He knows enough, the mariner, who knows Where lurk the shelves, and where the whirlpools boil, What signs portend the storm: to subtler minds He leaves to scan, from what mysterious cause Charybdis rages in the Ionian wave; Whence those impetuous currents in the main Which neither oar nor sail can stem; and why The roughening deep expects the storm, as sure As red Orion mounts the shrouded heaven.