Entitled: "Stowage of the British slave ship Brookes under the regulated slave trade act of 1788." Slave ships were large cargo ships specially converted for the purpose of transporting slaves, especially newly purchased African slaves to the Americas. In order to achieve profit, the owners of the ships divided their hulls into holds with little headroom, so they could transport as many slaves as possible. Unhygienic conditions, dehydration, dysentery and scurvy led to a high mortality rate, on average 15% and up to a third of captives. Slavery in the US was the legal institution of chattel slavery that existed in the USA in the 18th and 19th centuries. The US was polarized by slavery into slave and free states along the Mason-Dixon Line, which separated Maryland (slave) and Pennsylvania (free). Although the international slave trade was prohibited from 1808, internal slave-trading continued at a rapid pace, causing the forced migration of more than one million slaves from the Upper South to the Deep South in the antebellum years. The total slave population in the South eventually reached four million before abolition.