the African coast in 1820 and 1821. A total of 11 American slave ships were taken by the U.S. Navy over this period. Then American enforcement activity reduced. There was still no agreement between the United States and Britain on a mutual right to board suspected slave traders sailing under each other's flag. Attempts to reach such an agreement stalled in 1821 and 1824 in the United States Senate. A U.S. Navy presence, however sporadic, did result in American slavers sailing under the Spanish flag, but still as an extensive trade. The Webster-Ashburton Treaty of 1842 set a guaranteed minimum level of patrol activity by the U.S. Navy and the Royal Navy, and formalized the level of co-operation that had existed in 1820. Its effects, however, were minimal while opportunities for greater co-operation were not taken. The U.S. transatlantic slave trade was not effectively suppressed until 1861, during Lincoln's presidency, when a treaty with Britain was signed whose provisions included allowing the Royal Navy to board, search and arrest slavers operating under the American flag. Post-revolution Southern manumissions Although Virginia, Maryland and Delaware were slave states, the latter two already had a high proportion of free blacks by the outbreak of war. Following the Revolution, the three legislatures made manumission easier, allowed by deed or will. Quaker and Methodist ministers particularly urged slaveholders to free their slaves. The number and proportion of freed slaves in these states rose dramatically until 1810. More than half of the number of free blacks in the United States were concentrated in the Upper South. The proportion of free blacks among the black population in the Upper South rose from less than 1% in 1792 to more than 10% by 1810. In Delaware, nearly 75% of blacks were free by 1810. In the United States as a whole, the number of free blacks reached 186,446, or 13.5% of all blacks, by 1810. After that period, few slaves were freed, as the development of cotton plantations featuring short-staple cotton in the Deep South drove up the internal demand for slaves in the domestic slave trade and high prices being paid for them. South Carolina made manumission more difficult, requiring legislative approval of every instance of manumission. Several Southern states required manumitted slaves to leave the state within thirty days. Domestic slave trade and forced migration The growing international demand for cotton led many plantation owners further