In chapter twenty-three of Harper Lee's To Kill a Mockingbird, Atticus has a conversation with Jem and Scout (though Jem does most of the talking) about what took the jury so long to come back with a verdict in Tom Robinson's trial. The truth is that in any other trial in which a black man was accused of raping a white woman, the jury would barely have deliberated at all, as their minds would have been made up before the trial ever began. The fact that this one deliberated for several hours is actually a victory, despite the guilty verdict.
In Tom Robinson's trial, the jury was out for an extraordinarily long time. The day after the trial, Jem starts to talk with his father about juries in general and then about Robinson's jury.
“Tom’s jury sho‘ made up its mind in a hurry,” Jem muttered. Atticus’s fingers went to his watchpocket. “No it didn’t,” he said, more to himself than to us. “That was the one thing that made me think, well, this may be the shadow of a beginning. That jury took a few hours. An inevitable verdict, maybe, but usually it takes ‘em just a few minutes. This time—” he broke off and looked at us.
Atticus then reveals another piece of information that surprises the children. One of the jurors "took considerable wearing down—in the beginning he was rarin’ for an outright acquittal." What is shocking is who the lone holdout on the jury was--a Cunningham.
Atticus took a chance and let one of the Old Sarum Cunninghams stay on the jury. He knows the Cunninghams have been stubbornly determined to do things they want and not be pushed around for many generations. Atticus also said:
the other thing about them was, once you earned their respect they were for you tooth and nail.
Atticus has the slightest hope that the Cunninghams had left the jail the night before (after walking away from a kind of stand-off with Scout and the other Finches) with at least a tiny bit of respect for the Finch family. Atticus hoped that would translate to some fairness on the jury.
Then too, he said, it took a thunderbolt plus another Cunningham to make one of them change his mind. “If we’d had two of that crowd, we’d’ve had a hung jury.”
Jem is the one who really understands all of this; Scout simply reports their conversation. What we know from that conversation is that things might, just might, be starting to change in terms of racial prejudice. Those few hours of deliberation give us hope for the future.