In chapter 21, Scout does her best to stay awake while the jury takes a few hours to deliberate on the Tom Robinson case. Scout says that she gets a premonition or "impression" that makes her shiver even though the courtroom is hot and it is summertime (210). The best way Scout can describe this shivering feeling is to compare it to times in her life when the mockingbirds are still, carpenters aren't working on Maudie's house, and everyone on the street shuts themselves in their homes on a cold winter day. These images create a sense of coldness and desertion as people wait for the cold to go away. In the courthouse that night, everyone is waiting for the same thing—a break in the cold. For a year or so, the whole county has been anxiously waiting for the outcome of this trial, so it's as if the whole world stops.
Scout then says that the courtroom is like "A deserted, waiting, empty street," much like the one when everyone closed their doors and waited for Atticus to kill the mad dog Tim Johnson on that February day. It's as if everyone is waiting for the metaphorical mad dog, Tom Robinson, to be officially shot with a verdict. Everyone knows what the outcome will be, but they wait anyway. White men wait to get the verdict over with so they can move on with life and black men wait to support Tom in his predictable plight.
Scout's reference to the mockingbirds can certainly point towards the theme that people shouldn't shoot them because they are harmless and good. People shouldn't convict harmless Tom Robinson, either, but they do. The sin is on their heads when they convict him because it's a sin to kill a mockingbird.
While waiting for the verdict, Scout recalls the incident back in February when her father shot dead the mad dog, Tim Johnson, with a single gunshot. Now, when Atticus returns to hear the verdict along with the rest of the court, she feels somehow that it was like that earlier time, but with a crucial difference:
...it was like watching Atticus walk into the street, raise a rifle to his shoulder and pull the trigger, but watching all the time knowing the gun was empty.
The earlier incident of the mad dog takes on an interesting symbolism when Scout recalls it at the climax of the trial. In the earlier incident, Atticus was able to eliminate the danger: he triumphed in an extremely tense, heightened situation. This time, however, he will be defeated. The linking of the two incidents in Scout's mind is understandable as in both cases she is watching her father taking on the odds, but this time he is deprived of ammunition. The mad-dog incident could also be taken as symbolic of the evil of racism which Atticus attempts to shoot down, as he did the dog. He is not able to prevail against the prejudiced majority however.
Scout also remembers how 'the mockingbirds were still' back on that cold day in February. This also reminds us of the novel's main themes as the mockingbird functions as a symbol of innocence. Now Tom, as an innocent victim of prejudice, is about to be convicted, and will eventually be killed, 'stilled' like the mockingbirds.
In Chapter 21, while Scout is waiting for the verdict, she thinks about something Jem had told her about the power of human concentration, and she tries to act on it. (Much of the novel focuses on young people learning to act on what they've been taught, and on forming community.)
She also thinks about the mockingbirds and how they fall silent in February. (The physical world sometimes echoes the emotional world in the book. Also, winter is a time when the world is dark and despair seems likely; that's the case now, when Atticus loses the case.)
She thinks of the carpenters stopping their hammering on Miss Maudie's new house, and the workmen building a new house after the old burns down is like Maycomb must rebuild into a new community after this travesty of justice.