In and almost filling the space, at least a mile in width, between the inner and the outer walls were huge, seven-sided structures--featureless, squat, forbidding heptagons of dull green metal. No thing living was to be seen in that space. Its pavement was of solid metal and immensely thick, and that metal, as well as that of the walls, was burned and blackened and seared as though by numberless exposures to intolerable flame. In a lower compartment of one of these enormous heptagons Vortel Kromodeor, First Projector Officer, rested before a gigantic and complex instrument board. He was at ease--his huge wings folded, his sinuous length coiled comfortably in slack loops about two horizontal bars. But at least one enormous, extensible eye was always pointed toward the board, always was at least one nimble and bat-like ear cocked attentively in the direction of the signal panel. A whistling, shrieking ululation rent the air and the officer's coils tightened as he reared a few feet of his length upright, shooting out half a dozen tentacular arms to various switches and controls upon his board, while throughout the great heptagon, hundreds of other Vorkuls sprang to attention at their assigned posts of duty. As the howling wail came to a climax in a blast of sound Kromodeor threw over a lever, as did every other projector officer in every other heptagon, and there was made plain to any observer the reason for the burns and scars in the tortured space between the lofty inner and outer walls of Vorkulia. For these heptagons were the monstrous flying fortresses which Czuv had occasionally seen from afar, as they went upon some unusual errand above the Jovian banks of mist, and which Brandon was soon to see in his visiray screen. The seared and disfigured metal of the pavement and walls was made so by the release of the furious blasts of energy necessary to raise those untold thousands of tons of mass against the attraction of Jupiter, more than two and a half times the gravity of our own world! Vast volumes of flaming energy shrieked from the ports. Wave upon wave, flooding the heptagons, it dashed back and forth upon the heavy metal between the walls. As more and more of the inconceivable power of those Titanic generators was unleashed, it boiled forth in a devastating flood which, striking the walls, rebounded and leaped vertically far above even those mighty ramparts. Even the enormous thickness of the highly conducting metal could not absorb all the energy of that intolerable blast, and immediately beneath the ports new seven-pointed areas of disfigurement appeared as those terrific flying fortresses were finally wrenched from the ground and hurled upward. * * * * *