For the people forced aboard, the ocean crossing was the most notorious part of the ordeal. It lasted from a few weeks to three months, and it was designed around one goal: delivering as many living bodies as possible at the lowest cost.
Triangles and shuttles
Textbooks describe a triangle: ships left Europe with trade goods, sailed to Africa to buy captives, crossed to the Americas to sell them, and returned with sugar, tobacco or cotton. The crossing was called the Middle Passage because it was the middle leg of that three-part route. The triangle fits the British and French trades fairly well, but not the largest one. Brazil's trade with Angola ran directly back and forth between the two, with Brazilian tobacco and cane brandy going east and captives coming west. Ships from Rhode Island carried rum to Africa. The trade was a web, not a single shape.
Aboard the ship
Men were usually chained in pairs, ankle to ankle, and packed below deck on shelves with less headroom than a person needed to sit up. Women and children were often kept apart and less tightly confined, which left them more exposed to sexual assault by the crew. In good weather captives were brought up on deck, fed, and forced to "dance" for exercise. Those who refused food could be beaten or force-fed. Ships stretched netting along their sides to stop people from leaping into the sea, a choice that many made rather than face slavery.
Death at sea
Overcrowding, heat, filth and poor water bred disease. The most common killer aboard was dysentery, which sailors called the flux. Smallpox, measles and scurvy also struck. On average, between 12 and 15 percent of captives died during the crossing, with higher rates in the early centuries and on long or unlucky voyages. Across the whole trade, that means well over a million deaths at sea, on top of those who died in Africa before boarding and in the first months after landing. The crews suffered too, as later abolitionists would point out.
Resistance
Captives were not passive cargo. They refused food, jumped overboard and planned uprisings, often in the days when the coast was still in sight. Historians estimate that revolts broke out on roughly one voyage in ten. Most were crushed with great violence, but the threat shaped the whole business: ships carried extra crew, guns and chains, which raised costs and probably reduced the number of people carried. One famous case came late. In 1839 captives aboard the Spanish schooner Amistad seized the ship near Cuba; after it was taken off the American coast, the United States Supreme Court ruled in 1841 that they had been illegally enslaved, and the survivors returned to Sierra Leone.
The Zong
In late 1781 the British ship Zong, overcrowded and short of water after navigation errors, was approaching Jamaica. Its crew threw more than 130 captives into the sea, intending to claim their value from the insurers. No one was charged with murder. The case reached court in 1783 because the owners sued their insurers for the value of the dead. The judge, Lord Mansfield, ordered a new trial on the insurance question, observing that the law treated the case as if horses had been thrown overboard. Olaudah Equiano brought the news to the campaigner Granville Sharp, who tried in vain to bring a prosecution and spread the story.
Seeing and counting
In 1788 abolitionists in Plymouth published a plan of the Liverpool ship Brookes. The Brookes diagram showed how 454 people could be packed aboard under the law of 1788, which limited numbers by tonnage; the ship had carried far more on earlier voyages. Rows of tiny black figures, laid side by side like spoons, became one of the most reproduced images in history.
Today historians can count with unusual precision. The Trans-Atlantic Slave Trade Database, built over decades by scholars such as David Eltis and David Richardson, records more than 36,000 voyages, with ships' names, ports, dates and numbers carried and landed. It shows, for example, that by the 1790s most British slaving voyages sailed from Liverpool.
Worked example: reading the Brookes diagram
Treat the Brookes plan as a historian treats any source. Who made it, and why? A committee of campaigners, to shock the public and influence Parliament. What does it get right? The cramped spaces, the measurements and the sheer number of bodies are drawn from real dimensions. What does it leave out? The people are identical, silent and still. There are no faces, no families, no different languages, no illness, no revolt. The image makes the victims look like cargo, which is exactly what the trade did to them.
So the diagram is both powerful evidence and a limited one. Set beside the voyage database, which counts, and beside survivors' accounts such as Equiano's, which describe fear, smells and grief, it becomes part of a fuller picture. Asking what a source can and cannot show is the first step to understanding the Middle Passage, and it applies just as much to the numbers in the database as to the drawing.