ruin of certain crops, it could mean an increase in snowfall and frost that could very rapidly destroy the economies and habitability of many places on the Earth's teeming surface.

"But what," asked the Chairman of the Council, "is causing this decrease in solar energy?"

This the astronomers could not answer. But they pointed to one factor. The reports from the U.S. Moon Base did not agree with the observations from Earth. Moon instruments claimed no decrease whatsoever in the amount of sunlight reaching the arid, airless surface of the Earth's only satellite.

The cause was somewhere on Earth. And the Security Council requested the careful scanning of the Earth from space platforms and the Moon to determine the center of the trouble.

Burl Denning had not found the next valley of much interest, either. Evidence of an Inca road over the mountain had petered out. There were signs there had been human dwellings, but they were not Inca—just reminders of the onetime passage of an unknown band of primitives who had grazed their sheep, built temporary tents, and pulled up stakes perhaps a hundred years before.

So again at night, Burl, his father, and Gonzales took counsel. They were debating which way to proceed next; Mark Denning reasoning that they should go further inland, following tales natives had told; Gonzales urging that they retrack their path and proceed northward toward the regions where Inca ruins abounded.

For the past week Burl had not been able to get radio reception. The static had increased as they had gone eastward over the mountain, but not a word of news or any human voice came through. The Moon was rising on the horizon as Burl sat playing with the antenna. Finally he gave up and switched it off.

The discussion had died away and the three men were quiet. The Indian guides had retired to their own campfire, and one of them had taken out his pipes and was blowing a soft, plaintive tune.

Burl stared at the full Moon in silence, wondering if he would ever have a chance to walk its surface, or if his own future was to lie in probing mankind's past rather than surveying the grounds of his future. As he watched, he thought he saw a faint light among the brightening stars where none had been before.

He squinted, and, sure enough, he saw that one tiny white light was swinging more and more toward the center of the sky. He pointed 

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