There is a common misconception among landspeople that the surface of the ocean is a blank, unpopulated wasteland. I understand this misconception. I've had the same flawed impression of the desert as I've driven across its flat brown miles in a car. I am not a desert person. I've never learned how to differentiate among the russets and umbers and raw siennas of the desert landscape in order to discern the profusion of crawling things that populate the dust. But I've sat for a thousand hours at sea at the helm of a sailboat, climbing azure water-mountains, slipping into the purple canyons of troughs, staring at the watery terrain and learning, often without intending, how to observe and eventually to comprehend the maze of biomass that ranges across its surface.

The seabirds are the most obvious inhabitants of this place. Beaked and clawed and winged like feathered reptiles, they have evolved out of an entirely different family of living creatures than the human beings that sail here. But they share the air with us-which makes them accessible to our senses in a way that our closer relatives, the whales and dolphins, seldom are.

There are three types of birds that one encounters at sea. First are the land birds, who cross the open water only when required on their long migratory journeys. Next are the gulls and terns-partial residents of this place-who fish the sea fringes by day but return to their nesting islands every night. And finally are the pelagic birds, the wanderers, who live along the surface of the sea like fishes of the air, and for whom the ocean is home.

The land birds are wind swimmers that fly by pumping their wings. When they cross a stretch of open water during a long migration, they move like tunneled, driven creatures whose whole being seems focused into a predetermined destination. If they are blown off course or carried by a storm too far away from the land, they risk dehydration and eventual exhaustion. Then they have no choice but to continue their struggle-pumping, pumping, pumping, until their strength fails and they finally tumble into the water and drown.

Seagulls and terns also fly from time to time by pumping their wings - because, in truth, they are only partly seabirds. In the morning when they leave their rookery islands they often travel in straight lines, swimming against the wind like finches or blackbirds, heading out to the fishing grounds where they will spend the day. Once they are properly at sea they seem to relax, as if they've just remembered what a pleasure it is to be free of the land. At these times they resemble the pelagic birds, gliding and banking and diving for food. But as dusk approaches, they mount the wind again and turn resolutely homeward, pumping their wings and racing the sun toward shore.

The only true seabirds are the ocean wanderers: the albatrosses and fulmars, the shearwaters and skuas and petrels. These birds roam the waves, sometimes for years on end, only returning to the land for brief periods of nesting and raising their young. During all the rest of their lives they remain at sea, following routes that are poorly understood by human observers, traveling as wind sailors across untold thousands of miles.

Out in the ocean these birds are a mariner's constant companions. Along with the surface of the sea, they are what he watches. Their movement is mesmerizing. The patterns they trace across the surface of the water seem at once random and oddly predictable. In the daytime they rise from behind a breaking crest, bank into a stall, tip, dive, run down the belly of a trough, disappear behind the curl of the next crest. At night they chatter in the dark as their shadows dart in and out of the circles of the ship's running lights.

Many times when I've been at sea and watching the birds, I've found myself wondering about the mystery of their flight. How do these creatures, which are heavier than the air, conspire to leave the surface of the water? How do they rise without apparent effort into an invisible medium? How do they move against the flow of wind when common sense suggests that they should be pushed in the opposite direction?

The answer that any good engineer will give to questions like these has to do with the aerodynamics of an airfoil - a complex shape that occurs most perfectly in nature in the wings of soaring birds. This is the same shape that men in this century have learned to use for the construction of aircraft wings, the same shape that some nameless mariner stumbled upon three thousand years ago for use in fashioning the vertical wings of sail boats.

I know about airfoils. I've read the scientific explanation of how they work dozens of times. I've repeated this explanation to novice sailors on countless occasions. There is a diagram that appears in encyclopedias and flight manuals and sailing primers of an airfoil shape – a pair of lines flat on the bottom and curved on the top, with a series of vectors in the form of small arrows that represent the flow of the air as it moves along the two surfaces. The diagram looks like this:

According to aerodynamic theory, an airfoil works on the principle of lift: a force created by a pressure differential in the stream of air flowing past the foil. Because one of the foil's surfaces is curved and the other is straight, the air molecules that pass over them must travel unequal distances. The molecules that pass along the longer, curved surface must accelerate in order to keep up with the molecules that pass along the shorter, straight surface. As this acceleration occurs, the upper molecules spread apart, reducing their density and creating an area of relative low pressure. Meanwhile, the molecules on the flat surface are left with a positive pressure, so that they push upward against the area of low pressure, forcing the wing to rise.

Fine. . . just so. Now I understand the symbols in the diagram and I know, operationally, how an airfoil works. But does this knowledge bring me any closer to an understanding of the mystery of flight or to an appreciation of its beauty?

The answer is no-and the reason has to do with a fundamental incongruity between a complex phenomenon in nature and its scientific description. The problem with the diagrammatic rendering of an airfoil is that it is too fractured, too mechanical. It is a clever explanation of the interplay of forces at work on a physical object, but it captures nothing of the flow and feel of the thing it tries to describe. Like attempting to teach a novice sailor how to make a boat move to windward by issuing a series of verbal instructions, the scientific description of an airfoil fragmentizes an experience that is fluid. It tries to translate into words and symbols something that is-in the end-ineffable.

Watching seabirds tip and glide across the wake of a sailboat far out at sea has become, for me, a kind of avian magic show. The ease and grace of the birds' movements as they soar fractions of inches above the waves is like a sleight of hand performed by a troupe of master magicians-a dare repeated over and over again, challenging the audience to guess how the trick is done. Aerodynamic formulas and diagrams of airfoils notwithstanding, the magicians win the dare every time. Their flight remains a dazzling aerobatic display -and the dexterity of their wingtips makes our own much-vaunted fingers and opposing thumbs seem crude in comparison.

In the end, the birds serve as a point of contact with the ocean world. Their effortless movement speaks of their near-perfect adaptation to their natural setting. Their apparent aimlessness evokes a spirit of freedom that is worthy, if not always of our emulation, then at least of our wonder. And the careless ease with which they fly posits them as unselfconscious celebrants of their ocean world, appearing to all the rest of nature to be flying for the pure joy of it.