“Sure thing!” she exclaimed.  “That’s she, all right.  How in the world did you ever—­pardon me, Mr. Harleston, I shouldn’t have said that.”

“You’re not meddling, Miss Williams.  But it’s a long story—­too long to detail now.  Some day soon I’ll confide in you, for you’ve helped me very much in this matter and deserve to know.  In fact, you’ve helped me more than you can imagine.  Meanwhile mum’s the word, remember.”

“Mum, it is, Mr. Harleston,” she replied, “For once a telephone girl won’t leak, even to her best friends.”

“I believe you,” Harleston returned.  “Keep your eyes open, also your ears, and report to me anything of interest as to our affair.”

Miss Williams answered with a knowing nod and an intimate little smile, then swung around to answer a call.  Harleston returned to his rooms.  The happenings of the recent evening were quite intelligible to him now: 

When the episode of the cab of the sleeping horse occurred, Mrs. Spencer was in the Chartrand apartment.  Marston, in some way, had learned of Harleston’s participation in the cab matter, and with Sparrow had followed him to the Collingwood, entering by the fire-escape—­with the results already seen.  The noise on the fire-escape was undoubtedly made by them, and the long interval that elapsed before they entered his apartment was consumed in reporting to her, or in locating his number.

One thing, however, was not clear:  how they had learned so promptly of Harleston’s part in the affair, and that it was he who had taken the letter from the cab.  Either someone had seen him at the cab and had babbled to the Marston crowd, or else Mrs. Winton or Mrs. Clephane had not been quite frank in her story.  He instantly relieved Mrs. Clephane of culpability; Mrs. Winton did not count with him.  Moreover, it was no longer of any moment—­since Spencer’s people knew and had acted on their knowledge, and were still acting on it—­and were still without the letter.  The important thing to Harleston was that it had served to disclose what promised to be a most serious matter to this country, and which, but for the trifling incident of the cab, would likely have gone through successfully—­and America been irretrievably injured.

Madeline Spencer had assured him that the United States was not concerned; that the matter had to do only with a phase of the Balkan question.  But such assurances were worthless and given only to deceive, and, further, were so understood by both of them.  Maybe her story was true—­only the future would prove it.  Meanwhile you trust at your peril, caveat emptor, your eyes are your market, or words to similar effect.  Of course he could cause her to be apprehended by the police, yet such a course was unthinkable; it would violate every rule of the game; it would complicate relations with Germany, and afford her adequate ground for reprisals on our secret agents.  A certain code of honour obtained with nations, as well as with criminals.