Lincoln's practical sense and his understanding of human nature enabled him to save the life of the son of his old Clary's Grove friend, Jack Armstrong, who was on trial for murder.  Lincoln, learning of it, went to the old mother who had been kind to him in the days of his boyhood poverty, and promised her that he would get her boy free.
 
The witnesses were sure that Armstrong was guilty, and one of them declared that he had seen the fatal blow struck.  It was late at night, he said, and the light of the full moon had made it possible for him to see the crime committed.  Lincoln, on cross-examination, asked him only questions enough to make the jury see that it was the full moon that made it possible for the witness to see what occurred; got him to say two or three times that he was sure of it, and seemed to give up any further effort to save the boy.
 
But when the evidence was finished, and Lincoln's time came to make his argument, he called for an almanac, which the clerk of the court had ready for him, and handed it to the jury.  They saw at once that on the night of the murder there was no moon at all.  They were satisfied that the witness had told what was not true.  Lincoln's case was won.