Please explain the story of the mad dog in To Kill a Mockingbird, by Harper Lee.
In chapter ten of To Kill a Mockingbird, by Harper Lee, Scout and Jem Finch are convinced "Atticus was feeble: he was nearly fifty." Their father does not do many things that other fathers do, such as play football, and they are not happy to have such an old, useless man (in their eyes) for a father.
One Saturday, Jem and Scout are heading out for the day and get just past the Radley place when they notice an odd sight coming toward them. It is a dog, Tim Johnson, "a liver-colored bird dog, the pet of Maycomb." Jem can see that something is not right with the dog, so he quickly takes Scout back home.
Calpurnia is distracted and thinks nothing of Tim Johnson's walking down the street until Jem describes how the dog is walking:
Jem gulped like a goldfish, hunched his shoulders and twitched his torso. “He’s goin‘ like that, only not like he means to.”
Calpurnia fears the dog is suffering from hydrophobia (rabies), so she immediately goes out to see for herself.
Tim Johnson was not much more than a speck in the distance, but he was closer to us. He walked erratically, as if his right legs were shorter than his left legs.
She rushes the children back into the house and calls Atticus before asking the telephone operator to call Miss Stephanie and Miss Maudie to warn them about the dog. The operator is skeptical, as Atticus was, because she knows hydrophobia is not a typical occurence in February; however, Calpurnia knows what she saw. Next Calpurnia goes to warn the Radleys, though no one answers her frantic knocking.
Sheriff Heck Tate arrives with Atticus, and they watch Tim Johnson get close enoguh to shoot; unfortunately, the dog wanders too close to the Radley house and Heck Tate is hesitant to shoot the dog in case he misses and the bullet hits someone or something at the Radley place.
Tate hands the gun to Atticus, knowing that Atticus is an extremely accurate shot, which is exactly what is called for in this situation. Atticus protests, saying, “'I haven’t shot a gun in thirty years—'” Scout recounts what happens next:
Mr. Tate almost threw the rifle at Atticus. “I’d feel mighty comfortable if you did now,” he said. In a fog, Jem and I watched our father take the gun and walk out into the middle of the street. He walked quickly, but I thought he moved like an underwater swimmer: time had slowed to a nauseating crawl.... Atticus pushed his glasses to his forehead; they slipped down, and he dropped them in the street. In the silence, I heard them crack. Atticus rubbed his eyes and chin; we saw him blink hard. In front of the Radley gate, Tim Johnson had made up what was left of his mind. He had finally turned himself around, to pursue his original course up our street. He made two steps forward, then stopped and raised his head. We saw his body go rigid. With movements so swift they seemed simultaneous, Atticus’s hand yanked a balltipped lever as he brought the gun to his shoulder. The rifle cracked. Tim Johnson leaped, flopped over and crumpled on the sidewalk in a brown-and-white heap. He didn’t know what hit him.
Miss Maudie arrives and calls Atticus by his old nickname, "One-Shot Finch." Jem and Scout are amazed by all of this, as they had no idea their father could shoot a rifle, let alone with such precision. Suddenly both of the kids, but especially Jem, are proud of their father. Zeebo, Calpurnia's son, will come to collect Tim Johnson's body.