After Martin Hewitt had rushed off to St. Augustine's Hospital with the key, the envelope, and the cypher I had brought him, I heard nothing of him till dusk fell—about six. Then I received this telegram:—

I had to be at my office between eight and nine, and to keep Hewitt's appointment I should probably have to sacrifice my dinner. But I was particularly curious to know the meaning of that cypher, and just as curious to know how it could be read; and, moreover, I knew that any case that Hewitt called interesting would probably be interesting above the common. So I took my hat and sought a cab.

I was first at the meeting-place—indeed, a little before my time. No. 120 Broad Street was a great new building of offices, most, if not all, closed at this time—a fact indicated by the shutting of one of the halves of the big front door, where a char-woman was sweeping the steps under the board which announced that offices were to be let. I waited nearly a quarter of an hour, and then at last a hansom stopped and deposited Hewitt and another older gentleman before me.

"Hope we haven't kept you waiting, Brett," Hewitt said. "This is Mr. Bell, of Kingsley, Bell and Dalton; it took me a little longer than I expected to reach him. His offices are shut, and the clerks all gone, but we are going to turn up the lights for a bit. The lift man is gone too, I expect, so we shall have a good long stair-climb."

As to the lift man Hewitt was right, and during our long climb I received, briefly, an account of the loss Mr. Bell's firm had suffered. "I have told Mr. Bell," Hewitt said, "that it was you who happened across the key in such an odd fashion, and when I wired I was sure he would be glad to let you see the upshot of your strange bit of luck. I was also pretty sure that you would like to see it, too. For I really believe that this case—which I confess seemed pretty near hopeless a few hours ago—is coming to an issue now, and here."

"Did you get any information out of the man in the hospital?" I asked.

"Not a scrap," Hewitt replied. "He was still insensible, and though I saw his clothes, and they told me a good deal about the gentleman's personal habits—which are not dazzlingly noble, to put it mildly—they told me nothing else whatever, except that he had recently been knocked down in the mud, which I knew already. But the cypher has told me something, as I will explain presently."

By this time we had reached the high floor in which the offices stood, and Mr. Bell, all wonder and pale agitation, unlocked the outer door, and turned on the electric light.

"Now," cried Hewitt, "show me your ventilators!"

There were some, it seemed, in the top panes of the windows, but these were not what Hewitt wanted. There were others in the form of upright chambers or flues, made of metal, and painted the same colour as the walls about them. They rose from the floor in corners and wall angles, and could be shut or opened by means of lids over their upper ends. These were more to Hewitt's mind, and he went about from one to another, groping under the lids, and poking down into the flues with a walking-stick. There was a wire-grating, or diaphragm, it seemed, in each of them, two or three feet down, and we could hear the end of the stick raking on this at each investigation. One after another of these ventilators Hewitt examined, till he had examined them all, in outer and inner rooms, without result; and I could see that he was disappointed.

"There must be another somewhere," he said, and hunted afresh.

But plainly he had tried them all, and now he could do no more than try them all again, with as little result.

"It is a ventilator," he said, positively. "Unless——" he broke off thoughtfully and stood silent for a few moments. "Ah! of course!" he resumed presently. "We'll send for the housekeeper and a candle. Which is the nearest empty office—the nearest office to let? Is there one on this floor?"

"I think not," Mr. Bell answered. "But there's one on the floor below, just opposite the lift—I see the bill on the door every day as I come up."

"We'll try that, then. I'll rake out every ventilator in this palatial edifice before I'll call myself beaten. Come, call the housekeeper. Is there a speaking tube? Tell him to bring a light."

The housekeeper came, wonderingly, with a watch-man's oil-lantern, and we all went to the floor below. Opposite the lift was a glass door from which a bill had recently been torn.

"Why, it's let!" said Mr. Bell.

"Yes, sir," assented the housekeeper. "Let a day or two ago to a Mr. Catherton Hunt. Or, at least, a deposit was paid."

"But see—the door's not locked," Hewitt observed, pushing it open. "I think we'll trespass on Mr. Catherton Hunt's new offices, since they seem quite empty, and he hasn't taken possession. Come—ventilators!"

It was a small office—an outer room of moderate size, and one smaller inner room. Hewitt at once attacked the ventilators in the larger apartment—there were two of them—but retired disappointed from each. There was one ventilator only in the small room. Hewitt tilted the lid, which was at about the level of his eyes, thrust in his hand, and drew forth a bundle of folded papers; thrust in his hand again and drew forth another bundle; did it again, and drew forth more!

Mr. Bell fell upon the first bundle almost as a dog falls upon a bone; and now he snatched eagerly at each successive paper or bundle, till Hewitt raked the grating with his stick, and declared that there were no more. "Is that all?" he asked.

Mr. Bell went tremblingly from paper to paper, and, at last, said that he believed it really was. "I can verify it by the list upstairs," he added, "if you are sure there are no more."

"No more," repeated Hewitt, rattling his stick in the ventilator again. "Let us go and verify, by all means."

We sent the puzzled housekeeper away, and returned to the office above, and presently Mr. Bell, now beginning so far to recover from his amazement as to express incoherent gratitude, reported that the bonds were correct and complete to the last and least.

"Very well," said Hewitt, "then my part of the business is done, though I must say I've had luck, or rather, Brett has had it for me. But the police must come on now. I think, Mr. Bell, we'll go along to Scotland Yard when we leave here. They'll be wanting to see Mr. Catherton Hunt, I expect, whoever he is—and somebody in your office, too, if I'm not sadly mistaken."

"Who?" gasped Mr. Bell.

"That, perhaps, you can help to point out. See here—do you know whose figures they are?" and Hewitt produced the small slip of paper containing the cypher.

"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."

Here I handed Hewitt the pencilled note I had made at the hospital, with letters substituted for the figures, thus:—i, h, n, d, t, r, e, i; o, s, t, 0, c, i, h, e; c, w, 0, 0, e, m, n, s; s, t, 0, 0, 0, 0, f, a; e, t, 0, 0, 0, 0, c, v; a, o, 0, 0, 0, 0, r, e; a, h, t, k, r, i, e, t; l, e, w, n, n, a, a, t.

Hewitt took the paper and went on. "If that were all the thing would be childishly simple. But you will see that we seem as far from the solution as ever; for the letters as they stand mean nothing, though in fact they are in normal relative frequency; so that if they mean other letters, all the rules are upset, and we are at a standstill. I admit that for a long time the thing bothered me. But a peculiarity struck me. Not only were the figures, or letters, disposed in groups of eight, but there were also eight such groups—sixty-four altogether. What did that suggest? What but a chessboard?"

"A chessboard?" I queried.

"Just so—a chessboard. Eight squares each way—sixty-four altogether. So I drew a rough representation of a chessboard, and set out the letters on it, in their order, like this:—

"Now, there was my chessboard with my letters on it. I tried reading them downward, across, upward and diagonally, in the direction of the moves of different chess pieces—king, queen, rook and bishop. Nothing came of that, whatever I did; the thing was as unreadable as ever. But there remained one chess-move to try—the eccentric move of the knight; the move of one square forward, backward or sideways, and then one square diagonally, or, as it has sometimes been more concisely expressed, the move to the next square but one of a different colour from that on which it rests. I tried the knight's move, and I read the cypher.

"I began at the top left-hand corner, just as one does in reading a book. I read the moves downward—i to w, e and h, and found that led to nothing. So I took the one alternative move, and, with a little consideration, skipped along from i to t in the second line of squares, t in the top line, h in the second line, e in the third, r in the top and e in the second. That gave me an idea. There were the letters i, t, t, followed by the word here. I tried back from the i again, and taking in the reverse order the w, e and h which I had first given up, I read my own name, as you can see it, from the h on the bottom line but one, moving upward. So I had the words Hewitt here. I need not carry you through all the steps, which will now be plain enough to you. But I found that the message actually began in the right-hand corner, and read thus, the noughts counting for nothing—

"'Invent loss disc take at once Martin Hewitt here fear watch.'

"The noughts were plainly merely inserted to fill in unneeded squares, and keep the rest of the figures in their proper relative places when the cypher was written in line. At first I was a little puzzled to understand what seemed to be the first word invent. But it was quite clear that loss disc meant 'loss discovered,' so I concluded that here in the beginning was a contraction also, and that in was a separate word. In that case vent could be a contraction for no other word but 'ventilator,' in accordance with the sense of the words. So I concluded that the meaning of the whole sentence was simply this: 'The plunder is in the ventilator, the loss is discovered, take away the booty at once; Martin Hewitt is here, and I fear I may be watched.' There is the reading, and our little adventure this evening is what it has led to.

"Of course, the confederate wouldn't go groping about the squares so painfully as I have had to do. To him the reading would be simple enough, for the order of the moves would be preconcerted. Each of the conspirators would have, as a guide, both to reading and writing the cypher, a drawn set of squares, numbered in the order of the moves—1 where we have the i, 2 where we have the n, 3 where we have the v, and so on. With that before him, either reading or writing in this extraordinary cryptogram would be easy and quick enough. And now for Scotland Yard!"