In Chapter 9, Poirot displays a more sarcastic side when he uses verbal irony. It seems he doesn't like how Miss Van Schuyler bosses around her "poor relation" Cornelia:
"[Miss Van Schuyler] was frightfully kind and said it would be quite all right for me to come.”
“That was very gracious of her,” said Poirot dryly. The ingenuous Cornelia agreed unsuspectingly.
There is a lot of class-related tension in the novel, including in the relationship between cousins Cornelia and Miss Van Schuyler. Cornelia is grateful for her trip and does not notice or care that her much wealthier relative bosses her around and speaks poorly of her. Always observant yet polite, Poirot notes this with verbal irony: he says, "That was very gracious of her," although in actuality he thinks Miss Van Schuyler is being the opposite of gracious. He's dry enough that Cornelia thinks he's serious and agrees, and so he doesn't insult her or Miss Van Schuyler. But readers know he's employing verbal irony, in part because the narrator tells us Cornelia agrees "unsuspectingly," and in part because Miss Van Schuyler's behavior toward her cousin has thus far been overbearing and rude. Even if Cornelia doesn't notice it, Poirot does.
In Chapter 25, Poirot asks Pennington to come in for an interview after Mrs Otterbourne's murder. In his reluctant agreement, Pennington employs verbal irony:
"Will you come to my cabin in half an hour’s time?”
“I should be delighted.” Pennington did not sound delighted. He did not look delighted either. Race and Poirot exchanged glances and then abruptly left the room.
Pennington says something he clearly doesn't mean. This moment is not only an example of verbal irony but also a possible clue, as Race and Poirot's reaction tells the reader. Why is Pennington reticent to speak with the men? Is it because he's the murderer, or for some other reason? As the reader later discovers, he has something to hide, but he killed neither Linnet nor Mrs. Otterbourne. It may be that, at this point, he worries the murders will be pinned on him. After all, Poirot believes he tried to kill Linnet with the falling rock, and he has plenty of motive.
But in Chapter 27, Poirot ultimately determines that Pennington did not murder Linnet or anyone else aboard the boat:
[Pennington] had the motive, yes. He had the will to do it, yes. He got as far as attempting to do it. Mais c’est tout. For this crime, something was wanted that Pennington hadn’t got! This is a crime that needed audacity, swift and faultless execution, courage, indifference to danger, and a resourceful, calculating brain. Pennington hasn’t got those attributes. He couldn’t do a crime unless he knew it to be safe. This crime wasn’t safe! It hung on a razor edge. It needed boldness.
Poirot metaphorically says the crime "hung on a razor edge." It was planned but also highly risky, and Pennington isn't the sort of man who murders if there's any risk he'll get caught. Poirot uses his understanding of the suspects' psychology to draw conclusions that the more traditional criminologist Colonel Race would not have come to.
In Chapter 25, Poirot reveals to Miss Van Schuyler that the young communist upstart Ferguson is, in fact, a wealthy nobleman. The moment is full of situational irony—not only because Ferguson's background doesn't line up with his ideals, but also because Miss Van Schuyler has spent the entire trip trying to make powerful and wealthy acquaintances.
“You recognized him, I suppose?”
“Recognized him?”
“Calls himself Ferguson and won’t use his title because of his advanced ideas.”
“His title?” Miss Van Schuyler’s tone was sharp.
“Yes, that’s young Lord Dawlish. Rolling in money, of course, but he became a communist when he was at Oxford.”
Miss Van Schuyler has spent the entire novel trying to social-climb and brag about the nobility she knows, and she has treated Cornelia badly in part because Cornelia is poor. Ferguson, an outspoken communist who has nothing but nasty things to say about the rich, provokes Miss Van Schuyler's ire, and they argue. When Ferguson says he wants to marry Cornelia, Miss Van Schuyler tells him his social status is not adequate.
After Ferguson leaves, Poirot informs Miss Van Schuyler that Ferguson has a title and is extraordinarily wealthy. Ferguson is the exact sort of person she has been trying to impress the entire trip, but she's despised him openly under the impression he's unimportant. While Ferguson's lack of tolerance and outspokenness are undoubtedly character flaws, and there is irony in his communism given he's a wealthy nobleman and Oxford student, he does seem genuinely committed to his ideals. He never tries to win Cornelia over by telling her about his wealth or his rank.
In Chapter 25, Poirot asks Pennington to come in for an interview after Mrs Otterbourne's murder. In his reluctant agreement, Pennington employs verbal irony:
"Will you come to my cabin in half an hour’s time?”
“I should be delighted.” Pennington did not sound delighted. He did not look delighted either. Race and Poirot exchanged glances and then abruptly left the room.
Pennington says something he clearly doesn't mean. This moment is not only an example of verbal irony but also a possible clue, as Race and Poirot's reaction tells the reader. Why is Pennington reticent to speak with the men? Is it because he's the murderer, or for some other reason? As the reader later discovers, he has something to hide, but he killed neither Linnet nor Mrs. Otterbourne. It may be that, at this point, he worries the murders will be pinned on him. After all, Poirot believes he tried to kill Linnet with the falling rock, and he has plenty of motive.
But in Chapter 27, Poirot ultimately determines that Pennington did not murder Linnet or anyone else aboard the boat:
[Pennington] had the motive, yes. He had the will to do it, yes. He got as far as attempting to do it. Mais c’est tout. For this crime, something was wanted that Pennington hadn’t got! This is a crime that needed audacity, swift and faultless execution, courage, indifference to danger, and a resourceful, calculating brain. Pennington hasn’t got those attributes. He couldn’t do a crime unless he knew it to be safe. This crime wasn’t safe! It hung on a razor edge. It needed boldness.
Poirot metaphorically says the crime "hung on a razor edge." It was planned but also highly risky, and Pennington isn't the sort of man who murders if there's any risk he'll get caught. Poirot uses his understanding of the suspects' psychology to draw conclusions that the more traditional criminologist Colonel Race would not have come to.