A metaphor makes a direct comparison between two unlike things, stating that one thing is another. Metaphors serve to make abstract concepts more tangible and accessible to readers, or to introduce multiple ideas simultaneously.

Turner consistently employs vivid metaphors throughout his essay comparing frontier expansion to natural forces like waves, tides, and vital energies: “the frontier is the outer edge of the wave” (3). This metaphor conveys the perceived inevitability of westward settlement through the relentless motion of waves. Waves also evoke a cyclic process, fitting Turner’s view of the frontier’s perpetual remaking of American society.

Turner further compares growing settlements to “germs” (3) using organic metaphors to characterize expansion as instinctive and beyond human control. By framing the frontier in terms of natural processes and organisms subject to biological urges, Turner lends a scientific aura to his thesis that settling the frontier was the prime agent shaping American character and democracy.

Rhetorical appeals to authority involve citing the opinions of experts and authoritative sources to strengthen one’s own credibility and arguments. Moreover, appeals to authority can serve to contextualize an argument within a broader scholarly or professional