Complex sentence: Mansfield was previously the judge in Somersett's Case in 1772, which concerned the legality of keeping slaves in England, and he had ruled that slavery had never been established by statute in England and was not supported by common law.
Split into simpler sentences: Mansfield was previously the judge in Somersett's Case in 1772, which concerned the legality of keeping slaves in England. He had ruled that slavery had never been established by statute in England and was not supported by common law.