John Leyburn (1615 – 20 June, 1702) was an English Roman Catholic bishop who served as the Vicar Apostolic of England from 1685 to 1688 and then when it was divided served as the Vicar Apostolic of the London District from 1688 to 1702. He was not only a theologian, but also a mathematician, and an intimate friend of Descartes and Hobbes. Life He was the fourth son of John Leyburn and Catharine Carr, nephew of George Leyburn, and descended from Westmorland MP Sir James Leyburn. He was educated at the English College, Douai, where he was admitted a student on 20 June 1633. He received holy orders in 1646, and was engaged for some time in teaching the classics in the college. Richard Smith had served as chaplain to Viscountess Montague, wife of Anthony-Maria Browne, 2nd Viscount Montagu, at Battle Abbey in Sussex, from 1603 to 1609, when he left to go to Paris to study and write at Arras College. Smith was appointed Apostolic Vicar for the whole of England, Wales and Scotland in 1625, the same year that George Leyburn went to Arras. When Smith returned to England, he stayed in Turvey, Bedfordshire, at the house of Lord Montagu. In 1628 a warrant was issued for his arrest. He resigned his post in 1631, when he fled to Paris. After his ordination, John Leyburn was, for a time, secretary to Bishop Smith. During the time of the English Civil War Leyburn was tutor to Francis Browne, eldest son of Viscount Montague, and made the Grand Tour with his pupil, who later became the 4th Viscount Montagu. For about twelve years he resided in England as domestic chaplain in the family of Lord Montague, whose family home was at Cowdray House. (During the Civil War two thirds of the Cowdray estate was sequestered from the 3rd Viscount Montagu, and the house was garrisoned by Parliamentary forces.) Bishop Smith had resigned as Apostolic Vicar when he fled to France in 1631. The position remained vacant for some time, during which the church was governed by the Old Chapter, a body of archdeacons and deans created by Smith's predecessor, William Bishop, as a standing council for his own assistance, with power, during the vacancy of the see, to exercise episcopal ordinary jurisdiction. The Chapter was an anomaly in canon law; and Rome neither recognized nor censured it. The Chapter understood