From below there came a thunder of sound. Jim slid down the stairs. An irregular disk on the wall was glowing cherry-red from the heat of the blow-torch without, and the metal was quivering under the Mercurian's sledge-hammer blows. "Darl's right," he almost sobbed as he gazed helplessly. "They'll be through in no time. The Dome's gone, we're gone, the space ship's gone!" "Let me pass, Jim." Thomas' quiet voice sounded behind him. Holcomb turned. His leader was in a space suit, the helmet still unfastened. "Blazes! Where the devil are you going?" "Here, cover me with this till I reach the gyrocopter, then get back quick, and seal the air-lock." Darl thrust into Jim's hand the ray-gun he had previously reserved. "There's only one way to kill off the Martian and his mob. I'm taking it." Suddenly Jim Holcomb understood. "No, Darl, no--you can't do it! Not you! Let me go! I'm just a dumbhead. Let me go!" "Thanks, Jimmy, but it's my place." Darl's voice was low, and very calm. "I was in charge, and I lost the Dome. If I can save the boys on the ship, and you two, it's the least I can do. Good-by, old man. Give my regards to Earth." Thomas' face was gray-white. The thick bandages that still swathed him, Jim glimpsed them through the open neckpiece of the suit, gave him the semblance of a mummy. The helmet clicked shut. Swallowing a lump that rose in his throat, Jim pulled open the door. A wave of Mercurians surged in, to be seared into nothingness by his weapon. He was in the doorway, his ray sweeping the platform clear. Darl was out now, stepping into the flier that still hung by its hooked moorings. Jim caught a flash of blue and looked up. The Martian was hanging to a girder just above, his green tube pointing straight at Darl. A white ray spurted from Jim's gun. The Martian's weapon and the hand that held it vanished in the sizzling blast. The plane was loose! Jim leaped inside the air-lock, slammed the steel door shut, clamped it, and sprang for the quartz peer-hole. * * * * * Darl's gyrocopter was diving on a long slant for the Dome wall. Faster and faster it went, till all Jim could see was a white streak in the smoky dimness. And now he could see the vast interior, the teeming plain, the dwarf-festooned girders and roof-beams. He stood rigid, waiting breathlessly. Then the plane struck--fair in the center of a great panel of quartz. The wall exploded in a burst of flying, shattered splinters. A deafening crash rocked the Dome.