MLA
Mackall, William Whann, 1818-1891. Letter from Brigadier General MacKall (Richmond, Virginia) to "Dear General" - August 8, 1862. August 8, 1862. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/7032280hc3-cor.
APA, 6th edition
Mackall, William Whann, 1818-1891. (August 8, 1862)[Letter from Brigadier General MacKall (Richmond, Virginia) to "Dear General" - August 8, 1862]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/7032280hc3-cor.
Chicago
Mackall, William Whann, 1818-1891, Letter from Brigadier General MacKall (Richmond, Virginia) to "Dear General" - August 8, 1862, August 8, 1862, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/7032280hc3-cor.