, so arranged that if they are properly turned and the ship moves forward, it will dive, and continue to go down at an angle as long as the fins are properly set. If the vessel should stop moving the submarine would come to the top, because it is lighter than the water," responded the captain. CHAPTER V SOME OF THE MYSTERIES OF A SUBMARINE With a click the door of their prison cabin opened and a seaman informed them that their breakfast was ready. They passed through the narrow door, and edged their way along a tortuous path that led to the rear, where they entered what might be called a miniature galley, on one side of which was a narrow shelf containing food of various descriptions. There was room only for the attendant to pass while they were seated. An abundance of the best food was served, cereals, and even fruit, forming part of the menu. Each of these vessels carry from twenty-two to thirty men, but there were in sight in the dining room only ten, besides the cook and waiter.