y and hostile
sky domains.

Panting and weary with hours of climbing, Burl and his father made a
quick lunch in a sheltered jumble of rock near the top. Then,
shouldering their packs again, they trudged on. At last they reached a
point where the view of the other side spread out before them--a
breathtakingly clear vision of the little valley below.

As they looked down, the air seemed to shimmer and vibrate. Burl rubbed
his eyes. "It hurts," he said.

His father squinted. "There's a powerful vibrational effect. It may be a
very dangerous concentration of the invisible rays of the Sun as well as
of light."

Once Burl had gotten used to the odd visual effect, which was like
gazing into the twisting heat rays rising from an overheated oven, he
saw that there was a small flat region between the mountains. And in the
center of this valley was a large black structure of some sort. The
twisting effect of the light around it made it impossible to tell more.

"That's it," said Burl. His father nodded, shifted the pack to ease his
shoulders, unstrapped the hunting rifle slung over his back, and
carefully checked its loads.

Burl saw what his father was doing and suddenly understood the danger.
What could be doing a thing like this? What but something not of this
Earth? Something of distant space, of a science beyond that of man--and
_unfriendly_ besides. Now, for the first time, Burl realized what he had
not had time to before--this was an enemy he and his father were
facing--an enemy of all mankind--and utterly unknown.

He gulped, gripped his rifle, and followed his father down the sliding
rocky trail.

As they drew nearer the base of the mountain, the effects of the strange
vibrations grew more pronounced. Burl avoided looking directly ahead,
keeping his eyes on the ground before his feet, yet even so, he could
not help noticing how the stones around them seemed to shimmer in the
invisible waves. From the base of the valley the sky now seemed streaked
with black and gray rings, as if they were reaching the center of some
atmospheric whirlpool. Out of the mountains, after hours of arduous
scrambling, they started across the barren rocky plain.

Before them rose a vast circular structure several stories high,
ominously black and without any sign of windows or doors. Above the
building protruded two great projections ending in huge, shining discs.
One of the monstrous cuplike discs was facing the Sun, the other pointed