"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.