In the late 1780s an abolitionist group in Plymouth sent the campaigner Thomas Clarkson a diagram of the Liverpool-registered slave ship, the Brookes. This was adapted by Clarkson and his colleagues to demonstrate how 482 captives were crammed on board the ship during the 6-8 week Middle Passage from Africa to the colonies. It was known that on at least one voyage the Brookes had carried 609 enslaved Africans.

The Brookes diagram became one of the first images to be widely distributed in political propaganda. It appeared on 700 abolitionist posters printed in 1789 and in other material produced to illustrate the inhumane conditions in which enslaved people were transported. Another abolitionist image was that of a kneeling slave over whom the words 'Am I not a man and a brother' were written. This was used on posters, banners, crockery and as a commemorative medallion produced by Josiah Wedgwood (see the Wedgwood Museum: Slave Medallion web page for details).

The following extract is taken from a pamphlet distributed by the Plymouth abolitionist group describing the Brookes' diagram or 'plate':

In the men's apartment, the space allowed to each is six feet in length, by sixteen inches in breadth. The boys are each allowed five feet by fourteen inches; the women five feet ten inches, by sixteen inches; and the girls four feet by twelve inches. The perpendicular height between the decks is five feet eight inches.

The men are fastened together, two and two, by handcuffs on their wrists, and by irons rivetted on their legs. They are brought up on the main deck every day, about eight o'clock: and, as each pair ascends, a strong chain, fastened by ring-bolts to the deck, is passed through their shackles; a precaution absolutely necessary to prevent insurrection. In this state, if the weather is favourable, they are permitted to remain about one third part of the twenty-four hours, and, during this interval, they are fed, and their apartment below is cleaned; but when the weather is bad, even these indulgences cannot be granted them, and they are only permitted to come up in small companies, of about ten at a time, to be fed, where, after remaining a quarter of an hour, each mess is obliged to give place to the next, in rotation.

It may perhaps be conceived, from the crowded state in which the slaves appear in this plate, that an unusual and exaggerated instance has been produced; this, however, is so far from being the case, that no ship, if her intended cargo can be procured, ever carries a less number than one to a ton, and the usual practice has been to carry nearly double that number. The bill which has passed this last session of parliament (1789), only restricts the carriage to five slaves to three tons: and the Brookes, of Liverpool, a capital ship, from which the above sketch was proportioned, did, in one voyage, actually carry six hundred and nine slaves, which is more than double the number that appear in the plate. The mode of stowing them was as follows: platforms, or wide shelves, were erected between the decks, extending so far from the sides towards the middle of the vessel, as to be capable of containing four additional rows of slaves, by which means, the perpendicular height above each tier, after allowing for the beams and platforms, was reduced to two feet six inches, so that they could not even sit in an erect posture; besides which, in the men's apartment, instead of four rows, five were stowed, by placing the head of one between the thighs of another. All the horrors of this situation are still multiplied in the smaller vessels. The Kitty, of one hundred and thirty-seven tons, had only one foot ten inches; and the Venus, of one hundred and forty-six tons, only one foot nine inches perpendicular height, above each layer.

The above mode of carrying the slaves, however, is only one, among a thousand other miseries which those unhappy and devoted creatures suffer, from this disgraceful traffic of the human species, which, in every part of its progress, exhibits scenes that strike us with horror and indignation. If we regard the first stage of it, on the continent of Africa, we find, that a hundred thousand slaves are annually produced there for exportation, the greatest part of whom consist of innocent persons, torn from their dearest friends and connexions, sometimes by force, and sometimes by treachery. Of these, experience has shewn, that forty-five thousand perish, either in the dreadful mode of conveyance before described, or within two years after their arrival at the plantations, before they are seasoned to the climate. Those who unhappily survive these hardships, are destined, like beasts of burden, to exhaust their lives in the unremitting labours of slavery, without recompense, and without hope.

It is said by the well-wishers to this trade, that the suppression of it will destroy a great nursery for seamen, and annihilate a very considerable source of commercial profit. In answer to these objections, Mr Clarkson, in his admirable treatise on the impolicy of the trade, lays down two positions, which he has proved from the most incontestible authority - First, that so far from being a nursery, it has been constantly and regularly a grave for our seamen; for, that in this traffic only, more men perish in one year, than in all the other trades of Great-Britain in two years:

And, Secondly, that the balance of the trade, from its extreme precariousness and uncertainty, is so notoriously against the merchants, that if all the vessels employed in it, were the property of one man, he would infallibly, at the end of their voyages, find himself a loser.