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. * * * * * 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. * * * * *