M. Bouc cleared his throat.

The Wagon Lit conductor was recalled. He looked at them inquiringly.

"Michel," he said. "Here is a button from your tunic. It was found in the American lady's compartment. What have you to say for yourself about it?"

"That is very odd."

The conductor's hand went automatically to his tunic. "I have lost no button, Monsieur," he said. "There must be some mistake."

"I cannot account for it, Monsieur." The man seemed astonished, but not in any way guilty or confused.

As the significance of M. Bouc's word became plain to him, Pierre Michel flew into a violent state of agitation.

"She did not imagine it, Michael. The assassin of M. Ratchett passed that way -- and dropped that button."

"Let us hear what Pierre Michel has to say about this button," he said.

M. Bouc said meaningly: "Owing to the circumstances in which it was found, it seems fairly certain that this button was dropped by the man who was in Mrs. Hubbard's compartment last night when she rang the bell."

"But, Monsieur, there was no one there. The lady must have imagined it."

"Where were you when Mrs. Hubbard's bell rang?"

"We will send for him."

The conductor of the next coach was summoned. He immediately confirmed Pierre Michel's statement. He added that the conductor from the Bucharest coach had also been there. The three of them had been discussing the situation caused by the snow. They had been talking some ten minutes when Michel fancied he heard a bell. As he opened the doors connecting the two coaches, they had all heard it plainly. A bell ringing repeatedly. Michel had run posthaste to answer it.

"It is not true, Monsieur, it is not true!" he cried. "You are accusing me of the crime. Me? I am innocent. I am absolutely innocent. Why should I want to kill a Monsieur whom I have never seen before?"

"I told you, Monsieur, in the next coach, talking to my colleague."

"Do so, Monsieur, I implore you, do so."

"So you see, Monsieur, I am not guilty," cried Michel anxiously.

"And this button from a Wagon Lit tunic -- how do you explain it?"

"I cannot, Monsieur. It is a mystery to me. All my buttons are intact."

"Calm yourself, Michel," said M. Bouc, "and cast your mind back to the moment when you ran to answer Mrs. Hubbard's bell. Did you meet anyone at all in the corridor?"

"No, Monsieur."

"Did you see anyone going away from you down the corridor in the other direction?"

"Again, no. Monsieur."

"Odd," said M. Bouc.

"Not so very," said Poirot. "It is a question of time. Mrs. Hubbard wakes to find someone in her compartment. For a minute or two she lies paralysed, her eyes shut. Probably it was then that the man slipped out into the corridor. Then she starts ringing the bell. But the conductor does not come at once. It is only the third or fourth peal that he hears. I should say myself that there was ample time --"

"There are two courses open to our mysterious assassin," said Poirot slowly. "He could retreat into either of the toilets or he could disappear into one of the compartments."

Both of the other conductors also declared that they had not lost a button. Also that they had not been inside Mrs. Hubbard's compartment at any time.

"For what? For what, mon cher? Remember that there are thick drifts of snow all round the train."

"But they were all occupied."

"Yes."

With a gesture M. Bouc signified that the three conductors might depart.

Poirot nodded.

"You mean that he could retreat into his own compartment?"

"It fits, it fits," murmured M. Bouc. "During that ten minutes' absence of the conductor, the murderer comes from his own compartment, goes into Ratchett's, kills him, locks and chains the door on the inside, goes out through Mrs. Hubbard's compartment and is back safely in his own compartment by the time the conductor arrives."

"Who will you see first -- the Italian?"

Poirot murmured: "It is not quite so simple as that, my friend. Our friend the doctor here will tell you so."

"We have still to see eight passengers," said Poirot. "Five first-class passengers -- Princess Dragomiroff, Count and Countess Andrenyi, Colonel Arbuthnot and Mr. Hardman. Three second-class passengers -- Miss Debenham, Antonio Foscarelli and the lady's maid, Fräulein Schmidt."

"How you harp on your Italian! No, we will start at the top of the tree. Perhaps Madame la Princesse will be so good as to spare us a few moments of her time. Convey that message to her, Michel."

"Oui, Monsieur," said the conductor, who was just leaving the car.

But Princess Dragomiroff declined to take this course. She appeared in the dining car, inclined her head slightly and sat down opposite Poirot.

Her small toad-like face looked even yellower than the day before. She was certainly ugly, and yet, like the toad, she had eyes like jewels, dark and imperious, revealing latent energy and an intellectual force that could be felt at once.

"Tell her we can wait on her in her compartment if she does not wish to put herself to the trouble of coming here," called M. Bouc.

"You are most amiable, Madame," said Poirot.

"You need not offer apologies, Messieurs. I understand a murder has taken place. Naturally, you must interview all the passengers. I shall be glad to give all the assistance in my power."

Her voice was deep, very distinct, with a slight grating quality in it.

She cut short a flowery phrase of apology from M. Bouc.

"Not at all. It is a duty. What do you wish to know?"

"Your full Christian names and address, Madame. Perhaps you would prefer to write them yourself?"

"You can write it," she said. "There is nothing difficult -- Natalia Dragomiroff, 17 Avenue Kleber, Paris."

Poirot proffered a sheet of paper and pencil, but the Princess waved them aside.

"Yes, I have been staying at the Austrian Embassy. My maid is with me."

"You are travelling home from Constantinople, Madame?"

"Willingly. I directed the conductor to make up my bed whilst I was in the dining car. I retired to bed immediately after dinner. I read until the hour of eleven, when I turned out my light. I was unable to sleep owing to certain rheumatic pains from which I suffer. At about a quarter to one I rang for my maid. She massaged me and then read aloud till I felt sleepy. I cannot say exactly when she left me. It may have been half an hour, it may have been later."

"The train had stopped then?"

"You heard nothing -- nothing unusual during the time, Madame?"

"I heard nothing unusual."

"Would you be so good as to give me a brief account of your movements last night from dinner onwards?"

"The train had stopped."

"You consider her trustworthy?"

"No, no, she is alive, but she lives in complete retirement. Her health is very delicate, she has to lie on a sofa most of the time."

"You knew Colonel Armstrong well, then?"

"Fifteen years."

With some emotion in her voice the old lady said: "You speak of friends of mine, Monsieur."

"She has been with you long?"

"Were you at any time acquainted with a family of the name of Armstrong -- a family in which a tragedy occurred?"

"I knew him slightly; but his wife, Sonia Armstrong, was my god-daughter. I was on terms of friendship with her mother, the actress, Linda Arden. Linda Arden was a great genius, one of the greatest tragic actresses in the world. As Lady Macbeth, as Magda, there was no one to touch her. I was not only an admirer of her art, I was a personal friend."

"Hildegarde Schmidt."

The abrupt change of subject made the old lady raise her eyebrows. "Many times."

"What is your maid's name?"

"You have been in America, I presume, Madame?"

"She is dead?"

"Absolutely. Her people come from an estate of my late husband's in Germany."

"Yes, much younger than Mrs. Armstrong."

"I must ask you the reason of these questions. What have they to do with the matter in hand -- the murder on this train?"

"And she is alive?"

"It is most natural, Madame. And now to return to the question you did not answer. Where is the younger daughter of Linda Arden, the sister of Mrs. Armstrong?"

"Where is she?"

"In my view, then, this murder is an entirely admirable happening! You will pardon my slightly biased point of view."

"I honestly cannot tell you, Monsieur. I have lost touch with the younger generation. I believe she married an Englishman some years ago and went to England, but at the moment I cannot recollect the name."

"Ah!"

"They are connected in this way, Madame, the man who was murdered was the man responsible for the kidnapping and murder of Mrs. Armstrong's child."

The straight brows drew together. Princess Dragomiroff drew herself a little more erect.

"Certainly."

The old woman bent an acute glance at him.

"There was, I think, a second daughter?"

She was silent a minute, then: "Hercule Poirot," she said. "Yes. I remember now. This is Destiny."

She paused a minute and then said: "Is there anything further you want to ask me, gentlemen?"

"You will excuse me, Monsieur," she said, "but may I ask your name? Your face is somehow familiar to me."

"Only one thing, Madame, a somewhat personal question. The colour of your dressing gown."

"I am wondering," he said, "what she meant by Destiny."

"There is nothing more, Madame. I am much obliged to you for answering my questions so promptly."

She made a slight gesture with her heavily-beringed hand. Then, as she rose, and the others rose with her, she stopped.

She raised her eyebrows slightly. "I must suppose you have a reason for such a question. My dressing gown is of blue satin."

She walked away, very erect, a little stiff in her movements.

"Voilà une grande dame," said M. Bouc. "What do you think of her, my friend?"

"My name, Madame, is Hercule Poirot -- at your service."

But Hercule Poirot merely shook his head.