close after Nina Blake. I saw the squat, square-rigged figure of Capt. Mackensie come forward to greet the girl, his most distinguished passenger. Instantly she spoke to the men carrying the oblong box, and they set it on end beside her on the desk; and at her gesture, they moved away.

Nobody noticed me as I got up to that box and stood in its shadow. I had no particular motive, save perhaps my instinct as an S.S. Man to probe anything that puzzles me. And suddenly I heard Nina say softly:

"No. I'm not—too frightened, Captain. I'll be quite all right."

That stiffened me. But a far greater shock came almost instantly afterward. The girl was whispering now to Mackensie, their voices too low for me to hear. She was leaning partly against the upright box, and I saw her slim white hand furtively roving it, one of her fingers pressing what might have been a hidden lever. The sleek, polished side of the box was close to my head—and abruptly, from within it, I seemed to hear a faint muffled, ticking sound! A mechanism in the huge box which the girl's furtive hand had started! It was a slow, rhythmic tick, and a faint swishing.

"Oh, you, Penelle. Come here—I want you to meet Miss Blake."

Captain Mackensie had noticed me, and his gesture brought me to join them. For a moment we stood in a group as I was introduced. Nina's hand had darted to the box again, perhaps to stop the ticking.

"Your first flight, Penelle?" Mackensie was saying. His voice was booming, hearty, loud enough to carry to any of the passengers and crew who were near us here on the dim side-deck. Jamison had told me that of everyone on the X-87, only Captain Mackensie would be aware of my true identity and purpose. I caught his significant glance now as he shot it at me from under his heavy gray-black brows. And then abruptly he stepped nearer to me.

"Never talk secretly to me," he murmured. "No insulation here. You take care of Miss Blake. Say nothing—keep your eyes wide. When we get to the Moon—"

I stiffened, went cold, with my heart suddenly pounding. My hand darted out, gripped the captain's arm. "Wait!" I murmured.

On my chest, underneath my shirt, a flat, round little detector-grid was abruptly glowing warm against my flesh. An interference current was overcharging its low-pressure wires so that they were heating and burning me. An eavesdropping current! No one save a Government criminal-tracker may legally use an eavesdropping ray. But 

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