Thanks to Congress passing the "Act for the Relief and Protection of American Seamen" in 1796, historians know what American tattoos from the 19th century looked like. The act required sailor tattoos to be recorded for posterity, sort of like a social security number, so the military would know if the British tried to force an colonial seamen to fight for the wrong side.
Tattooing became far more common in the back half of the 19th century across the globe, especially after the invention of the electric tattooing machine in 1891. Photography became more common around this time as well, meaning there are quite a few photographs of old, old tattoos from this period, as well as a few disturbing skin samples. The gallery below also features sketches of historical tattoos, sometimes recorded by criminologists who naively thought tattoo placement and design had some kind of link with the criminal mind.