MLA
Brown, Joseph E. (Joseph Emerson), 1821-1894. Open letter from Joseph E. Brown, Governor of Georgia, to to A.H. Colquitt and others, The Federal Union (Milledgeville Weekly). 1860. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3418kprsbb-cor.
APA, 6th edition
Brown, Joseph E. (Joseph Emerson), 1821-1894. (1860)[Open letter from Joseph E. Brown, Governor of Georgia, to to A.H. Colquitt and others, The Federal Union (Milledgeville Weekly)]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3418kprsbb-cor.
Chicago
Brown, Joseph E. (Joseph Emerson), 1821-1894, Open letter from Joseph E. Brown, Governor of Georgia, to to A.H. Colquitt and others, The Federal Union (Milledgeville Weekly), 1860, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/3418kprsbb-cor.