Atticus faces the lynch mob in a steady manner. Even after he learns that Heck Tate has been drawn away from the jail on a pretext and that he will be facing the crowd alone, Atticus remains calm. When Scout bursts through the mob, however, Atticus is filled with "plain fear," fear that intensifies when Jem and Dill also appear:
Atticus got up from his chair, but he was moving slowly, like an old man. He put the newspaper down very carefully, adjusting its creases with lingering fingers. They were trembling a little.
While Atticus reacts so slowly, his mind is no doubt racing as he realizes the enormous danger at hand. Besides Tom's life being in jeopardy, he now has three children in deadly peril. The mood becomes even more menacing when one of the mob lays rough hands on Jem, Scout defends her brother by kicking the stranger, and Atticus then is given fifteen-seconds to send his children away.
Atticus knew the mob, some of whom had been drinking, was beyond logic or reason. He no doubt imagined the situation could spiral out of control at any moment. His children could be hurt--or killed--and barring those possibilities, they could see the kind of violence that would haunt them for the rest of their lives. Surely his terror and desperation grew as Jem refused to leave. When the mob disperses, after Scout's innocent conversation with Walter Cunningham, Atticus leans against the jail "with his face to the wall" because he is so weak with relief that the disaster had been averted. We can infer also that he had shed tears of relief since he wiped his face with his handkerchief and then "blew his nose violently."