necessary to conserve weight-lift costs and fuel reserves. They stood it upright and tried to turn the top. After a little resistance, it unscrewed slowly. Inside, they found a rolled document bearing the seal of the United States Air Force.

Burl took it out, and unfolded it with unsteady hands. His father read over his shoulder.

Gonzales poked at the empty cylinder, impatiently. Finally, he burst out, "What does it say? What do they want?"

Burl turned to him. "It's unbelievable! It's—it's just so darned surprising! The dimness of the days, the drop in temperature—it wasn't just around here! It was all over the world!"

Quickly, he went on to tell the Peruvian what they had just learned. The communication was from the U.S. Space Commission and it had been directed on its flight from California by the Moon Base, because only from the satellite could the exact location of the Dennings be spotted. It seemed that the Dennings were the only scientifically trained personnel close to the point on Earth where the disturbance originated. This also accounted for the blanketing of radio waves in their vicinity. Several airplanes had tried to locate them, but strange disturbances in the ether and atmosphere had made it impossible to establish contact. Also, the back reaches of the Andes were poorly mapped and treacherous in air currents, even in normal times.

"During the last week, a certain fraction of the Sun's light and energy reaching the Earth has been diverted. It has been bent or focused in much the same way that a lens bends light rays—and the point to which it has been directed is a spot only seven miles from here! Over that last mountain range," said Burl, pointing.

Gonzales followed his finger. "Just over the mountains lies the source of the trouble," said Burl excitedly. "And we're the nearest to it. They want us to go over there, see what it is, stop it, or report back. It took the telescopes in Moon Base to locate us and to track the center of the trouble!"

Mark Denning pursed his lips. "We'll have to start tomorrow, and we'll have to go fast. A loss of light and heat, however slight, could have very serious effects on life if continued too long. We can make it by tomorrow night, if we start early and leave the Indians and pack animals behind."

The other two nodded. Mark looked at them in the half-light of the Moon. "You'll have to stay with the equipment, Pedro, 

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