“That sure was a sudden dive,” agreed Jerry.
“They must have their machinery under pretty good control, and be able to work it quickly,” came from Bob. “Why, that old gentleman wasn’t down inside that hatch more than a quarter of a minute before the whole thing was under water. The hatch must have closed automatically when he went down it.”
“I guess that’s it,” said Jerry. “You can’t see so much as a bubble of her now.”
The boys gazed at the surface of the sea. The heaving and rolling waves were all that was visible.
“She must have gone down deep,” observed Ned. “You couldn’t even see her periscopes.”
“She didn’t have any,” asserted Jerry. “If she had they would have stuck up for a second or two, for usually they’re about twenty feet above the deck. She doesn’t use periscopes, that’s evident.”
“What are periscopes?” asked Bob, who usually didn’t take such an interest in mechanics as did his chums. When taunted with this Bob used to say it kept him so busy cooking for Ned and Jerry that he had no time to brush up on the latest inventions.
“Periscopes are the eyes of a submarine, when it is running in about twenty feet of water,” explained Jerry. “I mean at that depth below the surface. They are hollow tubes, and are just above the surface when the boat is down about twenty feet. They run through the deck, and into the pilot house. By looking into the lower end of them the observer can get a view all around him at the surface.”
“I don’t see how,” spoke the stout lad.
“It is done by means of reflecting mirrors, lenses and prisms,” Ned put in. “I looked through one once on a submarine that was being built. It’s great. It beats a telescope all to pieces. A telescope, you know, means an instrument by which you can see far off—‘tele,’ meaning afar, and ‘scope’ to look—Latin or Greek words, I guess.”