1790s: African Americans in Pennsylvania number 10,274; the white population numbers 424,099.

1990s: African Americans make up 1,157,000 of Pennsylvania's total population of 11,881,643.

1790s: Slavery is practiced in many places around the world. The United States, Great Britain, the West Indies, South Africa, France, Denmark, Mexico, Holland, Puerto Rico, Cuba, Brazil, and Chile, as well as most parts of Central America and most Spanish colonies in South America make it legal to hold slaves.

1990s: Most nations throughout the world have abolished slavery, although it is still practiced in some parts of Africa, Asia, and South America. The Anti-Slavery Society for the Protection of Human Rights in London estimates that forms of servitude affect more than 200 million poor people.

1790s: Severe yellow fever epidemics occur in the West Indies, the United States, Spain, and parts of southern Europe, South America, and Central America.

1990s: Populations in areas...