In addition to agriculture, slave labor was increasingly used in mining, shipbuilding and other industries. Soil exhausted by years of tobacco farming also pushed the plantation economy to expand westward. The failed slave uprisings of Gabriel Prosser in 1800 and Nat Turner in 1831 however marked the growing resistance to the system of slavery. One response to Nat Turner's rebellion by the Virginia government was to arrange for ships to transport free Blacks to Liberia. On October 16, 1859, abolitionist John Brown led a raid on an armory in Harpers Ferry in an attempt to start a slave revolt across the southern states. The polarized national response to his raid and execution marked a tipping point for many who believed the end of slavery would need to be achieved by force. By 1860, almost half a million people, roughly 31% of the total population of Virginia, were enslaved.