office.

The Fifth Assistant was sitting with his feet on his desk, a cigar in
his mouth, his gaze fixed on vacancy.

"Damn your old cipher, Harleston!" he remarked, coming out of his
abstraction. "It's bothered me more than anything I've tackled for
years. I can't make head nor tail of it. Its very simplicity--or seeming
simplicity--is what's tantalizing. It's in French. Of so much I feel
sure, though I've little more than intuition to back it. As you know,
this Vigenèrie, or Blocked-Out Square, cipher is particularly difficult.
I've tried every word and phrase that's ever been used or discovered. We
have a complete record of them. None fit this case. Can you give me
anything additional that will be suggestive?"

"Here's what I've brought," Harleston replied--and related, so far as
they seemed pertinent, the incidents of the previous afternoon and
evening.

"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 Vigenèrie cipher of the French Diplomatic Service and we shall have
the translation."

"You haven't that word?" Harleston asked.