They stared out across the city in the silence of ferocious mental calculation.
'We could have a real problem here,' said Colon eventually.
Carrot started to scribble furiously. When questioned, he explained at length about how you found the surface area of a dragon and then tried to estimate the chances of an arrow hitting any one spot. 'Aimed, mind,' said Sergeant Colon. 'I aim.' Nobby coughed.
'In that case it's got to be a lot less than a million-to-one chance,' said Carrot. 'It could be a hundred-to-one. If the dragon's flying slowly and it's a big spot, it could be practically a certainty.' Colon's lips shaped themselves around the phrase,
It's a certainty but it might just work. He shook his head. 'Nah,' he said.
'So what we've got to do, then,' said Nobby slowly, 'is adjust the odds ...'
Now there was a shallow hole in the mortar near the middle bar. It wasn't much, Vimes knew, but it was a start.
'You don't require assistance, by any chance?' said the Patrician.
'No.'
'As you wish.'
The mortar was half-rotted, but the bars had been driven deep into the rock. Under their crusting of rust there was still plenty of iron. It was a long job, but it was something to do and required a blessed absence of thought. They couldn't take it away from him. It was a good, clean challenge; you knew if you went on chipping away, you'd win through eventually.
It was the 'eventually' that was the problem. Eventually Great A 'Tuin would reach the end of the universe. Eventually the stars would go out. Eventually Nobby might have a bath, although that would probably involve a radical rethinking of the nature of Time.
He hacked at the mortar anyway, and then stopped as something small and pale fell down outside, quite slowly.
'Peanut shell?' he said.
The Librarian's face, surrounded by the inner-tube jowls of the Librarian's head, appeared upside down in the barred opening, and gave him a grin that wasn't any less terrible for being the wrong way up.
'Oook?'
The orangutan flopped down off the wall, grabbed a couple of bars, and pulled. Muscles shunted back and forward across its barrel chest in a complex pavane of effort. The mouthful of yellow teeth gaped in silent concentration.
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