When I had finished copying, William looked, unfortunately without lenses, holding my tablet at some distance from his nose. “It is unquestionably a secret alphabet that will have to be deciphered,” he said. “The signs are badly drawn, and perhaps you copied them worse, but it is certainly a zodiacal alphabet. You see? In the first line we have”—he held the page away from him again and narrowed his eyes with an effort of concentration—”Sagittarius, Sun, Mercury, Scorpio….”

“And what do they mean?”

“If Venantius had been ingenuous he would have used the most common zodiacal alphabet: A equals Sun, B equals Jupiter…. The first line would then read … Try transcribing this: RAIQASVL….” He broke off. “No, it means nothing, and Venantius was not ingenuous. He reformulated the alphabet according to another key. I shall have to discover it.”

“Is it possible?” I asked, awed.

“Yes, if you know a bit of the learning of the Arabs. The best treatises on cryptography are the work of infidel scholars, and at Oxford I was able to have some read to me. Bacon was right in saying that the conquest of learning is achieved through the knowledge of languages. Abu Bakr Ahmad ben Ali ben Washiyya an-Nabati wrote centuries ago a Book of the Frenzied Desire of the Devout to Learn the Riddles of Ancient Writings, and he expounded many rules for composing and deciphering mysterious alphabets, useful for magic practices but also for the correspondence between armies, or between a king and his envoys. I have seen other Arab books that list a series of quite ingenious devices. For example, you can substitute one letter for another, you can write a word backward, you can put the letters in reverse order, using only every other one; and then starting over again, you can, as in this case, replace letters with zodiacal signs, but attributing to the hidden letters their numerical value, and then, according to another alphabet, convert the numbers into other letters….”

“And which of these systems can Venantius have used?”

“We would have to test them all, and others besides. But the first rule in deciphering a message is to guess what it means.”

“But then it’s unnecessary to decipher it!” I laughed.

“Not exactly. Some hypotheses can be formed on the possible first words of the message, and then you see whether the rule you infer from them can apply to the rest of the text. For example, here Venantius has certainly noted down the key for penetrating the finis Africae. If I try thinking that the message is about this, then I am suddenly enlightened by a rhythm…. Try looking at the first three words, not considering the letters, but the number of the signs … IIIIIIII IIIII IIIIIII…. Now try dividing them into syllables of at least two signs each, and recite aloud: ta-ta-ta, ta-ta, ta-ta-ta…. Doesn’t anything come to your mind?”

“No.”

“To mine, yes. ‘Secretum finis Africae’… But if this were correct, then the last word should have the same first and sixth letter, and so it does, in fact: the symbol of the Earth is there twice. And the first letter of the first word, the S, should be the same as the last of the second: and, sure enough, the sign of the Virgin is repeated. Perhaps this is the right track. But it could also be just a series of coincidences. A rule of correspondence has to be found….”

“Found where?”

“In our heads. Invent it. And then see whether it is the right one. But with one test and another, the game could cost me a whole day. No more than that because—remember this—there is no secret writing that cannot be deciphered with a bit of patience. But now we risk losing time, and we want to visit the library. Especially since, without lenses, I will never be able to read the second part of the message, and you cannot help me because these signs, to your eyes…”

“Graecum est, non legitur,” I finished his sentence, humiliated. “It is Greek to me.”

“Exactly; and you see that Bacon was right. Study! But we must not lose heart. We’ll put away the parchment and your notes, and we’ll go up to the library. Because tonight not even ten infernal legions will succeed in keeping us out.”

I blessed myself. “But who can he have been, the man who was here ahead of us? Benno?”

“Benno was burning with the desire to know what there was among Venantius’s papers, but I can’t see him as one with the courage to enter the Aedificium at night.”

“Berengar, then? Or Malachi?”

“Berengar seems to me to have the courage to do such things. And, after all, he shares responsibility for the library. He is consumed by remorse at having betrayed some secret of it; he thought Venantius had taken that book, and perhaps he wanted to return it to the place from which it comes. He wasn’t able to go upstairs, and now he is hiding the volume somewhere.”

“But it could also be Malachi, for the same motives.”

“I would say no. Malachi had all the time he wanted to search Venantius’s desk when he remained alone to shut up the Aedificium. I knew that very well, but there was no way to avoid it. Now we know he didn’t do it. And if you think carefully, we have no reason to think Malachi knows Venantius had entered the library and removed something. Berengar and Benno know this, and you and I know it. After Adelmo’s confession, Jorge may know it, but he was surely not the man who was rushing so furiously down the circular staircase….”

“Then either Berengar or Benno…”

“And why not Pacificus of Tivoli or another of the monks we saw here today? Or Nicholas the glazier, who knows about my glasses? Or that odd character Salvatore, who they have told us roams around at night on God knows what errands? We must take care not to restrict the field of suspects just because Benno’s revelations have oriented us in a single direction; perhaps Benno wanted to mislead us.”

“But he seemed sincere to you.”

“Certainly. But remember that the first duty of a good inquisitor is to suspect especially those who seem sincere to him.”

“A nasty job, being an inquisitor.”

“That’s why I gave it up. And as you say, I am forced to resume it. But come now: to the library.”