Harper's had many southern subscribers and, in the years leading up to the Civil War, it "took a decidedly neutral stance toward the issue of slavery, prompting some critics to label it Harper's Weakly. However, when the war began in 1861, the newspaper became an avid supporter of the Union war program. Indeed, the paper's news coverage and illustrations served as a powerful arm of the admittedly unorganized Union propaganda effort." (Pierpaoli 2013, 867).
This is a "choropleth" map, one marking geographic areas with different patterns or shading representing differing statistical values. The text on the verso says that "The tint, by its depth of shade, shows the comparative percentage of slaves to the total population in each county. . . . This map will be of use to the philosopher and student." Indeed, the principal impact of the map is to demonstrate the great extent of slavery in Georgia. "Machiavellian bias can easily manipulate the message of a choropleth map." (Monmonier 1989, 42). The use of relatively darker tones or denser patterns for relatively lower percentages of slaves would increase the impression of extensive slavery, and conversely.
For other maps in the collection using similar techniques, Search > "choropleth."