“I don’t quite understand their game,” Carpenter chuckled, as he turned away, “but it’s no matter.  I took all the tricks this morning and still have a few trumps left.  I thought he certainly would try for a copy of the letter, but he didn’t even attempt it.  He may have committed it to memory, but I’ll chance it.”

Returning to his office he gave the code-book another careful inspection and confirmed his impression as to its being authentic.  Then he laid it aside, and took up the letter and a l’aube du jour!

First he tried it in reverse position:  ruoj ud ebua’l a.  The translation was gibberish.  Then he wrote the first and last letters, the second and next to last, the third and the third from last, and so on.  The result, too, was gibberish.  Next he dropped the first word, ‘a’ and tried the rest—­still gibberish.  He dropped also the ’l’—­still gibberish.  Then, in turn, the ‘a’ of the third word the ‘d’ of the fourth, the ‘j’ of the last word—­all gibberish.  Next he wrote the key-word entire but transposed the ‘a’ from the first letter to the last—­still gibberish.  He began with the aube—­still gibberish.

“Damn!” said he.

He was persuaded that the key-word was in the sentence before him; the code-book, Crenshaw’s slip of paper, and his own hunch were convincing, yet the combination was slow in coming.

Du jour a l’aube was the next arrangement.  He wrote it under the printed words and began to apply the Square.

The D and the A yielded A; the U and the B yielded V; the J and the C yielded E; the O and the D yielded R; the U and the E yielded T; the R and the F yielded I.

“Averti!”

Carpenter gave a soft whistle of satisfaction.  French, it was—­his hunch had not deceived him.  The key-word was found!

Swiftly he worked out the rest of the cipher, setting down the letters of the translation without regard to words. “Averti” was evident because it was the first word.  At the end, he had this result: 

  AVERTIQUELALLEMAGNEAENGAG
  EUNOFFICIERADECELERLAFORM
  ULESECRETEDESETATSUNISEMP
  LOYEEACOLLODONNIERLAFULMI
  COTONPOURLAPOUDRESANSFUME
  EALARTILLERIEDEGROSCALIBR
  EETQUEMADELINESPENCEREMIS
  SAIREDELALLEMAGNEAPARISPH
  OTOGRAPHIECIINCLUSEAETECH
  ARGEEDELARECEVOIRNESEPEUT
  DECOUVRIRLENOMDUTRAITRESP
  ENCERESTPARTIEPOURNEWYORK
  SURLALUSITANIAQUIDOITARRI
  VERLEQUATORZEATOUTEFORCEI
  NTERCEPTEZLAFORMULEOUEMPE
  CHEZAMOINSQUELALLEMAGNENE
  LOBTIENNESPENCERSIMPORTAN
  TEALAFRANCE

There was not the least doubt as to it being in French—­the last three words, as well as the first, proved it; also that he had the correct key-word.  It only remained now to separate the result into words.  And this puzzle presented no difficulties to Carpenter; he quickly marshalled it into form: