“Fastened outside the hull of this boat is one of the Fessenden oscillators: a steel disk eighteen inches in diameter, that can be vibrated very rapidly by electricity. These vibrations travel through the water, like wireless waves through the ether, till they strike the oscillator on another vessel and set it to vibrating in sympathy. To send a message, I start and stop the oscillator with this key so as to form the dots and dashes of the Morse code. To receive, I sit here with these receivers over my ears and ‘listen in,’ just like a wireless operator, till I pick up our call ‘X-4,’ ‘X-4.’” “How far can you send a message under water?” “Ten miles is the furthest I’ve ever sent one. Professor Fessenden has sent messages more than thirty miles. The invention only dates back to 1913 and what it will do in the future, there is no telling.” [Illustration: Courtesy of the American Magazine. Professor Fessenden receiving a message sent through several miles of sea-water by his “Oscillator.”] “Even now, couldn’t a surface vessel act as eyes for a whole flotilla of submarines and tell them where to go and when to strike by coaching them through the Fessenden oscillator?” The operator nodded. “We’re doing it to-day, in practice. But don’t forget that an enemy’s ship carrying a pair of oscillators can hear a submarine coming two miles away. You can make out the beat of a propeller at that distance every time.” “But how can you tell how far away and in what direction it is?” “I can’t, with a single oscillator like ours. But a ship carries two of them, one on each side of the hull, like the ears on a man’s head. And just as a man knows whether a shout he hears comes from the right or left, because he hears it more with one ear than the other, so the skipper of a surface craft can look at the indicator that registers the relative intensity of the vibrations received by the port and starboard oscillators and say, “‘There’s somebody three points off the starboard bow, mile and three quarters away, and heading for us. Nothing in sight, so it must be one of those blamed submarines.’ “And away he steams, full speed ahead and cutting zigzags. Or maybe he gets his rapid-fire guns ready and watches for Mr. Submarine to rise--like the _X-4_’s doing now.” Freed of the dead weight of many tons of sea water blown from her ballast-tanks by compressed air, the submarine rose to the surface like a balloon. Ventilators and hatch-covers were thrown open and we swarmed up on deck to fill our grateful lungs with the good sea air. Three motor-boats from the tug throbbed up alongside with the four torpedoes we had discharged.