Like all modern libraries the visible bit of the Radcliffe was the tip of an iceberg; the bulk of the actual collection was submerged under Radcliffe Square in chambers filled with books and the intrusive hum of modern climate control. Postmartin led us down a series of whitewashed brick passages to a no-nonsense metal security door marked NO ADMITTANCE. Postmartin used a swipe card on the security pad and punched in a combination. The door unlocked with a solid clunk and we trooped in to find a chamber with exactly the same shelves and climate control as the rest of the collection. There was a single institutional desk with a top bare except for what looked like the product of a loveless marriage between an early Mac and an IBM PC.

“It’s an Amstrad PCW,” said Postmartin. “Before your time, I suppose.” He sat down on a purple molded plastic chair and booted up the antique. “No hardware connections, no USB ports, three-inch floppy disks that they don’t make anymore—this is security through obsolescence. Much like the Folly itself. They cannot hack, if I’m using the term correctly, what they cannot access.”

The screen was an alarming green color, monochrome I realized, like something from an old film. The three-inch disk actually clunked when the machine started to access it.

“Do you have the copy of the Principia?” asked Postmartin.

I handed it over and he started to leaf slowly through the pages. “Every copy at the library was marked in a unique way,” he said and stopped at a particular page and showed it to me. “You see there, that word has been underlined.”

I looked; it was the word regentis. “Is that significant?” I asked.

“We shall see,” he said. “Perhaps you should write it down.”

I wrote the word in my police notebook and as I did I noticed Postmartin furtively scribbling something on a pad he thought was out of my sight. When I was done he flicked through the pages until he found another mark, and again I noted down the word, pedem, and again I saw him write something else on his pad. We repeated the process three more times and then Postmartin asked me to read the words back.

“Regentis, pedem, tolleret, loco, hostium,” I said.

Postmartin regarded me over the rim of his glasses. “And what do you think that signifies?” he asked.

“I think it signifies that the page numbers were more significant than the words,” I said.

Postmartin looked crestfallen. “How did you know?”

“I can read your mind,” I said.

Postmartin looked to Nightingale. “Can he?”

“No,” said Nightingale. “He spotted you writing the numbers down.”