"They're very small," remarked Mr. Bell, putting on his glasses; "very small indeed; but I think—why they're Henning's, I do believe!"
"Ah! one or two other little things seemed to point that way. Henning is your correspondence clerk, I believe, and I expect this thin little slip is a specimen of your typewriter paper. Have you any of his written figures for comparison?"
"Well no—I hardly think—you see he typewrites his letters, and although I know his writing very well I can't at the moment put my hand on any figures of his."
"Never mind—it's mere matter of curiosity; the police will ask him questions in the morning. What I believe has happened is this. Our friend Henning—if he's the man—has a friend outside a great deal cleverer than himself—though he would seem to have his share of cunning, too. Between them they resolved to rob you in the way they have done—temporarily. Henning was to take advantage of his position in that little inner room to get at the safe some day when it was open and when you were engaged in your own private inner room with a client, so leaving the safe unwatched. He was provided with a spare patent padlock and key, of the sort you used on that black box, and his confederate had drilled him in the trick of breaking that particular sort of padlock open, with other spare specimens. He got his opportunity this morning."
"Only this morning?"
"This morning, I think, else we should never have got these bonds back, nor even have heard of them again. I think you said you were engaged with a client for half an hour?"
"Yes, from about half-past ten to eleven."
"That was his chance, and he took it. He broke the padlock, took out the bonds, substituted the dummies he had already prepared in his own desk, and locked the box again with the new padlock. Meantime Hunt had paid a deposit, pending references, on the office below—the nearest empty room. Of course, he wouldn't get the key until the tenancy was finally accepted—which he never intended it should be. But he easily arranged to have the door left unlocked for a day or two, on some convenient excuse—arranging decorations, or what not. And the bill was taken down, so that prospective and prospecting tenants were kept away. The bonds being stolen, Henning took the first opportunity of carrying them to the empty office—probably piecemeal—a thing he could easily manage almost under your nose, before you were aware of your loss. There he was to conceal them, either in the chimney, under the boards, or in the ventilator, as he might find convenient—and he found the ventilator most convenient. Then he was to apprise his confederate of the fact that the robbery had been effected in order that Hunt might come and quietly fetch the plunder away. The message was to take an ingenious form. Hunt was to have a fellow waiting about in the street, and as soon as Henning could get out—say to lunch—he was just to send the key by this messenger—the key with which he had locked the new padlock on the black box. You see the advantages of that simple arrangement. First, the key, which is evidence, is got rid of in a safe and effectual way—a thing that couldn't be done as well by merely flinging it away on or near the premises, where it might be found. Next, the message is perfectly secret—the messenger could never guess what the key meant, nor could any other person not in the confederate's confidence. And, at the same time, the key tells all that is necessary; the robbery has been effected—come and remove the plunder.
"But something unforeseen happens. No sooner are the bonds stolen and safely hidden than you go to the box, find something wrong with the lock, break it open and discover the loss. This was a thing that they trusted would not happen till after the bonds were safely got away. More, I am sent for, the clerks are kept in from lunch, and so on. Henning gets into a funk, and resolves to send a message of special urgency to his confederate. For that purpose he uses a cypher which the two have agreed upon—the most ingenious cypher I have ever seen used for the purpose. He doesn't wish to make his message any more conspicuous than he need, so he writes his cypher on this scrap of paper and rolls it inside the key—probably another expedient agreed upon in case of necessity. Then the key goes into an envelope, for greater security of the cypher message, and the messenger gets it when Henning is at last released for lunch. What happened to the message we know; and here it is.
"Now I will not weary you with a detailed account of the different ways in which I attacked this cypher, but I will take the shortest possible cut to the true interpretation. A very short examination of the cryptogram shows that while no number is included above 23, the numbers, in their relative frequency, roughly agree with the relative frequency of the corresponding letters of the alphabet, a for 1, b for 2, and so on."