For everywhere around it, on all the roads, the dead lay piled on top of one another. For miles--all the inhabitants, rich and poor, business men, factory hands, negroes. There had been a mad rush as the fatal gas drove onward upon its lethal way, and all the fugitives had been overwhelmed simultaneously.

Here were golf links, with little groups strewn on the gra.s.s and fairways; here, at one of the holes, four men, their putters still in their hands, crouched in death. Here was the wreckage of a train that had collided with a string of freight cars at an untended switch, and from the shattered windows the heads and bodies of the dead protruded in serried ranks.

d.i.c.k looked back. His flight was driving on behind him. He guessed their feelings. They had sworn, as he had sworn, that none of them would return without stamping out that abomination from the earth forever.

He signaled to the flight to rise, and zoomed upward to twelve thousand feet. He did not want to look upon any more of those horrors.

At that height, the peaceful landscape lay extended underneath, in a checker-board of farms and woodlands. One could pretend that it was all a vile dream.

He avoided Charleston, and winged out above the Atlantic, striking a straight course along the coast toward the Bahamas. The sh.o.r.es of Georgia vanished in the west. d.i.c.k began to breathe more freely. His mind shook off its weight of horror. Only the blue sea and the blue sky were visible The aftermath of the gale remained in the shape of a strong head breeze and white crests below.

d.i.c.k glanced at the guinea-pigs. They were busily gnawing their cabbage and carrots. The gas had evidently been entirely dissipated by the wind.

Toward sunset the low jutting fore-land of Canaveral on the east coast of Florida, came into view. d.i.c.k shifted course a little. Three hours more should see them over Abaco.

His flight had explicit instructions. As soon as the black gas had rendered visible the headquarters of the Invisible Emperor, they were to circle above, dropping their bombs. When these were exhausted, the machine guns would come into play. There was to be no attention paid to signals of surrender. They were to wipe out the headquarters, to kill every living thing that showed itself--and the navy and the marines would mop up anything left over.

The sun went down in a blaze of gold and crimson. Night fell. The moon began to climb the east. The black sea, stretching beneath, was as empty as on the day when it was created. Nothing in the shape of navigation appeared.

Two hours, three hours, and old Evans turned round in his c.o.c.kpit and pointed. On the horizon a black thread was beginning to stretch against the sky. It was Abaco Island, in the Bahama group. They were nearly at their destination. An hour more--perhaps two hours, and the deadly menace that threatened America might be removed forever. d.i.c.k breathed a silent prayer for success.

They were over Abaco. A long, flat island, seventy miles or so in extreme length, and fairly wide, covered with a dense growth of tropical brush and forest, with here and there open s.p.a.ces, near the seacoast an occasional farm-house. d.i.c.k dropped to five thousand, to three, to one. The moon made the whole land underneath as bright as day.

There were no evidence of destruction by the hurricane. The farmhouses stood substantial and well roofed. If death had struck Abaco Island, it had been the work of man, not Nature.

d.i.c.k zoomed almost to his ceiling, until, in the brilliant moonlight, he could see Abaco Island from side to side. For the most part it was heavily wooded with mahogany and lignum vitae: toward the central portion there was open land, but there was not the least sign of any construction work.

Again he swooped, indicating to his flight to follow him. At a thousand feet he examined the open district intently. Here, if anywhere upon the island, the Invisible Emperor had his headquarters.

Was it conceivable that a gas factory, hangars, ammunition depots could exist here invisibly, when he could look straight down upon the ground?

d.i.c.k's heart sank. The hideous fear came to him that Graves had been mistaken, 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 gra.s.s 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 d.i.c.k 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 gra.s.s does not grow without sunlight.

d.i.c.k saw old Luke Evans nodding and pointing downward. The old man picked up his end of the speaking-tube, but d.i.c.k 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 d.i.c.k 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 phosph.o.r.escent 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 a.s.sail them invisibly out of the air. They must get to work quickly. But had they understood the significance of those bare patches?

d.i.c.k 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," d.i.c.k 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 d.i.c.k zoomed again, his flight wheeling up behind him.

Higher and higher, banking and going round in a succession of tight spirals, d.i.c.k 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 d.i.c.k 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 s.p.a.ces of the sky, blotting out the land in impenetrable darkness. That darkness, above which d.i.c.k 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! d.i.c.k 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 phosph.o.r.escent 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 a.s.sume 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, and was evidently constructed from some gas-impregnated materials. On this tarmac was a flight of shining airplanes, ready to take off. There were the odd, ovoid figures of the aviators in their silken overalls. More figures appeared, running out from the buildings. It was clear that the sudden raid had taken them all by surprise.

Luke Evans yelled and pointed. "We've got them now, sir!" d.i.c.k heard above the whine of the helicopter engine. "We've--"

But of a sudden the old man's voice died away, though his mouth was still moving.

d.i.c.k leaned out of his c.o.c.kpit and fired a single red Very light, the signal for the attack. And from each plane of his flight, beneath him, a bomb slid from its rack and went hurtling down upon the gang below, while the airplanes circled and hovered, each taking up its station.