But Darl wasn't thinking of coming down. Something he had seen through the translucent repair sheet was sending him to the look-out tower within the air-lock. Hand over hand he swung, tiny above that vast immensity of space. In his forehead a pulse still jumped as his heart hurried new oxygen to thirsty cells. He held his gaze steadily to the roof. A moment's vertigo, a grip missed by the sixteenth of an inch, the slightest failure in the perfect team-play of eye and brain, and rippling muscle, and he would crash, a half mile beneath, against hard rock. At last he reached the curving side of the landing lock. But the platform at the manhole entrance jutted diagonally below him, fifteen feet down and twelve along the bellying curve. Darl measured the angle with a glance as he hung outstretched, then his body became a human pendulum over the sheer void. Back and forth, back and forth he swung, then, suddenly, his grasp loosened and a white arc flashed through the air. Breathless, Jim saw the far-off figure flick across the chasm toward the jutting platform. He saw Darl strike its edge, bit his lip as his friend teetered on the rim and swayed slowly outward. Then Darl found his balance. An imperative gesture sent the watcher back to his post, his sorrel-topped head shaking slowly in wonderment. * * * * * Darl Thomas ran headlong up the staircase that spiralled through the dim cavern. "No mistake about it," he muttered. "I saw something moving outside that hole. Two little leaks before, and now this big one. There's something a lot off-color going on around here." Quickly he reached the little room at the summit. He flung the canvas cover from the peri-telescope screen. Tempered by filters as it was the blaze of light from outside hit him like a physical blow. He adjusted the aperture and beat eagerly over the view-table. Vacation jaunts and travel view-casts have made the moon's landscape familiar to all. Very similar was the scene Darl scanned, save that the barren expanse, pitted and scarred like Luna's, glowed almost liquid under the beating flame of a giant sun that flared in a black sky. Soundless, airless, lifeless, the tumbled plain stretched to a jagged horizon. The Earthman depressed the instrument's eye, and the silvered outside of the Dome, aflame with intolerable light, swept on to the screen disk. The great mirror seemed alive with radiant heat as it shot back the sun's withering darts. The torrid temperature of the oven within, unendurable save to such veterans of the far planets as Darl and Jim Holcomb, was conveyed to it through the ground itself. The direct rays of the sun, nearer by fifty million miles than it is to Earth, would have blasted them, unprotected, to flaked carbon in an eye-blink.