MLA
Huntingdon, Selina Hastings, Countess of, 1707-1791. Letter from Selina Hastings Huntingdon to Sarah (Gwynne) Wesley, Spa-Fields, March 2, 1791. March 2, 1791. Selina Hastings Letters, 1774-1791, undated. Pitts Theology Library. https://digital.library.emory.edu/purl/238w9ghxm6-cor.
APA, 6th edition
Huntingdon, Selina Hastings, Countess of, 1707-1791. (March 2, 1791)[Letter from Selina Hastings Huntingdon to Sarah (Gwynne) Wesley, Spa-Fields, March 2, 1791]. Selina Hastings Letters, 1774-1791, undated, Pitts Theology Library. https://digital.library.emory.edu/purl/238w9ghxm6-cor.
Chicago
Huntingdon, Selina Hastings, Countess of, 1707-1791, Letter from Selina Hastings Huntingdon to Sarah (Gwynne) Wesley, Spa-Fields, March 2, 1791, March 2, 1791, Selina Hastings Letters, 1774-1791, undated, Pitts Theology Library. https://digital.library.emory.edu/purl/238w9ghxm6-cor.