"God, but the air's going fast," Darl groaned. The accident he had feared through all the months he had captained Earth's outpost on Mercury had come at last. The Dome's shell was pierced! A half-mile high, a mile across its circling base, the great inverted bowl was all that made it possible for man to defy the white hell of Mercury's surface. Outside was an airless vacuum, a waste quivering under the heat of a sun thrice the size it appears from Earth. The silvered exterior of the hemisphere shot back the terrific blaze; its quartz-covered network of latticed steel inclosed the air that all beings need to sustain life. Darl tugged desperately at the control-stick, thrust the throttle over full measure. A little more of this swift outrush and the precious air would be gone. He caught a glimpse of the Dome floor beneath him and the shaft-door that gave entrance to the mine below. Down there, in underground tunnels whose steel-armored end-walls continued the Dome's protection below the surface, a horde of friendly Venusians were laboring. If the leak were not stopped in a few minutes that shaft door would blow in, and the mine air would whisk through the hole in its turn. Only the Dome would remain, a vast, rounded sepulcher, hiding beneath its curve the dead bodies of three Earthmen and the silent forms of their Venusian charges. * * * * * Darl's great chest labored as he strove to reach the danger spot. Invisible fingers seemed to be clamped about his throat. His eyes blurred. The gyrocopter was sluggish, dipped alarmingly when it should have darted, arrow-like, to its mark. With clenched teeth, the Terrestrian forced the whirling lifting vanes to the limit of their power. They bit into the fast thinning air with a muffled whine, raised the ship by feet that should have been yards. By sheer will he forced his oxygen-starved faculties to function, and realized that he had reached the wall. He was drifting downward, the hole draining the Dome's air was five feet above him, beyond his reach. The driven vanes were powerless to stem the craft's fall. One wing-tip scraped interlaced steel, a horizontal girder, part of the vault's mighty skeleton. Darl crawled along the wing, dragging with him a sheet of flexible quartzite. The metal foil sagged under him and slanted downward, trying like some animate thing to rid itself of the unwonted burden. He clutched the beam, hung by one leg and one arm as his craft slid out from beneath him. The void below dragged at him. He put forth a last tremendous spurt of effort.