But surely the utmost ingenuity of man had not contrived to render a modern plane, with its metalwork and machinery, absolutely transparent? * * * * * And, again, how was it possible to have silenced the sound of engines, the whir of a propeller, so that there should be no auditory indication whatever of a plane's presence? Dick looked all about him. Nothing was in the air--he could have sworn it. He replaced the soaring lever and banked in a close circle, his glance piercing the night. No, there was nothing. Crash! Boom! The plane rocked violently, tossing upon gusts of air. A huge, gaping hole of blackness had suddenly appeared in the middle of the White House lawn. The tents were flat upon the ground. Through the rising smoke clouds Dick saw tongues of flame. No shell that, but a bomb, and dropped from the skies less than five hundred feet from where Dick hovered. Yet there was nothing visible in the skies save the round orb of the moon. A rush of wind past Dick's face! One of the vanes of the helicopter crumpled and fluttered away into the night. Dick needed no further persuasion. The dead soldier had not lied. Von Kettler had begun the fulfillment of his threat! CHAPTER V _The Enemy Strikes_ As Dick's airship veered and side-slipped, he kicked hard on the left rudder and brought the nose around. Furiously he sprayed the air with a leaden hail from his quick-firer. He heard a rush of wind go past him, and realized that his unseen antagonist had all but rammed him. Yet nothing was visible at all, save the moon and the empty sky. He had heard the rush of the prop-wash, but he had seen nothing, heard nothing else. Incredible as it seemed, the pilot was flying a plane that had attained not merely invisibility but complete absence of all sound. Dick side-slipped down, pancaked, and crashed. He emerged from a plane wrecked beyond hope of early repair, yet luckily with no injury beyond a few minor bruises. He rushed toward the hangar, to encounter a bevy of scared mechanics. "Another plane! Rev one up quick!" he shouted. Planes were already being wheeled out, pilots in flying suits and goggles were striding beside them. Dick ordered one of them away, stepped into his plane, and in a moment was in the air again. In the minute or two that had elapsed since the encounter, the enemy had been active. Crash after crash was resounding from various parts of Washington. Buildings were rocking and toppling, débris strewed the streets, fires were springing up everywhere. A thousand feet aloft, Dick could see the holocaust of destruction that was being wrought by the infernal missiles. Bombs of such power had been the unattained ambition of every government of the world--and it had been left to the men of the Invisible Emperor to attain to them. Whole streets went into ruin at each discharge and from everywhere within the city the wailing cry of the injured went up, in a resonant moan of pain.