“A French message in an English envelope, inclosing an unmounted photograph of Madeline Spencer, a well-known German Secret Agent in Paris,” Carpenter remarked slowly; “and the letter is borne by Madame Durrand to the French Ambassador.  You see, my intuition was right? the letter is in French; and as it is of French authorship the key-word is French.  That narrows very materially our search.  Find the key-word to the Vigenerie cipher of the French Diplomatic Service and we shall have the translation.”

“You haven’t that word?” Harleston asked.

“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:  a l’aube du jour.”

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

“Where did you get this ‘at the break of day,’ Harleston?” he asked as he wrote.

“Found it in Crenshaw’s pocket-book when he returned to hold me up,” Harleston replied.

“Only this isolated phrase?”

“Yes—­and signed with the single initial ‘M.’”

“Hump!” Carpenter commented.  “Mrs. Spencer’s name, I believe you said, is Madeline.  I tell you there are too many women in this affair.”

Suddenly he threw down the pen.  “What’s the use in going on with it.  If you can supply a key to this key we may arrive.  Such an array of unpronounceables may be Russian, it assuredly isn’t French or English.  Look at it!” and he handed the translation to Harleston, who read: 

  AGELUMTONZUCLPMUHRHUNBARGPUH
  PJICLWYIAOIWFPHLUOZFRXUFJWH
  WASNVDPS

“Good Lord!” said Harleston.  “I pass.  Did you ever see so many consonants.  I reckon my key-word isn’t the key.”