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.

An exclamation burst from Darl. A half-inch from the Dome's blazing arc, a hundred yards in actuality, the screen showed a black fleck, moving across the waste! Darl quickly threw in the full-power lens, and the image leaped life-size across the table. The black fleck was the shadow of a space-suited figure that lumbered slowly through the viscous, clinging footing. How came this living form, clad in gleaming silver, out there in that blast-furnace heat? In one of the space suit's claw-like hands a tube flashed greenly.

Darl's hand shot out to the trigger of the beam-thrower. Aimed by the telescope's adjustment, the ray that could disintegrate a giant space flier utterly flared out at his finger's pressure. Against the lambent brown a spot glowed red where the beam struck. But, warned by some uncanny prescience, the trespasser leaped aside in the instant between Thomas' thought and act. Before Darl could aim and fire again the foe had dodged back and was protected by the curve of the Dome itself.

Two white spots showed on either side of Darl's nostrils. His mouth was a thin white slit, his eyes gray marbles. Standing against the wall beside him was a space suit, mirror-surfaced and double-walled against the planet's heat. In a few moments he was encased within it, had snatched a pocket ray-gun from the long rack, and was through the door to the auxiliary air-lock. The air soughed out in response to his swift thrust at a lever, a second door opened, and he was on the outside, reeling from the blast of that inferno of light and heat.

For a moment the Earthman was dazzled, despite the smoked quartz eye-pieces in his helmet. Then, as his eyes grew used to the glare, he saw, far below, the erect figure of the stranger. The man was standing still, waiting. His immobility, the calm confidence with which he stood there, was insolently challenging. Darl's rage flared higher at the sight.

Scorning the ladder that curved along the Dome to the ground, he threw himself at the polished round side of the great hemisphere. With increasing speed he slid downward, the gleaming surface breaking only slightly the velocity of his fall. On Earth this would have been suicidal. Even here, where the pull of gravity was so much less, the feat was insanely reckless. But the heat-softened ground, the strength of his metal suit, brought Darl safely through.

He whirled to meet the expected onslaught of the interloper. The green tube was aimed straight at him! The Earthman started to bring his own weapon up when something exploded in his brain. There was a moment of blackness; then he was again clear-minded. But he could not move—not so much as the tiny twist of his wrist that would have brought his own weapon into play.

Frozen by this strange paralysis, Darl Thomas saw the giant figure approach. The apparition bent and slung him to its shoulder. Glowing walls rose about him, dimmed. The Terrestrian knew that he was being carried down into one of the myriad openings that honeycombed the terrain. The luminescence died; there was no longer light enough to penetrate to his helmet's darkened goggles.

Frantic questions surged through the captured Earthman's mind. Who was his captor? From where, and how, had he come to Mercury? Jim, Angus McDermott, and himself were the only Terrestrians on the planet; of that he was certain. Only one or two of the reptile-skinned Venusian laborers had sufficient intelligence to manipulate a space suit, and they were unquestionably loyal.

This individual was a giant who towered far above Darl's own six feet. The Mercurian natives—he had seen them when ITA's expedition had cleaned out the burrows beneath the Dome and sealed them up—were midgets, the tallest not more than two feet in height. Whatever he was, why was the stranger trying to destroy the Dome? Apparently Thomas himself was not to be killed offhand: the jolting journey was continuing interminably. With enforced patience the Earthman resigned himself to wait for the next scene in this strange drama.

In the headquarters tent Jim's usual grin was absent as he moved restlessly among the switches and levers that concentrated control of all the Dome's complex machinery. "Darl's been gone a devilish long time," he muttered to himself. "Here it's almost time for shifts to change and he's not back yet."

A bell clanged, somewhere up in the mass of cables that rose from the control board. For the next ten minutes Holcomb had no time for worry as he rapidly manipulated the innumerable wheels and handles in accord with the vari-colored lights that flickered on a huge ground-glass map of the sub-Mercurian passages. On the plain outside there was a vast rustling, a many-voiced twittering and squeaking that was not quite bird-like in tone. Through the opened tent-flap one could see the stream of Venusian workers, their work-period ended, pouring out of the shaft-head and filing between the ordered ranks of others whose labors were about to begin.

They were queer-looking specimens, these gentle, willing allies of the Earthmen. Their home planet is a place of ever-clouded skies and constant torrential rains. And so the Venusians were amphibians, web-footed, fish-faced, their skin a green covering of horny scales that shed water and turned the sharp thorns of their native jungles. When intrepid explorers discovered in the mazes of Mercury's spongy interior the surta that was so badly needed as a base material for synthetic food to supply Earth's famine-threatened population, it