They probably learned to transform heat to power direct. You can take power—electricity—and make heat. Why not the other way about? For maybe fifty thousand years, and maybe more, they had to live without even thinking of the surface of their world. But as the dwarf star cooled off, they needed its heat again."

He stopped. He seemed to listen intently. But there was no sound in the icy night. There were only bright, unwinking stars and an infinity of sand—and cold.

"So they dug up to the surface again," he went on. "Air had come back, molecule by molecule from empty space, drawn by the same gravitation that once had kept it from flying away. And the fused-solid rock of the surface, baked by day and frozen by night, had cracked and broken down to powder. When air came again and winds blew, it was sand. The whole planet was desert. The people couldn't live on the surface again. They probably didn't want to. But they needed power. So they built that monster grid they're so jealous of."

"You mean," Esther demanded incredulously, "that's a generator?"

"A transformer," corrected Stan. "Solar heat to electricity. Back on Earth the sun pours better than a kilowatt of energy on every square yard of Earth's surface in the tropics—over three million kilowatts to the square mile. This checkerboard arrangement is at least a hundred and fifty by two hundred miles. The power's greater here, but, on Earth, that would mean ninety thousand million kilowatts. More than a hundred thousand million horsepower—more than the whole Earth uses even now!

"If those big slabs convert solar radiation into power—and I charged up the skid from one of them—there's a reason for the checkerboard, and there's a reason for dumping the sand—it would hinder gathering power—and there's a reason for getting upset when somebody started to meddle with it. And they're upset! They'll have the conservation of moisture down to a fine point, down below, but they made those leggy machines to haul more water, from the poles. When they set them all to hunting us, they're very much disturbed! But luckily they'd never have worked out anything to fly with underground and they're not likely to have done so since—considering the storms and all."

There was a short silence. Then Esther said slowly, "It's—very plausible, Stan. I believe it. And they'd have no idea of space travel, so they'd have no idea of other intelligent races, and actually they'd never think of castaways. They wouldn't understand, and they'd try 

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