A moment later Carpenter entered; a scholarly-looking man in the fifties; bald as an egg, with the quiet dignity of bearing which goes with a student, who at the same time is an expert in his particular line—­and knows it.  He was the Fifth Assistant Secretary, had been the Fifth Assistant and Chief of the Cipher Division for years.  His superior was not to be found in any capital in Europe.  His business with the secret service of the Department was to pull the strings and obtain results; and he got results, else he would not have been continued in office.  His specialty, however, was ciphers; and his chief joy was in a case that had a cipher at the bottom.  Ciphers were his recreation, as well as his business.

The Secretary with a gesture turned him over to Harleston—­and Harleston handed him the letter.

“What do you make out of it, Mr. Carpenter?” he asked.

Carpenter took the letter and examined it for a moment, holding it to the light, and carefully feeling its texture.

“Not a great deal cursorily,” he answered.  “It’s a French paper—­the sort, I think, used at the Quay d’Orsay.  Have you the envelope accompanying it?”

“Here it is!” said Harleston.

“This envelope, however, is not French; it’s English,” Carpenter said instantly.  “See! a saltire within an orle is the private water-mark of Sergeant & Co.  I likely can tell you more after careful examination in my workshop.”

“How about the message itself?” Harleston asked.

“It is the Vigenerie cipher, that’s reasonably certain; and, as you are aware, Mr. Harleston, the Vigenerie is practically impossible of solution without the key-word.  It is the one cipher that needs no code-book, nor anything else that can be lost or stolen—­the code-word can be carried in one’s mind.  We used it in the De la Porte affair, you will remember.  Indeed, just because of its simplicity it is used more generally by every nation than any other cipher.”

“I thought that you might be able to work it out,” said Harleston.  “You can do it if any one on earth can.”

“I can do some things, Mr. Harleston,” smiled Carpenter deprecatingly, “but I’m not omniscient.  For instance:  What language is the key-word—­French, Italian, Spanish, English?  The message is written on French paper, enclosed in an English envelope.—­However, the facts you have may clear up that phase of the matter.”

“Here are the facts, as I know them,” said Harleston.

Carpenter leaned back in his chair, closed his eyes, and listened.

* * * * *

“The message is, I should confidently say, written in English or French, with the chances much in favour of the latter,” he said, when Harleston had concluded.  “Everyone concerned is English or American; the men who descended upon you so peculiarly and foolishly, and who showed their inexperience in every move, were Americans, I take it, as was also the woman who telephoned you.  Moreover, she is fighting them.”