He gained a world; he gave that world
Its grandest lesson: “On! sail on!”

The couplet frames achievement as both conquest and bequest: the subject “gained a world” (won fame, territory, or mastery) but his greater legacy is moral and inspirational—teaching perseverance. The imperative “On! sail on!” evokes seafaring as a metaphor for human striving: keep moving forward despite storms, uncertainty, or exhaustion. The line suggests that the highest form of success is not possession but example—leaving behind a lesson that enlarges others’ courage. In Miller’s romantic, frontier-inflected idiom, progress is cast as motion, risk, and refusal to turn back.