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INCEDVM

'There is our key. So we construct our code. I tried out a forward substitution, but succeeded with a backward…' He scribbled rapidly.

'Now we reconstruct our message. That first B becomes a P, the M becomes R…'

Saladin stared at the new string, unreasonably disappointed. 'It's still nonsense.'

Bacon smiled, a magician with another trick to show. 'A simple transposition would be too easy. Our puzzle involves numbers as well as letters. Look at the "sentence" again. Nine "words" of five letters, and one of four. What sentence is as regular as that? What we have here is a simple string of letters, of length – how many, Thomas?'

'I'm not one of your Parisian students,' Thomas growled.

'Just answer,' Joan murmured.

'Forty-nine, then.'

'Good. What's significant about the number forty-nine?'

'Seven sevens,' said Saladin immediately.

'Very good!' said Bacon.

Thomas looked surprised. Saladin said, 'Some of the villagers think it's a lucky number. Seven times seven. That's how I know.'

'Seven squared,' Bacon said. 'That is surely a clue. So now, if we write out the decoded message again, not in these arbitrary blocks of five or four, but in a grid of seven by seven…'

'It still means nothing to me,' Joan said.

But Thomas was tracing the letters with a chalky fingertip. 'But if you read, not across, but down – else why put them in a grid at all? P – E – R… Give me that chalk, Roger.' He wrote out the letters, column by column, as a single line.