March 18, 1861
According to the 1860 census tables found on S. Augustus, Mitchell's 1861 Map of the United States... the population of the United States was 31,429,891 million, an increase of 8,239,016 as recorded in the 1850 census. Of those 31 million, as also reported on the tables accompanying the map, 3,952,838 were slaves. The map also provides statistics on the free and slave populations in each state as recorded in the 1850 and 1860 census.
Also published in 1861, Edwin Hergesheimer's landmark map entitled Map showing the distribution of the slave population of the southern states of the United States... also provides statistics on the slavery in the southern portion of the country. The differences between the two maps, both published in 1861 and both based on 1860 census data, are the methods in which the data is portrayed. Hergesheimer's map breaks down the data into the percentage of slaves per county as opposed to the total county population. The choice of shading from lower percentages per county in light grey to higher percentages illustrated in darker tones, including a contrasting type color, provides a dramatic representation of slavery in the southern states.
Mitchell, S. Augustus (Samuel Augustus). Map of the United States, and territories. Philadelphia, Pa., Together with Canada &c. Mitchell's map publication office, 1861. 1 map, col., 32 x 53 cm., on sheet 61 x 65 cm.
Catalog record: http://lccn.loc.gov/99447041
Hergesheimer, E. (Edwin). Map showing the distribution of the slave population of the southern states of the United States. Compiled from the census of 1860 Drawn by E. Hergesheimer. Engr. by Th. Leonhardt. Washington Henry S. Graham 1861. 1 map 69 x 86 cm.
Catalog record: http://lccn.loc.gov/99447026
Schulten, Susan. The Cartography of Slavery and the Authority of Statistics. Civil War History, Volume 56, Number 1, March 2010, pp. 5-32.
Catalog record: http://lccn.loc.gov/57002353