And then--the miracle! Dick 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 phosphorescent 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 assume 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!" Dick 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. Dick leaned out of his cockpit 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. * * * * * Dick was too late. By a whole minute he had missed his chance. He realized that immediately, for before the red light had flared from his pistol, the hostile planes were in the air. He had flown too low, and given the alarm. It meant a fight now, instead of a mad dog destruction, and Dick did not underestimate the power of the enemy. But he felt a thrill of furious satisfaction at the prospect of battle. From every plane the bombs were falling. Underneath, ruin and destruction, and leaping flames--and yet darkness, save for the phosphorescent outlines of the buildings. And the lines of these were broken, converging into strange criss-crosses of luminosity, as the beams fell in shapeless heaps. Dark fire, sweeping through the headquarters of the Invisible Emperor, a veritable hell for those below! A taste of the hell that they had made for others! Then a strange phenomenon obtruded itself upon Dick's notice. _Nothing was audible!_ The bombs were falling, but they were falling silently. No sound came up from beneath. And, except for the throbbing of his engine, Dick would have thought it had stopped. He could no longer hear it.