“The copper tube,” he resumed mechanically to us, though his real attention was on something else, “has an alternating current induced in it. It is attached to solid disks of steel, which in turn are attached to a steel diaphragm an inch thick. Surrounding the oscillator is a large watertight drum.”
“Then it makes use of sound-waves in the water?” queried Hastings, almost incredulously.
“Exactly,” returned Craig. “I use the same instrument for sending and receiving—only I’m not doing any real sending. You see, like the ordinary electric motor, it is capable of acting as a generator, too and a very efficient one. All I have to do is to throw a switch in one direction when I want to telegraph or telephone under water, and in the other direction when I want to listen.”
“Talking through water!” exclaimed Burke, awestruck by the very idea, as though it were scarcely believable.
“That’s not exactly what I’m doing now,” returned Kennedy, indulgently, “although I could do it if there was any one around this part of the country equipped to receive and reply. I rather suspect, though, that whoever it is is not only not equipped, but wouldn’t want to reply, anyhow.”
“Then what are you doing?” asked Burke, rather mystified.
“Well, you see I can send out signals and listen for their reflection—really the echo under water. More than that, I can get the sounds direct from any source that is making them. If there was a big steamer out there I could hear her engines and propellers, even if I couldn’t see her around the point. Light travels in straight lines, but you can get sounds around a corner, as it were.”
“Oh,” I exclaimed, “I think I see. Even if that little scout cruiser did disappear around the point, you can still hear her through the water. Is that it?”
“Partly,” nodded Kennedy. “You know, sound travels through water at a velocity of about four thousand feet a second. For instance, I find I can get an echo from somewhere practically instantaneously. That’s the bottom of the bay—here. Another echo comes back to me in about a twentieth of a second. That, I take it, is reflected from the sea wall on the shore, back of us, at high tide. It must be roughly a hundred feet—you see, that corresponds. It is a matter of calculation.”
“Is that all?” I prompted, as he paused again.