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.

Jim clung to his port-hole, tears rolling down his cheeks, unashamed. The plane, and Darl, vanished. Jim saw the black smoke masses whirl through the jagged hole in the Dome's wall as the air burst out in a cyclonic gust. He saw the vast space filled with falling Mercurians, saw a blue form plunge down and crash far below. He knew that in all that huge hemisphere, and in the burrows beneath it, there was no life save himself, and Angus, and the faithful Ran-los. For only in this compartment that clung to the roof of the Dome was there left air to breathe. And, from the void beyond, the silver space ship sped on toward Mercury, sped on to a safe landing that, but for Darl Thomas's sacrifice, would have been her doom....

Guided by Jim and Angus, a party of men from the battle-flier, equipped with oxygen respirators, went to the aid of Darl. They dug him out from under his crumpled plane and the piled splinters of quartz. His metal was dented and twisted, but unpierced. They carried him tenderly to the space ship, and carefully set him down. The ship's physician listened long with his stethoscope, then looked up and smiled.

"He's alive," the doctor said, "just barely alive. The thick padding of bandages must have saved him from the full shock of the crash. They're hard to kill, these ITA men. I'll be able to bring him around, God willing."