Sentence:  [T]he ships departed with equipment to keep the captives in check. Indeed it is that equipment – notably the chains and <WSD>manacles</WSD> – which to this day symbolizes the slave ships themselves. The repressive machinery of the slave ship – cannons, swivel guns, hand guns, cutlasses, chains, <WSD>manacles</WSD>, thumbscrews – speaks not only of the fear of the ship's owners and crew but points us directly to the essential reality of the slave ships. 
Meaning of the word marked with <WSD> tags:  A shackle for the wrist, usually consisting of a pair of joined rings; a handcuff;  a similar device put around an ankle to restrict free movement.