MLA
Huntingdon, Selina Hastings, Countess of, 1707-1791. Letter from Selina Hastings Huntingdon to [Wilks Student], January 5, 1775. January 5, 1775. Selina Hastings Letters, 1774-1791, undated. Pitts Theology Library. https://digital.library.emory.edu/purl/3655dv424k-cor.
APA, 6th edition
Huntingdon, Selina Hastings, Countess of, 1707-1791. (January 5, 1775)[Letter from Selina Hastings Huntingdon to [Wilks Student], January 5, 1775]. Selina Hastings Letters, 1774-1791, undated, Pitts Theology Library. https://digital.library.emory.edu/purl/3655dv424k-cor.
Chicago
Huntingdon, Selina Hastings, Countess of, 1707-1791, Letter from Selina Hastings Huntingdon to [Wilks Student], January 5, 1775, January 5, 1775, Selina Hastings Letters, 1774-1791, undated, Pitts Theology Library. https://digital.library.emory.edu/purl/3655dv424k-cor.