ent at my wild-eyed appearance, and the ash jarred
from his cigar.

"Gregg! What in the devil--"

I tried to grin. "I'm on my way to bed--worked all night helping Snap
with those damn Earth messages."

I went past him, out the door into the main interior corridor. It was
the only way the invisible prowler could have gone. But I was too late
now--I could hear nothing. I dashed forward into the main lounge. It
was empty, dim and silent, a silence broken presently by a faint
click--a stateroom door hastily closing. I swung and found myself in a
tiny transverse passage. The twin doors of A 22 and A 20 were before
me.

The invisible eavesdropper had gone into one of these rooms! I listened
at each of the panels, but there was only silence within.

The interior of the ship was suddenly singing with the steward's
siren--the call to awaken the passengers. It startled me. I moved
swiftly away. But as the siren shut off, in the silence I heard a soft,
musical voice:

"Wake up, Anita--I think that's the breakfast call."

And her answer: "All right, George. I hear it."

CHAPTER IV

_A Burn on a Martian Arm_

I did not appear at that morning meal. I was exhausted and drugged with
lack of sleep. I had a moment with Snap, to tell him what had occurred.
Then I sought out Carter. He had his little chart-room insulated. And we
were cautious. I told him what Snap and I had learned: the Gamma rays
from the moon, proving that Grantline had concentrated a considerable
ore-body. I also told him the message from Grantline.

"We'll stop on the way back, as he directs, Gregg." He bent closer to
me. "At Ferrok-Shahn I'm going to bring back a cordon of Interplanetary
Police. The secret will be out, of course, when once we stop at the
moon. We have no right, even now, to be flying this vessel as unguarded
as it is."

He was very solemn. And he was grim when I told him of the invisible
eavesdropper.

"You think he overheard Grantline's message?"

"I don't know," I said.

"Who was it? You seem to feel it was George Prince?"

"Yes."

I was convinced that the prowler had gone into A 20. When I mentioned
the purser, who seemed to have been watching me earlier in the night,
and again was sitting in the smoking room when the eavesdropper fled
past, Carter looked startled.

"Johnson is all right, Gregg."

"Is he? Does he know anything about this Grantline affair?"

"No--no," said the captain hastily. "You haven't mentioned it,