that he had come on a wild-goose chase. This could not be
the place. It was quite incredible.

Again and again he circled, studying the ground beneath. Now he could
see that the tough grass and undergrowth marked curious geometrical
patterns. Here, for example, was an oblong of bare earth around which
the vegetation grew, and it was obviously the work of man.

Here were four squares of bare ground set side by side, with thin
strips of vegetation growing between them.

Then of a sudden Dick knew! Those squares and parallelograms of bare
ground indicated the foundations of buildings. _He was looking down on
the very site of the Invisible Emperor's stronghold!_

He shouted, and pointed downward. Luke Evans looked round and nodded.
He understood. He patted the camera-box with a grim smile on his old
face.

CHAPTER VIII

_The Magnetic Trap_

Upon those squares and oblongs of bare earth, incredible as it seemed,
rose the structures of the Invisible Empire, themselves both invisible
and transparent, so that one looked straight down through them and saw
only the ground beneath them.

Every interior floor and girder must have been treated with the gas.
They had been cunning. They must have discovered some permanent means
of charging paint with the shadow-breaking gas, so that the buildings
would remain invisible for months and years instead of hours.

But they had not been cunning enough. It had not occurred to them that
the foundations would still be visible underneath, for the simple
reason that grass does not grow without sunlight.

Dick saw old Luke Evans nodding and pointing downward. The old man
picked up his end of the speaking-tube, but Dick ignored the gesture.
He signaled to his flight to rise, and zoomed up, circling, and
studying the land beneath.

That oblong was evidently the central building. Those four squares
probably housed airplanes, and each would hold half a dozen. That
elliptical building might contain a dirigible. That round patch was
probably the gas factory.

Now Dick could see more patches of bare ground, extending in the
direction of the sea. He gunned his ship and followed the gap among
the trees to the ocean, a few miles distant. Yes, there were more
evidence of activity here. Beside the water, in what looked like a
deep natural harbor, was what seemed to be the foundations of a dock.
Perhaps even vessels of war floated on the phosphorescent Bahama sea.

* * *