The fashionable Mulligan wears a "Panama hat" as he walks from the tower to the swimming hole, its "brims . . . quivering" in the shore breeze. Shortly after, the narrative makes it a "Mercury's hat quivering in the fresh wind," the moving brims having suggested a connection with the ancient messenger god.
Panama hats are actually made in Ecuador, woven from a ten-foot-tall, reed-like palm plant that grows in the hills there. People passing through the isthmus to the California gold rush in the nineteenth century, and workers constructing the canal in the early twentieth, encountered these stylish hats in Panama and made them famous throughout the world. The hat became iconic from the 1920s through the 1940s, on heads like those of Al Capone, Franklin Delano Roosevelt, and Humphrey Bogart.