After the three children have been discovered in the courtroom, they convince Atticus to let them stay for the final verdict of the Tom Robinson trial--after they have returned for supper with Calpurnia. Upon their return, Jem expresses his confidence about an acquittal, but Reverend Sykes expesses doubt. "I ain't ever seen any jury decide in favor of a colored man over a white man." Jem, Scout and Dill then begin their own vigil.
Scout observes Atticus wandering the courtroom and realizes that things don't seem normal. "I had never seen a packed courtroom so still." After more than a four hour wait, she takes a short nap herself. When she awakes, in typical Maycomb style, nothing has changed; the sleepy crowd accurately represents the town: nothing else important is happening, there is nowhere else to go.
Scout remembers one of Jem's "psychical" gems: if enough people were to concentrate on one thing, the power of positive thinking would surely cause that thing to occur.
I toyed with the idea of asking everyone below to concentrate on setting Tom Robinson free, but thought if they were as tired as I, it wouldn't work.
Then another thought crept upon her. It was like one she had had the previous winter, and she shivered even though the courtroom was hot. The stuffy courtroom reminded her of a cold February morning when all the mockingbirds were still.
A deserted waiting empty street, and the courtroom was packed with people. A steamy summer night was no different from a winter morning.
The above refers to the day Atticus killed the mad dog in the street. Things would be no different here. Scout has a premonition that this great change that they hope for--Tom's acquittal--will still happen. Things stay the same in Maycomb, be they cold mornings or hot summer nights. If Atticus can slay the rabid dog, then he can convince the racist jury.
But Scout then
... saw something only a lawyer's child could be expected to see... and it was like watching Atticus walk into the street, raise a rifle to his shoulder and pull the trigger, but watching all the time knowing that the gun was empty.
A jury never looks at a defendant it has convicted, and when this jury came in, not one of them looked at Tom Robinson.
Scout recognizes that Atticus does not have the ammunition needed to convince this jury, and she sees beforehand that Tom will be convicted. Convincing a white jury to put aside it racial biases is not as easy as killing a mad dog after all.