The Science of Discworld Revised Edition

not simply sinks for dissolved minerals, one-way streets into which minerals flow and get trapped. They are mineral-processing machines. The geological evidence of the similarity of ancient and modern sedimentary rocks suggests that the inflow and the outflow pretty much balance each other.
    So do we have ancient seas in our blood? In a way. The
proportions
of magnesium, calcium, potassium, and sodium are exactly the same as they were in the ancient seas from which our blood may have evolved – but cells seem to prefer a salt concentration of 1%, not 3%.

NINETEEN
THERE IS A TIDE …

    ‘ HE’S RIGHT ABOUT the rain,’ said the Senior Wrangler, who was at the omniscope. ‘You’ve got clouds again. And there’s lots of volcanoes.’
    ‘I’m moving him on further … Oh. Now he says it’s dark and cold and he’s got a headache …’
    ‘Not very
graphic
, is it?’ said the Dean.
    ‘He says it’s a splitting headache.’
    H EX wrote something.
    ‘Oh,’ said Ponder. ‘He’s under water. I’m sorry about that, I’m afraid he’s a little hard to position accurately. We’re still not sure what size he should be. How’s this?’
    The trumpet rattled. ‘He’s still under water, but he says he can see the surface. I think that’s as good as we’re going to get. Just walk forward.’
    As one wizard, they turned to watch the suit.
    It hung in the air, a few inches above the floor. As they watched, the figure inside made hesitant walking motions.
    It was not a nice day.
    It was still raining, although it had slackened off recently, with sporadic outbreaks during the early part of the millennium and scattered showers during the last couple of decades. Now ten thousand rivers were finding their way to the sea. The light was grey and gave the beach a flat, monochrome, and certainly very damp look.
    Whole religions have been inspired by the sight of a figure emerging, miraculously, from the sea. It would be hard to guess at what strange cult might be inspired by the thing now trudging out of the waves, although avoidance of strong drink and certainly of seafood would probably be high on its list of ‘don’ts’.
    Rincewind looked around.
    There was no sand underfoot. The water sucked at an expanse of rough lava. There was no seaweed, no seabirds, no little crabs – nothing potentially dangerous at
all
.
    ‘There’s not a lot going on,’ he said. ‘It’s all rather dull.’
    ‘It’ll be dawn in a moment,’ said Ponder’s voice in his ear. ‘We’ll be interested to see what you think of it.’
    Strange way of putting it, Rincewind thought, as he watched the sun come up. It was hidden behind the clouds, but a greyish-yellow light picked its way across the landscape.
    ‘It’s all right,’ he said. ‘The sky’s a dirty colour. Where is this? Llamedos? Hergen? Why aren’t there
any
seashells? Is this high tide?’
    All the wizards were trying to speak at once.
    ‘I can’t think of
everything
, sir!’
    ‘But
everyone
knows about tides!’
    ‘Perhaps some mechanism for raising and lowering the sea bed would be acceptable?’
    ‘If it comes to that, what causes tides
here
?’
    ‘Can we all please stop
shouting
?’
    The babble died down.
    ‘Good,’ said Ridcully. ‘Over to you, Mister Stibbons.’
    Stibbons stared at the notes in front of him.
    ‘I’m … there’s … it’s a puzzler, sir. On a round world the sea just sits there. There’s no edge for it to pour off.’
    ‘It’s always been believed that the sea is in some way attracted to the moon,’ the Senior Wrangler mused. ‘You know … the attraction of serene beauty and so on.’
    Dead silence fell.
    Finally, Ponder managed: ‘No one said anything to me about a moon.’
    ‘You’ve
got
to have a moon,’ said Ridcully.
    ‘It should be easy, shouldn’t it?’ said the Dean. ‘
Our
moon goes around the Disc.’
    ‘But where can we put it?’ said Ponder. ‘It’s got to be light and dark, we’ve got to move it for phases, and it’s got to be almost as big as the sun and we
know
that if you try to make things sun-sized here they, well, become suns.’
    ‘Our moon is closer than the sun,’ said the Dean. ‘That’s why we get eclipses.’
    ‘Only about ninety miles,’ said Ponder. ‘That’s why it’s burned black on one side.’
    ‘Dear me, Mister Stibbons, I’m surprised at you,’ said Ridcully. ‘The damn great sun looks pretty big even though it’s a long way away. Put the moon