How does Patrick O'Brian use imagery in The Yellow Admiral?

O'Brian uses vivid and detailed imagery in Mr. Whewell's descriptions of the horrific conditions aboard slave ships. He describes their cramped spaces, rampant disease, and high mortality rate, which acts as a shocking invective against the practice. These images are reinforced firsthand when Jack's squadron captures a slave ship, the Nancy, and Jack and Stephen see for themselves the state of the vessel.