James Boswell's uncle John was a member of a sect known as the Glasites.
A catholic actress. She performed from ca. 1749, and in the 1750s played at Bath, Richmond and Drury Lane. In 1759 she was one of the main attractions at the theatre in Edinburgh. Little else is known about her. From a letter from Temple to Boswell (ca. January 1760) it can be gathered that she had performed in Newcastle in the fall of 1759, and that she was (had been?) married to her music master.
Life with Boswell:
It is known that Boswell at some point during 1759 had an affair with Mrs. Cowper, who must have been about 10 years older than him. Boswell referred to her as Sylvia, which it is suggested in Correspondence of James Boswell and John Johnston (p. 5-6) merely refers to her role as Sylvia in a performance of Farquhar's The Recruiting Officer. Among other small things, Boswell's regard for Mrs. Cowper is taken as evidence that he authored A View of the Edinburgh Theatre in 1759.