A rusty nail placed near a faithful compass, will sway it from the truth, and wreck the argosy.

The image contrasts a “faithful compass” (a reliable moral or intellectual guide) with a “rusty nail,” a small, seemingly negligible influence that can nonetheless distort direction. The metaphor suggests how minor corruptions—petty self-interest, a bad adviser, a small lie, a hidden bias—can deflect judgment from “the truth” and lead to catastrophic consequences (“wreck the argosy,” i.e., a richly laden ship). Read this way, the line warns that integrity and clear perception are fragile: even trivial, unnoticed forces can bend them, so vigilance about small compromises matters as much as resisting obvious dangers.