MLA
Johnston, Joseph E. (Joseph Eggleston), 1807-1891. Letter from Joseph Eggleston Johnston (St. Augustine[?], Florida) to John Mackay (Fort Foster, near Tampa, Florida), during the Second Seminole War - April 28, 1837. April 28, 1837. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/433dz08msd-cor.
APA, 6th edition
Johnston, Joseph E. (Joseph Eggleston), 1807-1891. (April 28, 1837)[Letter from Joseph Eggleston Johnston (St. Augustine[?], Florida) to John Mackay (Fort Foster, near Tampa, Florida), during the Second Seminole War - April 28, 1837]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/433dz08msd-cor.
Chicago
Johnston, Joseph E. (Joseph Eggleston), 1807-1891, Letter from Joseph Eggleston Johnston (St. Augustine[?], Florida) to John Mackay (Fort Foster, near Tampa, Florida), during the Second Seminole War - April 28, 1837, April 28, 1837, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/433dz08msd-cor.