Isaac nodded. “That would fit the riddle. If it is patron saints, then I’d guess ‘ancient walls’ refers to the city. London has several.”

“Saint Paul,” Tom said.

“Michael as well. Though he’s an archangel, not a saint.”

“There’s another M name,” I said. “Master Benedict showed me a book of them once. Mel-something… Mellitus?”

“Who’s that?” Sally said.

“First bishop of London, I think.”

Tom was frowning. “There’s another, too. Something to do with a pilgrimage. I can’t remember his name. It sounded funny. Like Erwin, or something.”

We all looked at one another, but no one knew.

“Solve the code,” Isaac suggested. “That may make the riddle clear.”

“You keep saying you know how,” Tom said to me. “Are you going to show us or not?”

“Sure,” I said. “The secret is this line, here: ‘la ruota dell’italiano.’ It means ‘the Italian’s wheel.’ ”

I opened the sack I’d taken from Blackthorn and pulled out the device inside.

CHAPTER 25

“THIS IS IT,” I SAID. “The Italian’s wheel is Alberti’s disk.”

“What is that thing?” Tom said.

“A decoding device. Two wheels, made of wood, joined in the center. Both have an alphabet on them. The outer disk is fixed in place, but the inner disk can move.”

I flicked the smaller wheel inside. It spun around a couple of times before coming to a stop.

“It’s basically a substitution cipher, like a simple letter shift. With Alberti’s disk, you match the coded letters on the inside wheel, to the true letters on the outside wheel. The solution depends on how the wheels are aligned. Change them by even one place”—I moved the inner disk—“and you get a totally different substitution.”

“That’s what makes Alberti’s disk so clever,” I said. “It changes position all the time.”

Sally leaned in, curious. “How do you know where to set the wheels?”

“There are two ways to do it,” I said. “The first is by using a key, like with Vigenère. But we haven’t been given a key. Instead, the first letter of the cipher is a capital letter. See?”

B s m v & h c t c q q t d k d & l x

“The B refers to the letter B on the outer disk. That tells you to align the B with the index letter on the inner disk. Alberti usually chose the letter k as the index. So we begin by aligning the B on the outside with the k on the inside.”

I moved the wheels so they were properly aligned.

“Now we decipher the message. s is matched with P, so the first letter in the message is P.”

Bridget marched across the paper as I wrote the letter down.

B s m v & h c t c q q t d k d & l x

P