“Gail told us her father stole this book from Paul Revere’s house. If we can assume that it’s from Revere’s era, that would date it around the late eighteenth to early nineteenth century. Which would make sense, if it’s somehow connected to the eagle—which we know was molded in 1814. Now I didn’t study much history growing up, and I don’t know much about Paul Revere or the postrevolutionary war period. But I do know a lot about math, and where it intersects with history. The era of Revere and the original American patriots was a golden age of cryptography.”

She glanced at him through her goggles in a way that made him feel small again. She obviously assumed he had no idea what she was talking about. She was mostly right, but he was going to go down swinging. He’d picked up things over the years—sometimes he even surprised himself. Breaking into places wasn’t always as easy as jimmying a car door, matching some wires to pop an electronic trunk, and unscrewing a car battery. He wasn’t an expert safecracker like some of the hoods he’d met at Shirley, but he could work a tumbler and short a keypad.

“Codes. Secret messages.”

“Exactly. People think that World War II was the high point of cryptography, with so many stories about German encryption methods, the famous Enigma project—especially here, at MIT, where a lot of sophisticated methods of code-hacking were developed. But the Germans didn’t hold a candle to the Founding Fathers. The Revolutionary War was fought over a backdrop of secret messages, complicated spy networks, and fairly complex systems of encryption. George Washington, for one, was famous for using an invisible ink he’d had developed by his own ‘scientists’ to communicate with his personal spy networks. Thomas Jefferson created something called the ‘Jefferson disk,’ which was a cipher for encoding messages that could be passed through the mail. So there’s good reason to believe that Paul Revere—who was basically a spy, if I remember correctly—might have been similarly possessed.”

Nick raised his eyebrows beneath his goggles.

“You think this book was Paul Revere’s?”

“Apart from riding around on horses shouting about the British, he was known for being a metalworker and an engraver. And he definitely worked with copper. But it doesn’t really matter who made this book if we can’t read it.”

She carefully unscrewed the top of the flask. Nick got a whiff of the battery acid, and even with the goggles, his eyes began to water.

“You’re going to want to lean back,” Hailey said.

She started to tilt the flask toward the open copper page.

“The liquid in a car battery,” she said, as the first drops left the flask, “is about thirty-five percent sulfuric acid.”