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 cockpit 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. Dick 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 spaces, near the seacoast an occasional farm-house. Dick 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. Dick 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? Dick'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 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.