MacMillan Encyclopedia of World Slavery: Volume II Paul Finkelman & Joseph C. Miller eds. (Macmillan Reference USA - Simon and Schuster Macmillan New York, 1998), p. xiii.
In 1800 the population of the United States included 893,602 slaves, of which only 36,505 were in the northern states. Vermont, Pennsylvania, Massachusetts, Rhode Island, Connecticut, New York, and New Jersey provided for the emancipation of their slaves before 1804, most of them by gradual measures.
The 3,953,760 slaves at the census of 1860 were in the southern states.