problem. In employing such a system of ciphers, some
combination, precisely similar to the combinations used on the locks of
safes, would have to be used. It was absolutely necessary, in order to
insure safety, to use not one cipher, but a large number, changing the
arrangement of the letters with each line written--even with each word,
in order to defy solution. Yet such an arrangement being purely
arbitrary, could not well be trusted to memory, for, once forgotten, the
translation of the document written, even by the writer himself, would
be absolutely impossible. It occurred to him that as there were six
different concentric lines of lettering, each constituting in itself a
complete cipher, the obvious way to use the box would be to place the
pearls in a given position, write six words, using a different alphabet
for each word, and then shift the ring of pearls to a new position, and
repeat the operation. This, of course, could be done indefinitely,
although half a dozen changes would be sufficient to insure a cipher
that would absolutely defy solution. Where, however, was the key? That,
after all, was the important matter; without it, the snuff box would be
as useless to Monsieur de Grissac as it would be to his enemies
themselves.

For many minutes Duvall puzzled over the matter, unable to reach any
satisfactory conclusion. Then he began to think of the song which had so
clearly been repeated, over and over, as a message to him from outside.
The words of the refrain began to run aimlessly through his mind, his
eyes upon the box. Suddenly he realized that the word cross, in its
repetitions, its position as the final word of the song, must have a
definite meaning. Before his eyes he saw the cross, so delicately carved
as to project scarcely an eighth of an inch above the thin and fragile
ivory surface. Instinctively he began to push at it, pressing it this
way and that, to discover, if possible, any spring or other means
whereby it might be made to turn or lift up. As he did so, his fingers
unconsciously pressed upon the large pearl at the top. In a moment the
upper surface of the cross slid to one side, disclosing a tiny shallow
cavity beneath it, some quarter of an inch in either direction, and no
deeper than the thickness of a piece of cardboard. Within this lay a bit
of tissue paper, tightly folded.

Duvall drew it carefully out and examined it. Upon it were written six
numbers: 12-16-2-8-20-4. There was nothin