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:—
| 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 | o | o | 0 | f | a |
| e | t | 0 | o | o | 0 | c | v |
| a | o | 0 | o | o | 0 | r | e |
| a | h | t | k | r | i | e | t |
| l | e | w | n | n | a | a | t |
"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!"