"It is bluff, my friend," he said dryly. "I haff seen many weapons. I am a scientist! You play der game of poker. You try a bluff! But I answer you with der heat-ray!" He moved his great bulk, and Thorn released his left hand. There was a sudden crack on Kreynborg's side of the room. A pebble a little over an inch in diameter fell to the floor. Kreynborg wavered, and toppled and fell. Three times more, his face merciless, Thorn drew back his arm, and three times Kreynborg's head jerked slightly. Then Thorn faced the panel on which the induction-screen switch was placed. Several times he thrust his hand through the screen and abruptly drew it back with pain, in an attempt to throw the switch. At last he was successful, and now he walked calmly across the room and bent over the motionless Kreynborg. "Skull fractured," he said grimly. "All right, Sylva." * * * * * He went through the narrow doorway beyond, picking up the pocket-gun as he went. There was a noise of whining machinery. Now Thorn was emptying pellets into the mechanism that controlled the dome of force. There was a crashing of glass. It stopped. There were blows and thumpings. That noise stopped too. Thorn came back, his eyes glowing. He flung open the outer door of the rocket-ship, and Sylva went to him. He pointed. Far away, the Fighting Force of the United Nations was swirling upward. Like smoke from a campfire or winged ants from a tree-stump, they went up in a colossal, twisting spiral. Beyond the domes and above them. The domes existed no longer. Up and up, and up.... And then they swooped down upon the suddenly fleeing enemy. Vengefully, savagely, with all the fury of men avenging not only what they have suffered, but also what they have feared, the combat-squadrons of the United Nations fell upon the invaders. Green hexynitrate explosions lighted up the sky. Ear-cracking detonations reverberated among the mountains. There was battle there, and death and carnage and utter destruction. The roar of combat filled the universe. Thorn closed the door and looked down at Kreynborg, who breathed stentorously, his mouth foolishly open. "Our men will be back for us," he said shortly. "We needn't worry." Then he said, "Huh! He called himself a scientist, and he didn't know a sling-shot when he saw one!" But then Thorn Hard dropped a weapon made of a forked stick and strong elastic from his chute-pack, and caught Sylva hungrily in his arms. * * * * *