evidence of human dwellings."

Burl nodded, and set the sifter down. He'd learned to curb his natural energies for the exacting tasks required of serious scientific research. "Okay," he said, "I was hoping you'd move on soon, Dad. This looked like a washout from the first. I'd say this place was sacked and ruined even before the Incas fell."

The older man nodded. "I suppose so. Well, let's wash up and see what's for supper."

They went down to the icy mountain stream to wash the dirt from their hands. "It's been a nice day," Burl commented. "In spite of the Sun being out steadily, it wasn't hot at all. Cooler than yesterday."

Mark Denning looked up at the sky and the Sun lowering toward the horizon. "There must have been some volcanic dust in the heavens," he said. "The Sun's been a bit dimmed, have you noticed?"

Burl squinted his eyes against the glare. "Wasn't any eruption around here. Maybe in Ecuador?"

His father shrugged. "Could have been thousands of miles away," was his slow reply. "Volcanic dust travels around the world, just as radioactive dust permeated the atmosphere from atomic testings. They say that the dust from the great Krakatoa explosion remained in the atmosphere for three years before the last of it settled."

When they had finished supper and the Sun was casting its last red rays over the rapidly purpling landscape, Burl got out the expedition radio, set up its antenna, plugged in its compact atomic battery, and tried to get the news from Lima. All he got was static.

He fiddled with the dials for a long time, twisting the antenna, ranging the wavelengths, but there was static everywhere. "Strange," he said to his father, "something's disturbed reception completely."

Pedro Gonzales, their official Peruvian guide, leaned over. "Could be the battery she is broken, eh?"

Burl shook his head. "Not this battery," he said. "It's a brand-new one, a real keen development. And I already checked the wiring. It's some sort of disturbance that's blocking reception. Maybe we're in a dead zone or something."

"Wasn't dead yesterday," said his father. "Maybe that eruption was radioactive."

Burl looked up sharply. "I'll check the Geiger counters, Dad. Something's blocking reception, something strong and powerful to interfere with this set." But when he returned, he had to admit he had found nothing.

When the Sun went down, they retired, for the temperature drops swiftly in the high, thin air of the Andes.

In the rest of the world people watched their color-vision shows without interruption. Reception was good with the Moon base, the space platforms had no difficulty making reports, and the radio news beamed out as usual. In Lima, there was a little static, and direct transmission with Brazil seemed partially disrupted, but that was all.

In the following five days, the Denning expedition had managed the difficult climb over the next range of mountains and had come down in the high plateau valley between. In this same period, the world began to realize that the dimness of the sky was not a temporary phenomenon.

Weather stations noted that the past few days had all been several degrees under the average. Reports had come in that farmers were querying the unusual drop in the temperatures at night. And astronomers, measuring the surface heat of the Sun, came up with strange discrepancies from previous data.

One astronomer communicated with another, and a general exchange of advice began. In a short while, a communication was laid on the desk of the President of the United States, who scanned it and had it immediately transmitted to the Secretary General of the United Nations. The Secretary General circulated the report among the scientific bureaus of all member nations, and this led in turn to a meeting of the Security Council. This meeting was held in quiet, without benefit of newspaper reporters or audience.

There was no longer any doubt. The radiation of the Sun reaching the face of the Earth had decreased. The facts were indisputable. Where a day should have registered, in some places, at least 90° in the Sun, a reading of only 84° was noted. Measurements definitely showed that the face of the Sun visible to man on Earth had dimmed by just that margin.

This might not prove serious at first, but as the scientists called in by the Security Council pointed out, it promised terrible things as the year went on. A difference of five or ten degrees all over the Earth could mean the 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 it out to his father and Gonzales. "Too fast to be a celestial object," he said. "Is it one of the space platforms or a sputnik?"

The two men gazed at it in curiosity. Suddenly it seemed to grow brighter and sharper and to twist toward them in its path.

"Look!" gasped Burl, but the others were already on their feet.

The light plunged down. There was a sudden outburst of yellow flame that caused the three to duck instinctively, and brought the Indians to their feet with yells. The glare brightened until they could see that something was just above them. The fire vanished as swiftly as it came, but a white spot of light remained.

"It's a parachute!" Burl shouted. "It's a rocket or something, braking to a stop above us, and coming down by parachute!"

In the pale light of the full Moon they saw that something metallic and glistening hung from the white mushroom of a parachute. There was a clanging sound as it hit the rocky earth with a soft, sighing whoosh. The cloth of the parachute settled.

They ran across the dry stone of the valley floor, but Burl's long, athletic