chpt 17: What evidence does Atticus reveal about Bob Ewell that may find him guilty of Tom Robinson's alleged crime?

Atticus points out that Tom Robinson can't use his left hand, "I can't use my left hand at all. I got it caught in a cotton gin when I was twelve years old. All my muscles were tore loose." Tom could not have held Mayella down and done the damage (rape) on her with one hand.