Where do the children find Atticus? What is his purpose for being there?

They look for Atticus in his office, but finally spy him sitting outside the county jail, with the light bulb providing light for him to read his book. The children stay a safe distance away so Atticus won't notice them. Jem feels reassured knowing where his father is, but as they are about to head home, four old cars come into town. It is a mob of men come to "deal" with Tom Robinson.