powdery stuff. When the smoke cleared, Burl and his father leaped to their feet, rifles in hand.

There was a crack in the side of the wall where the explosive had gone off. And the rip was large enough to get through!

Without a word, they charged across the ground, still smoking from the concussion, and squeezed through the mysterious walls of the enigmatic building.

The walls were thin, thin but hard, as befit masters of atomic engineering. Inside, they found a roomless building—one single chamber within the frame of the outer walls.

A dim, bluish light emanated from the curving ceiling. On the uncleared rocky ground which was the floor of the building were a number of huge machines.

They were spherical glassy inventions, many times the height of a man, connected by strings of thick metal bars and rows of smaller globes, none of which was familiar. There was a steady humming noise, and above, the two giant, metal masts penetrating the ceiling rotated slowly. Doubtless, the great Sun-trapping discs were affixed to the top of these masts.

There was no living thing in sight.

Burl and his father stood silently, half crouched, with rifles at the ready, but nothing moved to challenge them. There was only the humming of the Sun transmitters.

Burl called out, but there was no answer. They advanced cautiously, fearing a trap. The place did not have the look of living things about it. "An automatic station," said Mark under his breath. "I think it's strictly automatic."

It gradually became evident that Mark was right. Everything was automatic. Whoever had built this structure to divert the rays of the Sun had simply set it down, put it in motion, and left. There was no evidence of any provisions for a garrison or a director.

They studied the machines but could make nothing of them. They found what looked like controls, but although they pushed and pulled the levers and knobs, the humming did not cease. It seemed as if the controls were either dummies or had to be specially motivated.

"What do we do now?" asked Burl, after they had tried pulling all the levers on one particular switchboard without any results. "Do you have enough powder left to blow up the machinery?"

His father shook his head. "I had only those two cans with 

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