med apart
from the world. Once he stumbled against something soft and
yielding--a body flung down there in death, fingers at its throat. And
there were other white-clad figures, grimly marking off the length of
the cat-walk....

Chris's nerves were raw and his face sopping with sweat beneath its
mask when suddenly he stopped at sight of something that lay on the
cat-walk, with the main fuel tanks on the girders just above it and
the entrance to the control car just below.

* * * * *

It was a black box, perhaps two feet square and a foot in depth, made
of dull metal that did not reflect the rays of the light bulb placed
at the head of the ladder leading down in the control car. There were
three curious little dials on its face, and the trembling finger of
each one was mounting.

It had been strategically placed. An explosion at that point would rip
open the fuel tanks, split the largest gas bag, wreak havoc on an
intricate cluster of main girders, and destroy the control car with
its mechanism.

"No wonder the ZX-2 crashed!" Chris muttered.

Then his hands swept down. The next instant he was hugging the thing
tight to his chest and stumbling down into the control car, hearing
only a high-pitched, impatient whine that was coming from the box as
the fingers of its dials crept slowly upward.

The ZX-1 was wavering wildly as her rudders flopped from side to side,
and with every swing the bodies that lay in her control car, strangled
by gas, stirred slightly. The gray-haired commander was stretched
there, one arm limply rolling as his ship, which had gone so suddenly
from him, rolled. Subordinate officers were tumbled around him. Death
rode the control car.

But down to it and through it now came one who was alive, a figure
made grotesque by the mask it wore and the pack of the parachute
strapped to it, who threaded past the littered bodies, an ever-rising
whine wailing from the box clasped in his arms.

With a leap, he was at one of the car's port-holes, fingers fumbling
at the heavy bolts. The seconds seemed eternal, and the box's whine
had become a shattering, sinister scream when at last the bolts
loosened. The round pane of glass teetered back, swung open--and the
masked man slung his metal burden out, out from the ZX-1 into the gulf
between sea and sky.

It arced through the sunlight, went spinning down, became a dot, its
screaming faded. Then something synchronized within i