obey us, since we have no food to give them."

"You're the only hope," Bork agreed. "They've saved what they could of
the tools from the camp and what magical instruments are still useful.
They've held on only for your return."

Hanson stared at them and around at the collection of bric-a-brac and
machinery they had assembled for him. He opened his mouth, and his
laughter was a mockery of their hopes and of himself.

"Dave Hanson, world saver! You got the right name but the wrong man,
Sather Karf," he said bitterly. He'd been a pretender long enough, and
what punitive action they took now didn't seem to matter. "You wanted my
uncle, David Arnold Hanson. But because his friends called him Dave and
cut that name on his monument, and because I was christened by the name
you called, you got me instead. He'd have been helpless here, probably,
but with me you have no chance. I couldn't even build a doghouse. I
wasn't even a construction engineer. Just a computer operator and
repairman."

He regretted ruining their hopes, almost as he said it. But he could see
no change on the old Sather's face. It seemed to stiffen slightly and
become more thoughtful, but there was no disappointment.

"My grandson Bork told me all that," he said. "Yet your name was on the
monument, and we drew you back by its use. Our ancient prophecy declared
that we should find omnipotence carved on stone in a pool of water, as
we found your name. Therefore, by the laws of rational magic, it is
_you_ to whom nothing is impossible. We may have mistaken the direction
of your talent, but nonetheless it is you who must fix the sky. What
form of wonder is a computer?"

Dave shook his head at the old man's monomania. "Just a tool. It's a
little hard to explain, and it couldn't help."

"Humor my curiosity, then. What is a computer, Dave Hanson?"

Nema's hand rested on Hanson's arm pleadingly, and he shrugged. He
groped about for some answer that could be phrased in their language,
letting his mind flicker from the modern electronic gadgets back to the
old-time tide predicter.

"An analogue computer is a machine that ... that sets up conditions
mathematically similar to the conditions in some problem and then lets
all the operations proceed while it draws a graph--a prediction--of how
the real conditions would turn out. If the tides change with the
position of some heavenly body, then we can build cams that have shapes
like the effect of the moon's orbit