, and gear them together in the right
order. If there are many factors, we have a cam for each factor, shaped
like the periodic rise and fall of that factor. They're all geared to
let the various factors operate at the proper relative rate. With such a
machine, we can run off a graph of the tides for years ahead. Oh,
hell--it's a lot more complicated than that, but it takes the basic
facts and draws a picture of the results. We use electronic ones now,
but the results are the same."

"I understand," Sather Karf said. Dave doubted it, but he was happy to
be saved from struggling with a more detailed explanation. And maybe the
old man did understand some of it. He was no fool in his own subject,
certainly. Sather Karf pondered for a moment, and then nodded with
apparent satisfaction. "Your world was more advanced in understanding
than I had thought. This computer is a fine scientific instrument,
obeying natural law well. We have applied the same methods, though less
elaborately. But the basic magical principle of similarity is the
foundation of true science."

Dave started to protest, and then stopped, frowning. In a way, what the
other had said was true. Maybe there was some relation between science
and magic, after all; there might even be a meeting ground between the
laws of the two worlds he knew. Computers set up similar conditions,
with the idea that the results would apply to the original. Magic used
some symbolic part of a thing in manipulations that were to be effective
for the real thing. The essential difference was that science was
predictive and magic was effective--though the end results were often
the same. On Dave's world, the cardinal rule of logic was that the
symbol was not the thing--and work done on symbols had to be translated
by hard work into reality. Maybe things were really more logical here
where the symbol was the thing, and all the steps in between thought and
result were saved.

"So we are all at fault," Sather Karf said finally. "We should have
studied you more deeply and you should have been more honest with us.
Then we could have obtained a computer for you and you could have
simulated our sky as it should be within your computer and forced it to
be repaired long ago. But there's no time for regrets now. We cannot
help you, so you must help yourself. Build a computer, Dave Hanson!"

"It's impossible."

Sudden rage burned on the old man's face, and he came to his feet. His
arm jerked