What is the mood throughout paragraphs 12-15?
Different editions of Poe's stories arrange the paragraphs slightly differently. However, in most copies, including the one in the hypertext library curated by the University of Virginia (attached), paragraphs 12–15 are those immediately after the murder of the old man, dealing with the narrator's precautions in concealing the body.
The mood in these paragraphs is a heightened version of the mood throughout the story, in which the narrator keeps proclaiming how calm and careful he has been with a little too much manic insistence for the reader to believe him. He even ends paragraph 13 with a maniacal laugh at the reflection that a tub had caught all the old man's blood, as though this was a peculiarly cunning precaution to have taken.
The narrator ends paragraph 15 by pointing out that he placed his own chair directly over the old man's corpse "in the wild audacity of my perfect triumph." This phrase captures the mood perfectly as it combines his insane overconfidence with a feverish attitude completely at odds with the calm he continues to profess.
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