On this he started his three men, making them try to work out keys on squares and parallelograms, as well as in other well-established ways. Then the pounds figure proving too large for this, he tried adding the various digits of these figures together. In this way, 328 became 3+2+8 or 13, and so he compiled a second list beginning: 13—6 19—5 13—4 But in spite of all his own and his men’s efforts he was unable to find any clue to the key. They worked until long after the usual quitting time, and at length he had to agree to an adjournment for the night. Next day he again attacked the problem, but it was not until well on in the afternoon that he made an advance. Tired and dispirited, he had sent for a cup of coffee to clear his brain, and after it he had, contrary to his custom, lighted his pipe, while he leaned comfortably back in his chair still turning the matter over in his mind. He was beginning to think the puzzle insoluble, when suddenly an idea flashed into his mind, and he sat up sharply, wondering if he had hit on the solution. He had been considering numerical ciphers of which the key is some book. These consist usually of sets of three numbers, the first representing the page, the second the line on that page, and the third the word on that line. But he recognised that one of these latter numbers might be a constant, that is, that the word should always be on, say, the fifth line of the page, or that it should be the first or second of the line. In this way the cipher could be worked with pairs of numbers. The difficulty in these cases was of course to find the book which each of the communicating parties used. So far had he progressed when he got his great idea. Where had he seen a book which seemed strangely out of keeping with its fellows? Of course! That was it at last! The _Concise Oxford Dictionary_ in Mrs. Vane’s sitting-room! As he thought over this he felt more and more certain that he had reached the explanation. Not only was there the fact of the book being there, but a dictionary was obviously not only the kind of book best suited for the purpose, but also that best suited for a dual number system. The first number would represent the page and the second the word on that page. The idea, further, was confirmed by the fact that while the figure for the pounds—or pages—ran 1 to about 1000, that for the shillings and pence—or words on the page—never rose above 30. There was no doubt, French thought, that he had got it at last. At Scotland Yard all things are procurable at short notice. He rang up a subordinate and gave urgent instructions that a _Concise Oxford Dictionary_ was to be obtained immediately and sent up to him. Five minutes later he was eagerly turning over the leaves. It took but a second or two to find page 328, and another second to count down to the sixth word. It was “French.”