For four or five years after his return from Congress, Lincoln remained in Springfield, working industriously at his profession. He was offered a law partnership in Chicago, but declined on the ground that his health would not stand the confinement of a great city. His business increased in volume and importance as the months went by; and it was during this time that he engaged in what is perhaps the most dramatic as well as the best known of all his law cases--his defense of Jack Armstrong's son on a charge of murder. A knot of young men had quarreled one night on the outskirts of a camp-meeting, one was killed, and suspicion pointed strongly toward young Armstrong as the murderer. Lincoln, for old friendship's sake, offered to defend him--an offer most gratefully accepted by his family. The principal witness swore that he had seen young Armstrong strike the fatal blow--had seen him distinctly by the light of a bright moon. Lincoln made him repeat the statement until it seemed as if he were sealing the death-warrant of the prisoner. Then Lincoln began his address to the jury. He was not there as a hired attorney, he told them, but because of friendship. He told of his old relations with Jack Armstrong, of the kindness the prisoner's mother had shown him in New Salem, how he had himself rocked the prisoner to sleep when the latter was a little child. Then he reviewed the testimony, pointing out how completely everything depended on the statements of this one witness; and ended by proving beyond question that his testimony was false, since, according to the almanac, which he produced in court and showed to judge and jury, THERE WAS NO MOON IN THE SKY THAT NIGHT at the hour the murder was committed. The jury brought in a verdict of "Not guilty," and the prisoner was discharged. Lincoln was always strong with a jury. He knew how to handle men, and he had a direct way of going to the heart of things. He had, moreover, unusual powers of mental discipline. It was after his return from Congress, when he had long been acknowledged one of the foremost lawyers of the State, that he made up his mind he lacked the power of close and sustained reasoning, and set himself like a schoolboy to study works of logic and mathematics to remedy the defect. At this time he committed to memory six books of the propositions of Euclid; and, as always, he was an eager reader on many subjects, striving in this way to make up for the lack of education he had had as a boy. He was always interested in mechanical principles and their workings, and in May, 1849, patented a device for lifting vessels over shoals, which had evidently been dormant in his mind since the days of his early Mississippi River experiences. The little model of a boat, whittled out with his own hand, that he sent to the Patent Office when he filed his application, is still shown to visitors, though the invention itself failed to bring about any change in steamboat architecture.