“It is written in the regular pica type,” he remarked thoughtfully, “and on a machine that has seen considerable rough usage, although it is not an old machine.  It will take me a little time to identify the make, but after I have done that, I think I could identify the particular machine itself the moment I saw it.  You see, it is only a clue that would serve to fix it once you found that machine.  The point is, after all, to find it.  But once found, I am sure we shall be close to the source of the letter.  I may keep this and study it at my leisure?”

“Certainly.”

For a moment Carton was silent.  Then it seemed as though the matter of Betty Blackwell brought to mind what he had read in the morning papers.

“That robbery of Langhorne’s safe was a most peculiar thing, wasn’t it?” he meditated.  “I suppose you know what Miss Blackwell was?”

“Langhorne’s stenographer and secretary, of course,” I replied quickly.

“Yes, I know.  But I mean what she had actually done?  I don’t believe you do.  My county detectives found out only last night.”  Kennedy paused in his rummaging among some bottles to which he had turned at the mention of the safe robbery.  “No—­what was it?” he asked.

Carton bent forward as if our own walls might have ears and said in a low voice:  “She was the operator who took down the detectaphone conversations at the other end of the wire in a furnished room in the house next to Gastron’s.”

He drew back to see what effect the intelligence had on us, then resumed slowly:  “Yes, I’ve had my men out on the case.  That is what they think.  I believe she often executed little confidential commissions for Langhorne, sometimes things that took her on short trips out of town.  There is a possibility that she may be on a mission of that sort.  But I think—­it’s this Black Book case that involves her now.”

“Langhorne wouldn’t talk much about anything,” I put in, hastily remembering his manner.  “He may not be responsible—­but from his actions I’d wager he knows more about her than appears.”

“Just so,” agreed Carton.  “If my men can find out that she was the operator who ‘listened in’ and got the notes and the transcript of the Black Book, then she becomes a person of importance in the case and the fact must be known to others who are interested.  Why,” he pursued, “don’t you see what it means?  If she is out of the way, there is no one to swear to the accuracy of the notes in the record, no one to identify the voices—­even if we do manage finally to locate the thing.”

“Dorgan and the rest are certainly leaving nothing undone to shake the validity of the record,” ruminated Kennedy, accepting for the moment at least Carton’s explanation of the disappearance of Miss Blackwell.  “Have you any idea what might have happened to her?”

Carton shook his head negatively.  “There are several explanations,” he replied slowly.  “As far as we have been able to find out she led a model life, at home with her mother and sister.  Except for the few commissions for Langhorne and lately when she was out rather late taking the detectaphone notes, she was very quiet,—­in fact devoted to her mother and the education of her younger sister.”