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.