Complex sentence: He witnesses the whipping of a Moriori slave, Autua, who later stows away on Ewing's ship, and convinces Ewing to advocate for him to join the crew as a freeman.
Split into simpler sentences: He witnesses the whipping of a Moriori slave, Autua, who later stows away on Ewing's ship. Ewing advocates for Autua to join the crew as a freeman.