* *

He circled back, his flock wheeling like a flight of birds and
following him. He signaled to them to scatter. They had certainly been
observed; at any moment a hail of lead might assail them invisibly out
of the air. They must get to work quickly. But had they understood the
significance of those bare patches?

Dick saw Luke Evans still fidgeting impatiently with his end of the
speaking-tube, and picked it up.

"I'm thinking, Captain Rennell, we've got no time to lose if we want
to keep the upper hand of those devils," called the old man.

"Yes, you're right," Dick answered. "Lay a trail of gas bombs all
around those hangars and buildings, enough to hold them dark for some
time. And keep a bomb or two in reserve."

Luke Evans shouted back. The plane was again above the structures. The
old man dropped a bomb over the side, and Dick zoomed again, his
flight wheeling up behind him.

Higher and higher, banking and going round in a succession of tight
spirals, Dick flew. Every moment he expected the blow to fall. As he
rose, Luke Evans dropped bomb after bomb. A thousand feet beneath the
flight was taking up positions, hovering with the helicopters, looking
up to Dick for the signal, and waiting.

Then from beneath the cloud of black gas began to rise, as Luke Evans
dropped his bombs. It filled the lower spaces of the sky, blotting out
the land in impenetrable darkness. That darkness, above which Dick and
his flight were soaring, rose like a solid wall, built by some
prehistoric race that aimed to fling a tower into the heavens.

* * * * *

And then--the miracle! Dick gasped in sheer delight as he realized
that he had made no mistake.

At first all he could see was a number of criss-crossing
phosphorescent lines that appeared shimmering through the blackness
underneath. They ran luminously here and there, forming no particular
pattern, much like the figures on the radium dial of a watch when
first they come into wavering visibility at night.

Then the lines began to intersect one another, to assume geometric
patterns and curves. And bit by bit they took meaning and
significance.

And suddenly the whole invisible stronghold lay revealed upon the
ground beneath, a shining, dazzling play of weaving light.

Buildings and hangars stood out, clearly revealed; the rounded vault
of a dirigible hangar, and the shining ribbon of a road that ran
through a pitch-dark tarmac