The manuscript, arranged in 13 letters, addresses various aspects of the slave trade in the region that lies between the Gambia and Senegal Rivers, the region that historically comprised the three "Kingdoms" of Cayor, Sin and Sallum, and bordered by the "Kingdoms" of the Wolof (Oualo) and Bambara. From this region, Clarkson estimated an annual trade of 2,240 slaves, of whom approximately 1,790 passed through the French Fort St. Louis and 450 through Gorée. Like Mungo Park, Clarkson found that the most common method employed to capture slaves is "pillage," or the organization of forces by the King of a region for secret raids on neighboring villages from which men and women are kidnapped.
Clarkson's letters include geographic and, to a degree, ethnographic notes on the region, plus detailed information on the means of acquisition, transport, and handling of enslaved individuals in Africa and on the Middle Passage. While Clarkson is strongly concerned with the moral issues raised by the slave-trade, the manuscript is designed partially to sway the opinion of politicians and often assumes an informational tone. He constructs his narrative so that the moral issues arise "naturally" from a consideration of the "facts" presented.
The manuscript contains nine illustrations, including a map of the region under study, several illustrations of implements used to restrain captives, two hand colored copper-plate engravings of African scenes, and a printed version of Clarkson's well-known diagrammatic cross section of a slave ship. There is at least one reference in the text to an illustration no longer present.
The association of this manuscript with Mirabeau is primarily circumstantial, and there are a number of differences between this version and the loose translation published in 1791. On the supporting side, however, a slip of paper in contemporary hand notes "title and table of contents in the hand of Mentelle," referring to Edmé Mentelle, close associate of Mirabeau. Secondly, one page of notes (p. 1) appears to indicate "cet oeuvrage appartient au Citoyen Mentelle," though Mentelle is strongly effaced, and makes reference to comments on the text by Geoffroy de Villeneuve. In the published English language version of his letters to Mirabeau, Clarkson cites Villeneuve, aide-de-camp to the Governor of Gorée, as his source of information for the African sections.