In Chap.10, who told Atticus about Harry Johnson's dog?

In chapter 10 the children see the dog coming down the street falling sideways like he is drunk.  The dog is going slow and acting strange.  They run to Calpurnia and tell her.

Calputrnia calls Atticus and also tells the operator to call the people and tell them to shut their doors.  She even runs to Boo Radley's house to warn them.

Atticus shows up and demonstrates his abilities as a crack shot.  The children are amazed as they did not think that their father could do anything.  They think of him as being old.  He kills the dog with one shot.