Finally Craig pressed down a key which seemed to close a circuit including the connection in the electric-light socket and the arrangement that had been let over the edge of the float. Standing where we were we could feel a sort of dull metallic vibration under our feet, as it were.
“What are you doing?” inquired Hastings, looking curiously at a headgear which Kennedy had over his ears.
“It works!” exclaimed Craig, more to Watkins than to us.
“What does?” persisted Hastings.
“This Fessenden oscillator,” he cried, apparently for the first time recognizing that Hastings had been addressing him.
“What is it?” we asked crowding about. “What does it do?”
“It’s a system for the employment of sound for submarine signals,” he explained, hurriedly. “I am using it to detect moving objects in the water—under the water, perhaps. It’s really a submarine ear.”
In our excitement we could only watch him in wonder.
“People don’t realize the great advance that has been made in the use of water instead of air as a medium for transmitting sounds,” he continued, after a pause, during which he seemed to be listening, observing a stop watch, and figuring rapidly on a piece of paper, all at once. “I can’t stop to explain this apparatus, but, roughly, it is composed of a ring magnet, a copper tube which lies in an air gap of a magnetic field, and a stationary central armature. The magnetic field is much stronger than that in an ordinary dynamo of this size.”
Again he listened, as he pressed the key, and we felt the peculiar vibration, while he figured on the paper.