Why does Jem say "We got him" in To Kill a Mockingbird?

In Chapter 17, Atticus is questioning Bob Ewell, who is on the witness stand. Atticus asks Bob several questions regarding what happened on the evening of November 21st, and asks him why he didn't find it necessary to call a doctor right away. Bob Ewell says that he never called a doctor in his entire life and didn't have the money to pay for one anyhow. Atticus goes on to ask Bob Ewell about the location of Mayella's injuries. Bob says that he agrees with Heck Tate's testimony that Mayella's right eye was blackened, and the right side of her face was badly beaten. Then, Atticus asks Bob Ewell if he can read and write, and asks Ewell if he could sign his name on the back of an envelope. Mr. Gilmer asks what is so interesting, and Judge Taylor is quick to point out that Bob is left-handed. Bob Ewell gets upset at Atticus and calls him a tricky lawyer before Atticus excuses him. Mr. Gilmer stands up to ask Mr. Ewell one more question. Mr. Gilmer asks Bob if he is ambidextrous. Bob Ewell says, "I most positively am not" (Lee 238). Scout mentions that Jem was softly pounding on the balcony rail and whispers, "We've got him" (Lee 238).

Jem whispers "we've got him" because he feels that Atticus has clearly proved that Bob Ewell was the person who beat up Mayella. The fact that Mayella was beaten on the right side of her face indicates that a left-handed man more than likely would have beaten her. Scout says, "Sherlock Holmes and Jem Finch would agree" (Lee 238). Scout goes on to mention that Tom Robinson could have easily been left-handed, and she believes that Jem is "counting his chickens."