Once upon a time, people made their way across the sea by reading the surface, shapes, and colors of the water. On clear nights, they took their directions from the stars; by day, they sailed by the wind and waves. In the Homeric world there were four reigning winds: Boreas blew from the north, Notus from the south, Eurus from the east, Zephyrus from the west.

Wind made itself most useful for navigational purposes by generating swells. Whatever the fickle gusts of the moment, the prevailing seasonal wind was registered in the stubborn movement of the sea. Swell continues for many days, and sometimes thousands of miles, after the wind that first raised it has blown itself out. Islands, because they deflect the direction of swell, can be "felt" from a great distance by a sensitive pilot. As the depth of the sea decreases, the swell steepens, warning of imminent landfall.

Sailing by swell entailed an intense concentration on the character of the sea itself. Wave shape was everything. A single wave is likely to be molded by several forces: the local wind; a dominant, underlying swell; and, often, a weaker swell coming from a third direction. Early navigators had to be in communion with every lift of the bow as the sea swept under the hull in order to sense each component in the wave and deduce from them the existence of unseen masses of land.

David Lewis, a New Zealand-born doctor who gave up his London practice to become a freelance ocean adventurer, sailed in the 1960s with some of the last traditional Polynesian navigators in their outrigger-canoes. We the Navigators is his firsthand report, from the Pacific Ocean in the mid-twentieth century, on how sailors like Odysseus crossed the Mediterranean circa 700 B.C., before the invention of the magnetic compass. Most importantly, Lewis's book conveys how the open sea could be as intimately known and as friendly to human habitation, as a familiar stretch of land, to those seamen who lived on its surface, as gulls do, wave by wave. . . .

Sailing with no instruments, the primitive navigator knew his local sea in the same unselfconscious way that a farmer knows his fields. The stars supplied a grand chart of paths across the known ocean, but there was often little need of these since the water itself was as legible as acreage farmed for generations. Color, wind, the flight of birds, and telltale variations of swell gave the sea direction, shape, character.

Here, where you feel the intersection of two swells, each deflected by islands far over the horizon, you make your turn . . . Now you search for toake, the tropic bird, and follow its homeward flight until the sea begins to brown with sand. . . . In Polynesian Seafaring and Navigation, Richard Feinberg, an anthropologist, includes a sequence of interviews with navigators from the island of Anuta in the Solomon Islands. One of these, Pu Maevatau, says of sailing under a cloudy sky that "the expert navigator . . . will make his bearer the ocean."

That sense of being borne along to your destination by the ocean itself is strong in Homer, whose voyagers are seen as creatures of nature assisted, or impeded, by the gods. When the gods are with you, the winds and the sea conduct you onward, like thistledown blown from wave to wave. For Odysseus, as for the Polynesian navigators in the books of Lewis and Feinberg, the ocean is a place, not a space; its mobile surface full of portents, clues, and meanings. It is as substantial and particular, as crowded with topographical features, as, say, Oxfordshire.

-- from Passage to Juneau: A Sea and Its Meanings, by Jonathan Raban