Answer:
By quickening the pace of the story.
Explanation:
Washington Irving's short story "The Adventure of The Mysterious Picture" is a part of the collection of short stories compiled in the book "Tales of A Traveler, By Geoffrey Crayon, Gent". This book consists of four parts, of which this story of the mysterious picture is from "Part I: Strange Stories By A Nervous Gentleman".
The story is about a traveler, our narrator who had been invited to be a part of a group of other men to stay at a mansion owned by one of their friends. There, the discussion about ghosts and haunted places led to the host of the house to declare that one of the rooms is indeed haunted. But without revealing which room it was, he said it will all be revealed in the morning, when they can see who the "her" of the night will be. The room that the narrator got had a picture that seems to be with some supernatural effects for he feels uncomfortable and even begins to think that it is this room that was mentioned. During the night, the narrator couldn't sleep so he went downstairs and slept on the sofa. This particular passage given in the question is from that scene where he had gone to sleep on the sofa. Irving quickens the pace of the story to build or provide more suspense.