Thus Judge Nelson built up a sound knowledge of the principles and practice of every branch of law which he was called upon to administer. An appeal or writ of error from his decisions was seldom taken. So familiar did he become with the jurisprudence involved in the administration of the Patent laws of this country, so thoroughly did he investigate questions of science and mechanics, and so sound a judgment was he known to form on these subjects, that his opinions concerning them were by courts and counsel accepted as of greater authority than those of any other judge. For many years before the close of his labors at the Circuit, patentees felt that when he had judicially passed upon their rights they were substantially settled, and hence there came before him repeatedly from distant points cases involving the validity of the most valuable patents in the country, and to his decision the parties generally submitted without appeal. On questions of admiralty and maritime law also he came to be considered a great authority. In his later years he was so adept in reaching the essential points of complicated cases that he was generally credited with a marvellous faculty of intuition. He was not guided by any intuition, however, but by the results of his careful study and legal experience.
In 1857 the Supreme Court of the United States rendered the famous Dred Scott decision, which became one of the contributory causes of the Civil War. Only two members of the court dissented. Justice Nelson concurred in the conclusion of Chief Justice Taney, who delivered the decision, dissenting on one point only, and adding that, in his opinion, the power of Congress could not be one-sided; if it existed to destroy slavery, it could also establish slavery.
Judge Nelson had gained some acquaintance with slavery in his own home town, for, when first he took up his residence in Cooperstown, in 1824, there were a number of slaves in the village. Some of the earliest settlers had negroes in bondage. Among these was James Averell, Jr., who worked his tannery by slave labor. One of his slaves, known as Tom Bronk, was for many years well known in Cooperstown as the servant of the former owner's son, William Holt Averell, and lived to a great age. The clumsily written bill of sale by which Tom Bronk became the property of James Averell, Jr., is still in existence: