"It's a difference engine," Rosie concluded. Tallie said nothing: she was holding a lantern so that Rosie could examine the mechanism she was supposed to duplicate. "In fact," she continued, looking around, "we're looking at about maybe a dozen different difference engines. All... close to two hundred years old. At a guess."
They were far back in the hidden area behind the Book - or, really, behind the Book's public face. There was a lot more Book back here than you could see from the front room.
The Book's workings were heavy and durable. Rosie couldn't quite approve of their primitive design - she was who she was, after all - but she had to admit that if you wanted a Clockwork Book to last for a very, very long time you would probably design it pretty much the same way, even today. She watched the gears turn and the barrels of the difference engines rotate along with them. A great long cam shaft did a complicated dance over her head. Yes. You could keep this thing going for ages without a lot of maintenance.
"You're only supposed to be looking at this part," Tallie reminded her. Rosie grinned at her. "Sure thing," she admitted. "Let's get back to your boss."
She told the Clockwork Book that, yes, she could make a duplicate of the difference engine. That she could improve it, too, not least by making it much smaller and lighter. With that done she could also build a connecting apparatus for whatever device the Book wanted to add. She paused meaningfully.
"Am I to understand that we have a bargain, then?" asked the Book, and she agree
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