In 1619, another race was brought here against their will and sold into slavery. This was the Negro, forced to leave his home near the African equator that he might contribute to the material wealth of shipmasters and planters. Slowly but surely chattel slavery took firm root in the South and at last became the leading source of the labor supply. The slave traders found it very easy to seize Negroes in Africa and make great profits by selling them in Southern ports. The English Royal African Company sent to America annually between 1713 and 1743 from 5,000 to 10,000 slaves.[16] After a time, when the Negroes were so numerous that whole sections were overrun, the Southern colonies tried ineffectually to curb the trade. Virginia in 1710 placed a duty of five pounds on each slave but the Royal Governor vetoed the bill. Bills of like import were passed in other colonies from time to time, but the English crown disapproved in every instance and the trade, so lucrative to British shipowners, went on. At the time of the Revolution, there were almost half a million slaves in the colonies.[17] The exact proportions of the slave trade to America can be but approximately determined. From 1680 to 1688 the African Company sent 249 ships to Africa, shipped there 60,783 Negro slaves, and after losing 14,387 on the middle passage, delivered 46,396 in America. The trade increased early in the eighteenth century, 104 ships clearing for Africa in 1701; it then dwindled until the signing of the Assiento, standing at 74 clearances in 1724. The final dissolution of the monopoly in 1750 led—excepting in the years 1754-57, when the closing of Spanish marts sensibly affected the trade—to an extraordinary development, 192 clearances being made in 1771. The Revolutionary War nearly stopped the traffic, but by 1786 the clearances had risen again to 146. To these figures must be added the unregistered trade of Americans and foreigners. It is probable that about 25,000 slaves were brought to America each year between 1698 and 1707. The importation then dwindled but after the Assiento rose to perhaps 30,000. The proportion of these slaves carried to the continent now began to increase. Of about 20,000 whom the English annually imported from 1733 to 1766, South Carolina alone received some 3,000. Before the Revolution the total exportation to America is variously estimated as between 40,000 and 100,000 each year. Bancroft places the total slave population of the continental colonies at 59,000 in 1714; 78,000 in 1727; and 293,000 in 1754. The census of 1790 showed 697,897 slaves in the United States. Not all the Negroes who came to America were slaves and not all remained slaves. There were the following free Negroes in the decades between 1790 and 1860: 1790 59,557 1800 108,435 1810 186,446 1820 233,634 1830 319,599 1840 386,293 1850 434,495 1860 488,070