Throwing off his overcoat, he took up the note and, tearing it open, read the few brief unsigned lines it contained. Then, replacing it upon the table, he drew his white hand across his brow, as though to clear his troubled brain. Afterwards he crossed to the small safe let into the wall near the fireplace and, unlocking it, took forth a little well-worn memorandum-book bound in dark blue leather. "Cipher Number 38, I think," he muttered to himself, as he turned over its pages until he came to that for which he was in search. Then he sat down beneath the reading-lamp and carefully studied the page, which, ruled in parallel columns, displayed in the first column the alphabet, in the second the key-sentence of the cipher in question-- one of forty-three different combinations of letters--and in the third the discarded letters to be interspersed in the message in order to render any attempt at deciphering the more difficult. In that cleverly-compiled little volume were forty-three different key-sentences, each easy of remembrance, and corresponding in its number of letters with about two-thirds, or so, of the number of letters of the alphabet. From time to time it changed automatically, according to the calendar and to a certain rule set forth at the end of the little volume. Hence, though the spy's code was constantly being changed without any correspondence from headquarters--"Number 70 Berlin"--yet, without a copy of the book, the exact change and its date could not be ascertained. Truly, the very best brains of Germany had, long ago, been concentrated upon the complete system of espionage in Great Britain, with the result that the organisation was now absolutely perfect. Taking a sheet of ruled paper from one of the compartments in the American rolltop desk before him, Lewin Rodwell, after leaning back wearily in his chair to compose himself, commenced, by reference to the pages of the little book before him, to trace out the cipher equivalents of the information contained in the note that had been left for him by an unknown hand in his absence. He opened the big silver cigarette-box at his elbow, and having taken a cigarette, he lit it and began reducing the information into cipher, carefully producing a jumble of letters, a code so difficult that it had for a long time entirely defied the British War Office, the Admiralty, the Foreign Office, and the French Secret Service. Though marvellously ingenious, yet it was, after all, quite simple when one knew the key-sentence. Those key-sentences used by "Number 70 Berlin" in their wonderful and ever-changing secret code--that code by which signal lights were flashed across Great Britain by night, and buzzed out by wireless by day--were quite usual sentences, often proverbs in English, such as "A little knowledge is a dangerous thing," "A man and his money are soon parted," "Give one an inch and he'll take an ell," "Money makes the world go round," and so on.