west in search of suitable land. In addition, the invention of the cotton gin in 1793 enabled profitable processing of short-staple cotton, which could readily be grown in the uplands. The invention revolutionized the cotton industry by increasing fifty-fold the quantity of cotton that could be processed in a day. At the end of the War of 1812, fewer than 300,000 bales of cotton were produced nationally. By 1820, the amount of cotton produced had increased to 600,000 bales, and by 1850 it had reached 4,000,000. There was an explosive growth of cotton cultivation throughout the Deep South and greatly increased demand for slave labor to support it. As a result, manumissions decreased dramatically in the South. Most of the slaves sold from the Upper South were from Maryland, Virginia and the Carolinas, where changes in agriculture decreased the need for their labor and the demand for slaves. Before 1810, primary destinations for the slaves who were sold were Kentucky and Tennessee, but, after 1810, the Deep South states of Georgia, Alabama, Mississippi, Louisiana and Texas received the most slaves. This is where cotton became "king." Meanwhile, the Upper South states of Kentucky and Tennessee joined the slave-exporting states. By 1815, the domestic slave trade had become a major economic activity in the United States; it lasted until the 1860s. Between 1830 and 1840, nearly 250,000 slaves were taken across state lines. In the 1850s, more than 193,000 enslaved persons were transported, and historians estimate nearly one million in total took part in the forced migration of this new "Middle Passage." By 1860, the slave population in the United States had reached four million. Of the 1,515,605 free families in the fifteen slave states in 1860, nearly 400,000 held slaves (roughly one in four, or 25%), amounting to 8% of all American families. The historian Ira Berlin called this forced migration of slaves the "Second Middle Passage" because it reproduced many of the same horrors as the Middle Passage (the name given to the transportation of slaves from Africa to North America). These sales of slaves broke up many families and caused much hardship. Characterizing it as the "central event" in the life of a slave between the American Revolution and the Civil War, Berlin wrote that, whether slaves were directly uprooted or lived in fear that they or their families would be involuntarily moved, "the massive deportation traumatized black