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."