"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; 0,
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 represe