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.