IN 1975, Phil Haynes was tending his dairy farm in Claremont, N.H., when two men stopped by to ask if they could fly off a hill on the farm with their hang glider. Mr. Haynes, who had never heard of hang gliding, gave them permission - on one condition: that they teach him how to fly. That day, for a few short minutes, Haynes flew through the air on a pair of wings.For years he had walked up that hill and stood on top wishing that somehow he could fly. When these men appeared on his doorstep and introduced him to the sport that would become his passion and livelihood, Haynes says he earnestly believed it was a sign from God. Sixteen years later, the sport of hang gliding has grown from a few hundred participants to an estimated 40,000, according to the United States Hang Gliding Association. Gliders have gone from primitive, do-it-yourself kits to wildly colored, high-performance models costing up to $4,000. And Phil Haynes's dairy farm has been transformed into Morningside Flight Park. On a windy Saturday recently, pilots and their friends came to learn about aero-towing from an expert, take a flying lesson, and enjoy the last days of summer. Each weekend at least a dozen or so pilots-to-be come to Morningside, one of the preeminent hang-gliding sites in the Northeast. Some of the new pilots claim to be afraid of heights. Many of them say they never thought they would be crazy enough to do something like this. All of them say they have always dreamed of flying. In two days' time they learn the history of hang gliding. They are taught to "read" the wind and are tested again and again on safety rules that are so crucial in this self-regulated sport. They discover how it feels to be suspended in a harness connected to the glider as they try to maneuver the triangle-shaped control bar that keeps them on course. They learn how to pick up the glider, run with it, and - often several lessons later - fly with it. Jeff Nicolay, a pilot and certified instructor who manages the flight park with Haynes, says that the pilots at Morningside are hang-gliding "purists." Their goal, he says, is to consistently get high enough in the air to find a lifting wind (called a "thermal") and maintain altitude. Ideally, they like to do this by launching off a mountainside without a tow or a motor. Increasingly, however, such aids are becoming more popular. Pilots want to spend less time on the ground and more time soaring through the skies. "Flying is addicting," says John Gavrial, a pilot from the early days of hang gliding who is a weekend regular at Morningside. "You can do just like the birds do." But unlike birds, hang-gliding pilots have been restricted by the relatively small number of flying sites, inclement weather, and the time it takes after one arrives at a launch site to get into the air. To overcome some of these problems, Bill Moyes, a native Australian who holds five world records in hang gliding, has reintroduced aero-towing. Mr. Moyes, who was the test pilot for the first hang glider in 1966 and manufactures of a popular line of gliders, says aero-towing will "give the [hang gliding] world 10 times more area to operate." Aero-towing uses an "ultralight" plane to tow gliders slowly to a point where they can be released to fly on their own. "Aero-towing is not new," says Moyes, who also helped develop the ultralight, but he points out that it has taken some trial and error to develop a plane that flies slowly enough for the glider to stay in control. Moyes says that the traditional method of teaching a beginning pilot to fly is tricky because of all that's involved just to get off the ground: "As an instructor, you're expected to take a student up a hill, teach him to pick the glider up, run with it, reach flying speed, jump in the air, climb in the harness, and fly - all in a matter of seconds," says Moyes. Aero-towing, on the other hand, can be done tandem in a two-person glider. A beginner may ride up with an expert-level instructor and fly his first time out, getting more experience with less hassle. And to a dedicated pilot, more flying time is what matters most. "I don't like being on the ground too long," says Gavrial. "In fact, I get mad if I'm ground-bound for long periods of time." Mr. Nicolay also would prefer to be up in the skies, but out of necessity is spending more time on the ground running his business. Even when he does fly, it is often to test some new equipment or flying method. Still, for Nicolay, the joy of flying is undiminished: "Hang gliding is not really a sport, it's an expression," he says. "Most of all, it's powerful entertainment."