"The combination of the safe," I interrupted. "Precisely. It never occurred to either of us that it might be a master-key to which all locks must yield. But so it was. "Not that I learned to use it without a lot of trouble and discouragement. It took months and months, and I only got it fully working on the train trip down from Stockbridge. "Of course, you have guessed that the whole story lay buried in that leather-bound book belonging to Fielding Thaneford which we found in the safe. I remembered all that you had told me about 'Le Chiffre Indéchiffrable,' but even granting that that particular cypher had been employed, how was I ever to stumble upon the indispensable key-word, or more likely, key-sentence? "One day I had an inspiration. There was the series of numbers: 1-4-2-4-8. Considered as numbers merely they could be of no use, since most cypher codes are built up on letters. But I might put the numbers into their written word equivalents, thus: One-four-two-four-eight It was certainly conceivable that these letters might form the key-sentence; it would be all the more easily memorized since, in its numerical form, it served as a combination to the safe. "I had with me the magic square which you had made for me, and I began very carefully to work out the problem according to your directions. "The initial procedure was to put down my theoretical key-sentences, thus:" O N E F O U R T W O F O U R E I G H T "Underneath I must write the cypher message, and half a dozen letters would be enough to show if I were on the right track. I opened Mr. Fielding Thaneford's old book, and copied down the first seven letters, ranging them vertically under the key-letters. That gave me this arrangement:" O N E F O U R Q W O T T U I "Now the rule goes on to say that you must find the letter O in the top horizontal column, and follow that column vertically downward until you come to the first cypher letter, in this case Q. The letter at the outside, left end of this second horizontal column, will be the first letter of the original message. "Well, I tried it, and got the letter B. The next pair yielded an I, which was encouraging, as one would expect a vowel in this position. But the third try gave me a J, and that was not so promising; then I got an N and an E. So far my decoded message read: BIJNE; not very enlightening. The next pair showed the letter U in both key-sentence and cypher. Such a combination is impossible on our magic square, and I had to put down a blank space. The final letter obtained was a Q, and the complete result read: BIJNE-Q. Pure gibberish of course. I tried out a few more pairs, and then gave up in disgust; my beautiful theory had fallen to pieces.