the
panel; a row of tubes glowed; the machinery hummed. Chris jerked off
his mask.

A last faint odor was present, but he hardly noticed it, for his lips
were at the mouthpiece and he was thrusting out a call for help.

"ZX-1 calling ... ZX-1 calling ... ZX-1--Hello!"

An answer from the flagship of the Black Fleet ahead had sounded.

"This is Travers, pilot on the ZX-1, speaking. We're coming dead for
you; full speed; you'll see us in minutes. Get some planes with men
capable of handling the dirigible up here immediately. The whole
crew's been laid out by gas; there was a contrivance planted aboard to
blow up the ship and send it down in flames as the ZX-2 was. The thing
that did it--"

_Crack!_

A gun barked out from behind; something crashed and splintered on the
radio panel. Chris felt a white-hot needle sear along the side of his
head. His brain reeled; with everything dancing queerly before him in
splotches of gray and black he toppled down off the seat, knowing the
radio-telephone had been put out of commission by the cessation of
sound in the ear-phones clamped to him.

He gripped his consciousness hard. It was like a delirium: he was
lying sprawled beside the seat, twisted round so that he saw, hanging
in the cubby's entrance door, an automatic, dribbling a wisp of
smoke--the automatic that had just fired, but hanging there by itself,
held by something he could not see!

He was only half conscious, for the scorching pain along his head was
throbbing his brain dizzily, but he realized that the service repeater
he had taken from the control car lay by his side, within easy reach.
But, while on the verge of risking a wild grab for it, he heard a
voice, speaking very softly and with a slight thickness of accent.

"Do not move," it said. "I fire if you do. Now, listen: What did you
do with the box that you found? Tell me quick, or die."

It was fantastic, unreal. There was--nothing, and yet a man, living,
breathing, but invisible, was speaking! Chris could not understand;
but it was at least a little relief to know he had a human to deal
with. For with humans, strategy can be used....

* * * * *

He groaned. He saw plainly that the unseen marauder had been aboard
when he had thrown the box over, and thus had not seen it explode in
midair: did not know whether it had been tossed out or merely rendered
harmless by being tampered with. If only the latter, it could be
quic