The American Revolution and the cause of liberty added impetus to the abolitionist cause. Even in Southern states, laws were changed to limit slavery and facilitate manumission. The amount of indentured servitude dropped dramatically throughout the country. An Act Prohibiting Importation of Slaves sailed through Congress with little opposition. President Thomas Jefferson supported it, and it went into effect on January 1, 1808, the first day the Constitution permitted Congress to prohibit importation of slaves. Benjamin Franklin and James Madison helped found manumission societies. Influenced by the Revolution, many slave owners freed their slaves, but some, such as George Washington, did so only in their wills. The number of free black people as a proportion of the black population in the upper South increased from less than 1 percent to nearly 10 percent between 1790-1810.