A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

B C D E F G H I J K L M N O P Q R S T U V W X Y Z A

C D E F G H I J K L M N O P Q R S T U V W X Y Z A B

D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

E F G H I J K L M N O P Q R S T U V W X Y Z A B C D

F G H I J K L M N O P Q R S T U V W X Y Z A B C D E

G H I J K L M N O P Q R S T U V W X Y Z A B C D E F

H I J K L M N O P Q R S T U V W X Y Z A B C D E F G

I J K L M N O P Q R S T U V W X Y Z A B C D E F G H

J K L M N O P Q R S T U V W X Y Z A B C D E F G H I

K L M N O P Q R S T U V W X Y Z A B C D E F G H I J

L M N O P Q R S T U V W X Y Z A B C D E F G H I J K

M N O P Q R S T U V W X Y Z A B C D E F G H I J K L

N O P Q R S T U V W X Y Z A B C D E F G H I J K L M

O P Q R S T U V W X Y Z A B C D E F G H I J K L M N

P Q R S T U V W X Y Z A B C D E F G H I J K L M N O

Q R S T U V W X Y Z A B C D E F G H I J K L M N O P

R S T U V W X Y Z A B C D E F G H I J K L M N O P Q

S T U V W X Y Z A B C D E F G H I J K L M N O P Q R

T U V W X Y Z A B C D E F G H I J K L M N O P Q R S

U V W X Y Z A B C D E F G H I J K L M N O P Q R S T

V W X Y Z A B C D E F G H I J K L M N O P Q R S T U

W X Y Z A B C D E F G H I J K L M N O P Q R S T U V

X Y Z A B C D E F G H I J K L M N O P Q R S T U V W

Y Z A B C D E F G H I J K L M N O P Q R S T U V W X

Z A B C D E F G H I J K L M N O P Q R S T U V W X Y

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Thus the plain-text letter B in the top, horizontal, line could be replaced by any one of the twenty-five characters arranged vertically beneath it in the twenty-five possible cipher alphabets. The decipherer knows which of these cipher alphabets to choose only by means of the keyword, those few letters whose structure is logical but whose effect is nothing short of magical, like a spell chanted over a base metal which then miraculously transmutes itself into ingots of gold. The spell works when the letters of the keyword are superimposed on those of the cipher in a series of repetitions, so that each letter of the keyword is paired, on each of its repetitions, with one in the cipher-text. Then, the transmutation. The values of the letters in the cipher change according to which alphabet the letters in the keyword instruct the decipherer to employ. What follows is a smooth, steady interchange of letters, a textual metamorphosis in which the hidden inscription crystallises like alum immersed in water, reassembling its structure according to an ordained pattern. The act of decipherment becomes as simple and certain as flipping over playing-cards to read their values, or removing the satin mask to expose the villain's face.

I found something deeply appealing about this idea of a key that can be used to unlock the most complex secrets, this word or phrase that, almost like a divine fiat, turns the random and chaotic into an ordered pattern. Vigenère was not a magician after all. No-his system belonged to our new age, that of Kepler, Galileo and Francis Bacon, one in which outer husks were cast off and the kernel of truth exposed for all to see. His system confirmed my faith in the powers of human reason to penetrate the depths of any mystery. And so was it any wonder that I believed my scrap of paper, combined with a few secret syllables, might penetrate that of Sir Ambrose Plessington?

Except that I did not yet know the keyword. Feeling overwhelmed, I set the books aside as the watchman was calling ten o'clock. My cousin Erasmus still seemed my best policy. Over the years, I had sold him many books on the subject of decipherment and I had even heard a rumour that he had deciphered papers for Cromwell. So I decided that he, of all people, would know what to make of the scrambled letters. But I would tell him nothing of my suspicions that it was a cryptogram devised in order to conceal the location of Sir Ambrose's fortune. 'My dear Erasmus,' I began, surprised by the slight tremor in my hand.

Darkness was complete by the time I finished the letter, and the bells of St. Magnus were announcing eleven o'clock. I would have to hurry, I realised, if I was to catch the night mail-coach. I reached for my coat, struck by a peculiar sense of urgency. But then I was struck, just as suddenly, by something else just as urgent.

There's nothing to fear, Mr. Inchbold. You will be quite safe. I promise…

As I shrugged into my coat and stared at the cipher on the bureau, the tiny crevice of doubt that had opened on the first night at Pontifex Hall now widened, and on a sudden impulse I knelt beside the bureau and prised up two loose floorboards, then tucked the slip of paper between the scantlings. After a moment's thought I added the inventory of missing books and the summons from Alethea, along with my down payment of twelve sovereigns-everything that could connect me with Pontifex Hall. Then I carefully replaced the boards, covered them with two stacks of books, and picked my way around other piles of books to the turnpike stair.

'Sir?'

I was halfway down the steps. Monk's face had appeared at the top, half hidden by his nightcap. He had given me a dreadful start.

'I shall be taking a turn in the street,' I called to him. Even in the gloom I could see his eyebrows rise in surprise. I rarely ventured outside after dark, and then usually only as far as the Jolly Waterman. If London was frightening by day, at night it was, from my limited experience, something else entirely. My resolve nearly deserted me. 'Only a short one,' I added. 'I have a letter to post.'

'Allow me, sir.' He started down the twisting steps. Posting letters was one of his many duties.

'No, no.' I shied a hand at him. 'All that sitting on the coach,' I explained, flexing my legs and patting my rump for his benefit. 'A walk will be just the thing for me. Now, please, Monk, to bed with you.'

The nightcap disappeared. A minute later I was stepping outside and on to the footway. The streets beyond the gate were empty and dark. The intermittent bull's-eye lanterns-a series of yellow haloes against the buildings-barely lit my way. From the distance came the sound of the bellman. I ducked my head and hurried after my shadow, moving as tentatively as if treading on eggshells.

The nearest receiving station to Nonsuch House was in Tower Street, near Botolph Lane. I found it without difficulty and, after dropping the letter through the posting-hole (a strong-box attached to the wall by means of a chain), I hurried back down Fish Street Hill to the sound of the curfew tolling. At its funereal call two sentries had stirred to life and were preparing to scrape shut the gates of the bridge. The portcullis had begun its descent. I scurried beneath in the nick of time, grateful once again to see the black-and-white hulk of Nonsuch House rising against the sky to meet me.

Thirty minutes later the letter was collected from the strong-box and delivered to the Inland Office, which occupied the upper floor of the General Letter Office in Clock Lane. There, by the light of a candle stub, among a litter of labels and hand stamps, the string was cut with a penknife, the wafer seal carefully broken, and the letter copied out word for word by a clerk. The clerk then carried the copy downstairs and into a larger room where a man sat behind a desk, thrumming the fingers of his right hand on its surface. His back was to the door.

'Sir Valentine,' murmured the clerk, whose name was Ottermole.

'What is it?'

'Another letter, sir. From Nonsuch House.'

The leather squeaked as Sir Valentine turned in his chair. The clerk placed the copy on the desk and, after climbing the stairs, folded the letter along its creases and carefully resealed it with a drop of wax. This, too, was delivered downstairs. A half-dozen brass-bound satchels sat by the doorway. By this time Sir Valentine had disappeared. Outside in the small coach-yard a team of horses was being hitched to the waiting mail-coach, due to arrive in Oxford some fifteen hours and five posts later.