Which half-niece of Newton married John Conduitt?

Catherine Barton, Newton’s half-niece, married John Conduitt. She was the daughter of Robert Barton and Hannah Smith, Newton’s maternal half-sister, making Barton Newton’s half-niece through his mother’s family.

Barton moved to London and helped run Newton’s household after he became Warden of the Mint. She was known in fashionable and intellectual circles for her wit and conversation, and she became associated with figures such as Jonathan Swift and Voltaire. She was also reputed to have conveyed the famous apple story about Newton to Voltaire, although later retellings simplified the episode.

Barton and Conduitt became engaged in July 1717 and married in August of that year. They had one daughter, also named Catherine. Conduitt later became Master of the Mint after Newton’s death, while Newton spent part of his final years living with the couple.

The family relationship is easy to muddle: Barton was Newton’s niece, but not his sister’s daughter in the usual full-sibling sense. Her mother, Hannah Smith, was Newton’s half-sister, which is why “half-niece” is the precise description.