“No, I’ve located the echo from the Sybarite and some others. But,” he added slowly, “there’s one I can’t account for. There’s a sound that is coming to me direct from somewhere. I can’t just place it, for there isn’t a moving craft visible and it doesn’t give the same note as that little cruiser. It’s sharper. Just now I tried to send out my own impulses in the hope of getting an echo from it, and I succeeded. The echo comes back to me in something more than five seconds. You see, that would make twenty thousand-odd feet. Half of that would be nearly two miles, and that roughly corresponds with the position where we saw the scout cruiser at first, before it fled. There’s something out there.”
“Then I was right,” I exclaimed excitedly. “I thought I saw something in the wake of the cruiser.”
Kennedy shook his head gravely. “I’m afraid you were,” he muttered. “There’s something there, all right. That wireless operator is up in our room and you have a wire from the boathouse to him?”
“Yes,” I returned. “Riley’s holding it open.”
Anxiously Kennedy listened again in silence, as though to verify some growing suspicion. What was it he heard?
Quickly he pulled the headgear off and before I knew it had clapped it on my own head.
“Tell me what it sounds like,” he asked, tensely.
I listened eagerly, though I was no electrical or mechanical engineer, and such things were usually to me a sealed book. Still, I was able to describe a peculiar metallic throbbing.
“Record the time for the echo from it,” ordered Kennedy, thrusting the stop watch into my hands. “Press it the instant you hear the return sound after I push down the key. I want to be sure of it and eliminate my own personal equation from the calculation. Are you ready?”
I nodded and an instant later, as he noted the time, I heard through the oscillator the peculiar vibration I had felt when the key was depressed. On the qui vive I waited for the return echo. Sure enough, there it was and I mechanically registered it on the watch.