on a screen. It's just as real as a painting. It's just different. Suppose we could make something like a magnetic field, with shape and coloring and solidity! Why not solidity? Given the trick, it should be as easy as shape or color!

"If we had a trick like that and wanted to stop some visitors from outer space, we'd simply construct a solid image of a can around them! It would be made of energy, and all the energy applied to it would flow to any threatened spot.

"It would draw power to fight any stress that tried to destroy it. Of course! And why should we build cities? We'd clear a place for them and generate them and maintain them simply by supplying the power needed to keep them in being! We'd make force-fields in the shape of machines, to dig canals or pile up dams."

He had raised his voice as he spoke. The solid walls and roof made echoes which clanged. He stopped talking.

"Then there wouldn't be any artifacts," Haynes said calmly. "When a city was abandoned, it would be wiped out as completely as the picture on a theatre-screen when the play is done with. But Wentworth!"

"Eh?"

"If we had that trick, and we'd captured some meddlesome strangers from outer space by clapping a can over them, what would we do?" He paused. "In other words, what comes next for us?"

"Get in the pilot's seat," he commanded. "Put your finger on the vertical flight-button. When you see light, stab it down so we'll shoot straight up! If we trapped somebody, and if we lifted the trap we'd have something worse than a trap to take care of them with. They'd do the same, and they've got what it should take!"

Silence followed.

"Such as?" Haynes asked at last.

"I saw one Thing this morning," said Wentworth grimly. "I don't like to think about it. If they're bringing it over to snap us up when this can is lifted off us, we're up against plenty of trouble. You keep your finger on the flight-button! That Thing was bigger than the Galloping Cow! I'll try to tip McRae off as to what's happened."

He settled down by the communicator. Every ten minutes he tried to call the expedition's ship. Every time there came a monstrous roar of static as the set came on, and no other sound at all. Aside from that, nothing happened. Absolutely nothing.

The flier lay on the 

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