MLA
Pierpont, Francis Harrison. Letter from Francis Harrison Pierpont (Wheeling, West Virginia) to Joseph M. Godwin and William Duff - September 19, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 20, Folder 11. https://digital.library.emory.edu/purl/474cnp5jw1-cor.
APA, 6th edition
Pierpont, F. H. (1861). Letter from Francis Harrison Pierpont (Wheeling, West Virginia) to Joseph M. Godwin and William Duff - September 19, 1861 (Box 20, Folder 11) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/474cnp5jw1-cor.
Chicago
Pierpont, Francis Harrison. “Letter from Francis Harrison Pierpont (Wheeling, West Virginia) to Joseph M. Godwin and William Duff - September 19, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 20, Folder 11. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/474cnp5jw1-cor.