"Why the silence, I wonder?" asked Seaton, while the futile shells of the enemy continued to waste their force some hundreds of feet distant from their goal, and while Crane and DuQuesne were methodically destroying the huge vessels as fast as they could aim and fire. At every report one of the monster warships disappeared--its shattered fragments and the bodies of its crew hurtling to the ground. His voice could not be heard in even the lessened tumult, but he continued: "It must be that our repellers have set up a partial vacuum by repelling even the air!" * * * * * Suddenly the shelling ceased and the Skylark was enveloped by a blinding glare from hundreds of great reflectors; an intense, searching, bluish-violet light that burned the flesh and seared through eyelids and eyeballs into the very brain. "Ultra-violet!" yelled Seaton at the first glimpse of the light, as he threw on the power. "Shut your eyes! Turn your heads down!" Out in space, far beyond reach of the deadly rays, the men held a short conference, then donned heavy leather-and-canvas suits, which they smeared liberally with thick red paint, and replaced the plain glasses of their helmets with heavy lenses of deep ruby glass. "This'll stop any ultra-violet ray ever produced," exulted Seaton, as he again threw the vessel into the Mardonalian fleet. A score of the great vessels met their fate before the Skylark was located, and, although the terrible rays were again focused upon the intruder in all their intensity, the carnage continued. In a few minutes, however, the men heard, or rather felt, a low, intense vibration, like a silent wave of sound--a vibration which smote upon the eardrums as no possible sound could smite, a vibration which racked the joints and tortured the nerves as though the whole body were disintegrating. So sudden and terrible was the effect that Seaton uttered an involuntary yelp of surprise and pain as he once more fled into the safety of space. "What the devil was that?" demanded DuQuesne. "Was it infra-sound? I didn't suppose such waves could be produced." "Infra-sound is right. They produce most anything here," replied Seaton, and Crane added: "Well, about three fur suits apiece, with cotton in our ears, ought to kill any wave propagated through air." The fur suits were donned forthwith, Seaton whispering in Crane's ear: "I've found out something else, too. The repellers repel even the air. I'm going to shoot enough juice through them to set up a perfect vacuum outside. That'll kill those air-waves." Scarcely were they back within range of the fleet when DuQuesne, reaching for his gun to fire the first shot, leaped backward with a yell. "Beat it!" Once more at a safe distance, DuQuesne explained.