t, and it was
gone--in a burst of weird, bluish light, whose fangs forked upwards
for a second, their unearthly flash dimming even the sunlight, and
then were gone, too....

* * * * *

Chris found that his whole body was shaking. For a moment he stood
there with his masked face through the port.

"Damn close," he muttered. "But what was it that left the box here?"

Then he jarred against the side of the car as the ship swung and came
back to realization of what was needed to be done, and done at once.
He shifted his gaze, drew his head back, and thrust it forth again,
staring.

"Good Lord!" he cried. "That plane's come back!"

His own craft was not alone under the rack. The same mysterious
machine hung there again, its cockpit empty, and the automatic spider
ladder was stretched down to it from the trap-door in the dirigible
above.

"Whatever flies it is aboard now." Chris thought aloud. "But it got
back too late to stop me. Well, this time--"

He felt uneasy, however, almost powerless. What was this thing that
had wiped out the crews of two dirigibles with deadly gas, and wrecked
one of them? He spun around. The control car looked the same. But what
might be moving in it?...

Chris carried no gun; but he extracted the service repeater from the
holster of a body at his feet. Gripping it, he leaped to the helm of
the dirigible. It was the work of a moment to clamp on the mechanical
"iron mike," which steadied the ZX-1's mad swaying and leveled her
ahead in a dead straight course. He could not cut down her speed,
unless he went to each one of the hull-enclosed engine stations, and
more urgent work awaited before he could afford to do that--work of
sending out an S.O.S. before the weird, unseen killer and wrecker came
to grips with him.

Though seeming hours, only minutes had passed since he had tooled his
scout into the rack. Ahead, he could see the smudge of the Black
Fleet's smoke on the horizon. Not so very far away, but a lot could
happen in the distance still separating dirigible and surface craft.

* * * * *

He ran back into the radio-telephone cubby, which was a division of
the control car. The operator was sprawled there, limp in his seat
before the shining, switch-studded panel. Chris removed the head-gear
of ear-phones: then he hauled one of the cubby's port-holes open,
letting in a rush of cleansing air. His fingers sped quickly over