platform above stiltlike legs. At the bottoms of the legs were wheels. The wheels ran on the cross-girders as on a railroad track, and the body of the thing was upraised enough to ride well above the sidewise-tilted slabs. There were other wheels to be lowered for travel on the girders which supported the slabs.

It was not a flying device, but a rolling one. It could travel in either of two directions at right angles to each other, and had been designed to run only on the great grid which ran beyond the horizon. It was undoubtedly a maintaining machine, designed to reach any spot where trouble developed, for the making of repairs, and it was of such weight that even the typhoonlike winds of a normal day on this world could not lift it from its place.

It came hurtling toward them at terrific speed. It would roll irresistibly over anything on the girders which were its tracks.

"Get on!" snapped Stan. "Quick!"

Esther moved as swiftly as she could, but space suits are clumsy things. The little skid shot skyward only part of a second before the colossus ran furiously over the place where they had been. A hundred feet beyond, it braked and came to a seemingly enraged stop. It stood still as if watching the hovering, tiny skid with its two passengers.

"It looks disappointed," said Stan dourly. "I wonder if it wants to chase us?"

He sent the skid darting away. They landed. In seconds the vibration caused by the huge machine's motion began and grew loud. They saw it race into view. As it appeared, instantly a deafening clamor began. Slabs in all directions rose to their vertical position, so that the two humans could not dodge from one row of girders to another. And then with a roar and a rush the thing plunged toward them once more.

Again the skid took off. Again the huge machine overran the spot where they had been, then stopped short as if baffled. Stan sent his odd craft off at an angle. Instantly the gigantic thing was in motion, moving in lightning speed in one direction, stopping short to move on a new course at right angles to the first, and so progressing in zigzag but very swift pursuit.

"'Won't you land so I can crush you?' said the monster to us two," said Stan dryly. "They won't let us have any more power, and we haven't any more to waste. But still—"

He listened to his suit-radio, twisting the tuning dials as he sent the 

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