Captain Luke Collingwood was used to grim voyages across the Atlantic, but this one had been worse than most. Dysentery, diarrhoea and smallpox had already claimed the lives of seven of the crew aboard the slave ship Zong. The slave cargo had suffered a far higher mortality rate. More than sixty had died since leaving the shores of Africa.

As Captain Collingwood searched in vain for the coast of Jamaica, he grew increasingly alarmed. He knew that the ship’s insurers would not cover the cost of his lost human cargo. Since each slave was worth about £30, he stood to lose a fortune.

On 29 November 1781, he was struck by a macabre idea, one that could turn loss into profit. At a meeting with the Zong’s officers, he suggested that they throw the slaves overboard. There was a sinister logic to his reasoning. If his slaves died of illness, their insurance value was lost. But if they were thrown overboard in order to preserve the ship’s scant supply of water (and thereby save the lives of others), an insurance claim would be valid under a legal principle known as the ‘general average’. It allowed the captain of a ship to sacrifice some of his ‘passengers’ in order to save others.

The First Mate of the slave ship Zong, James Kelsall, was appalled by the captain’s proposal. He said it was cold-blooded murder. But Collingwood disagreed, insisting that it would be ‘less cruel to throw the sick wretches into the sea than to suffer them to linger out a few days, under the disorder with which they were afflicted’.

After much persuasion, Kelsall changed his mind and reluctantly agreed with the captain and other officers. The weakest slaves were to be pitched overboard that very day.

Collingwood went below decks to select his first ‘parcel’ of victims. He decided to concentrate on the women and children, probably because he knew that they would put up less of a struggle. A total of 54 were hurled off the ship and could be seen flailing in the sea before eventually weakening and drowning.

Two days later, on 1 December, Collingwood elected to throw out another ‘parcel’: this time, his forty-two victims were all men. They drowned so quickly, and with such little effort on the part of the captain and his crew, that Collingwood decided to pitch even more slaves overboard. He was intent on getting the largest possible sum of money from the ship’s insurers. He ordered another thirty-six to be thrown into the ocean.

Chief Justice Mansfield

But this third batch of victims were made of stronger stuff and vehemently refused to go to their deaths without a struggle. Collingwood’s men were forced to chain them by the ankle and weigh their feet with balls of iron so they would sink immediately.

‘The arms of twenty-six were fettered with irons and the savage crew proceeded with the diabolical work, casting them down to join their comrades of the former days.’ So reads a contemporary account of the massacre.

Ten of the slaves were so terrified by their fate that they leaped overboard before the captain had the chance to have them chained.

Three weeks after the last murders, the slave ship Zong finally reached Jamaica with 208 slaves still aboard. They sold for an average price of £36 each, earning Collingwood a substantial profit even before he made his insurance claim. But he did not have long to enjoy his money: he died within three days of making landfall.

His death might have been the end of a sordid and macabre tale, but there was to be an extraordinary postscript, one that caused a sensation in Georgian England.

Olaudah Equiano brought news of the killings to Granville Sharp

The ship’s owners expressed their full support for what the late Captain Collingwood had done and filed an insurance claim for the 132 slaves that had been thrown overboard. They hoped to recuperate nearly £4,000 in jettisoned ‘cargo’.

Thus began a court case that was marked by callousness, cynicism and sheer human greed. The jury were in agreement with the owners and insisted that the insurers pay up the money for the drowned slaves. But the insurers appealed against the decision and asked for the case to be retried. This time, it was to be heard before the Lord Chief Justice, Lord Mansfield.

Those who hoped for a more enlightened approach from Lord Mansfield were quickly disappointed. ‘The case of slaves,’ he said, ‘was the same as if horses had been thrown overboard. The question was, whether there was not an absolute necessity for throwing them over board to save the rest.’

Granville Sharp

A hitherto unknown fact was now brought before the court. The ship’s owners had argued that the slaves were killed because there was not enough water on board. But this was not true. When the ship arrived at Jamaica, it still had more than 420 imperial gallons of stored water.

This ought to have proved the turning point: Collingwood and his crew were clearly guilty of cold-blooded murder. Yet the new evidence was deemed to be of no consequence and the Zong’s owners ultimately won the day. The insurers were forced to pay up for the ‘cargo’ that had been dumped at sea.

The English abolitionist Granville Sharp was appalled by the verdict and tried to bring forward a case of murder. This was brushed aside by Lord Mansfield.

‘What is this claim that human people have been thrown overboard?’ he said. ‘This is a case of chattels or goods. Blacks are goods and property; it is madness to accuse these well-serving honourable men of murder.’

This excerpt is from Fascinating Footnotes From History by Giles Milton, published by John Murray.