What event at the beginning of chapter 15 foreshadows the end of the chapter, and what prompts Jem to go downtown?
At the beginning of Chapter 15, Scout mentions that a "nightmare was upon us." This ominous comment foreshadows the dangerous events that take place later on in the chapter. Earlier in the day, a group of Atticus's friends visits his home to warn him about the Old Sarum bunch. After dinner, Atticus takes a light attached to a long extension cord with him as he leaves the house. Jem becomes curious and decides to follow his father. When Scout asks Jem why he wants to follow Atticus, he tells her, "I've just got this feeling" (Lee 92). Jem can sense that Atticus is putting himself in harm's way. Scout and Dill end up tagging along, and the children find Atticus sitting outside of Tom Robinson's cell. Suddenly, the Old Sarum bunch arrives, and the men surround Atticus. Scout cannot help herself and runs out into the middle of the group of men. Jem and Dill follow her out, and Atticus tells Jem to leave. Jem refuses to leave his father's side because he understands that Atticus is in danger. Fortunately, Walter Cunningham sees the error in his behavior and tells the men to leave.
There are two "mirrored" scenes in this chapter when Atticus is faced with a mob of men and one of his children defuses the situation. The first scene is at the beginning of the chapter, when Heck Tate and others come to the Finch home and stand outside. They are unhappy that Atticus is defending Tom Robinson at trial. The men disperse when Jem yells to his father that the phone is ringing.
Later, that night, Jem, Scout, and Dill go to find their father downtown after Jem gets a "feeling" something is wrong. They find Atticus sitting outside the jail, guarding Tom Robinson from an angry mob. This time, it is Scout's words to Mr. Cunningham that make the men scatter. She reminds him that she is friends with his son, Walter.
In both cases, the children humanize the situation and their father to the angry people around him. They can no longer function as a "blind mob" but instead must see the Finches as people, and as their neighbors.