William MaxwellBorn 1728
4th of Dalswinton. Son of Hugh Maxwell, 3rd of Dalswinton and Jane (or Jean) Douglas, a daughter of Helen Erskine, Boswells mothers half-sister, and William Douglas, 2nd of Kelhead. Married to Mary Boscawen, a daughter of the Viscount Falmouth. Brother of George Maxwell. Due to financial troubles he had to sell Dalswinton in 1786.
Maxwell eventually achieved the rank of Major in the army, so he might have been a Captain in 1763, although Boswell's use of the title "Captain" is not always to be taken quite literally.