"We've got quantities of keys to French ciphers, and numerous ones to
the Blocked-Out Square, but they won't translate this letter." He took
up a small book and opened it at a mark. "Here are samples of the
latter: _ecclesiastiques, coeur de roche, a deau eaux, fourreau, chateau
d'eau_, and so on. But, alas, none of them fits; the French Government
has a new key. Indeed, she changes it every month or oftener; sometimes
she changes it just for a single letter."

"Then we must apply ourselves to obtaining the French key-word,"
Harleston remarked. "Can you--do it?"

"Maybe we can pilfer it and maybe we can't. At least we can make a brisk
attempt. I will give orders at once. In the meantime, if you'll keep me
advised of what happens, we may be able to piece your and my information
together and make a word."

"I'll do it!" Harleston replied and started toward the door. Half-way
across the room he suddenly whirled around. "Lord, Carpenter. what an
imbecile I am!" he exclaimed. "I fancy I've had the key-word all the
while and never realized it."

"There are too many petticoats in this case," Carpenter shrugged.

"Never mind the petticoats!" Harleston laughed. "Get out the letter and
try this phrase on it: _à l'aube du jour_."

Without a word of comment, Carpenter set down the cipher message, letter
by letter, and wrote over it _à l'aube du jour_. Then he took up a
printed Blocked-Out Square and with incredible swiftness began to write
the translation.