Crouched in the open doorway of the plane, gazing at the patchwork of farmland below, I found my thoughts returning, again and again, to an old parachuting joke: On his first trip up, a student reviewed out loud the three major points he had learned: "1. My static line automatically pulls open the canopy. 2. In case of malfunction, I activate the spare chute. 3. Once on the ground, a truck will pick me up." Finally screwing up his courage, the student jumped, only to discover his static line wasn't connected. Pulling the emergency ripcord likewise didn't work. And plummeting toward the ground he thought to himself, "With my luck, that truck won't be down there, either."

Somehow the joke didn't seem so funny 2,500 feet off the ground.

"Get ready," shouted jumpmaster Joe Lagoia. One hand on the door frame, one hand on the floor, I faced into the wind and waited. Joe made a sign to the pilot; the de Havilland's engine noise cut back."Go!"

My "Geronimo!" didn't get past the first vowel as I executed a perfect blind leap, survived an adrenaline attack as the static line snapped open my chute, and looked up at the most beautiful orange-and-white canopy anyone could ever hope to see.

Of course, I had totally blown the proper exit and countdown procedure, but ecstasy leaves little room for guilt. Instead I had visions of the boys breaking out champegne and cigars back at Houston Mission Control.

Half a dozen colorful circles of nylon drifted beneath me, slowly settling below the hazy Maryland horizon toward a small gravel target. Each of us had shelled out 50 bucks, trained for four hours and sat around for what seemed an eternity just waiting for this moment of quiet isolation. Now I had two minutes to enjoy it.

Though sport parachuting has only recently caught on, the original concept can be blamed on Leonardo da Vinci. A Frenchman made the first jump in 1783 from a tower, and a century and a half later international competition began in Yugoslavia with the 1951 World Championships. Since then the number of people willing to hurl themselves from airplanes has grown at a rate that might prove alarming even to a colong of lemmings.

One estimate claims that worldwide participation increased from about 1,000 in the early '60s to over a quarter of a million today. For this country, establishing exact numbers is difficult, according to William Ottley, executive director of the United States Parachuting Association, but an educated guess would be that 35,000 enthusiasts make over a million jumps a year.

As the only national organization for the sport, the USPA publishes skydiving information and guidelines, organizes competition, trains and certifies instructors and sponsors the national team. Of the 200 to 250 drop zones across the country, 146 voluntarily meet USPA suggestions.

"Since they can't regulate you, this business is really based on the reputation of the individual operator," explains George Kabeller, who with Betty Johnston manages the USPA-sanctioned Souhtern Cross Parachuting Center in Downsville, Md. "We do everything by the book, even a little better, because we want the public to know us as a safe organization. They always remember you for the one bad thing you do, so that makes the job that much more difficult."

Fortunately, that one bad thing doesn't happen often any more. With recent safety advances, chute malfunctions are almost nonexistent and injuries are rare. "We've only had two breaks and a sprain this year," says Kabeller, "and those have been from people not keeping their feet together when they land. All a student has to do is follow instuctions."

A beginner at Southern Cross finds himself in with anywhere from 15 to 30 fellow pupils. Most are young, usually college students, but the sport attracts everyone from grandmothers to doctors to truck drivers.

"Skydrivers are often scuba divers and skiers," notes Kabeller. "The out-going, aggressive type person." On weekend afternoons, dozens of these experienced aggressive type persons loll about the hangar in colorful jump suits, nonchalantly repacking chutes, cracking jokes and sipping Mountain Dew. Hours of such low-key loitering separate each "hop and pop."

Our class began with an introduction to the equipment. "This is an airplane," pointed out instructor Ken Adams. "We're going up in about 15 minutes and jump out." A nervous chuckle spread through the group. "Not really," he added with a grin. Funny. I nervously noted the number of students today: 13.

During the morning session, we simulated airplane exits, donned main and emergency chutes, and learned what to do in case of malfunction. "Four Hail Marys," suggested one of the local "sky gods."

After lunch at the general store across the road (popsicles still a nickel) we gathered at the target for a grueling session of practice falls from a bench to a gravel circle.The idea is to absorb the impact through a roll on one side of the body from the feet to the hip to the shoulder without using the hands. Unfortunately all that absorbed impact comes back the next day to haunt you.

Since the Cross will not "throw" students if the wind is more than 10 mph, we had to wait an hour or so for favorable conditions. Finally seven jumpers, a pilot and jumpmaster Joe crammed in per planeload. Joe promised he wouldn't push.

Southern Cross has perfected its technique to the point where getting down is almost as safe as getting up - and certainly less crowded. Dince manually ripping a ripcord is a tricky task, a student's first five jumps are with a static line - a stout cord hooked to the plane that pulls open his chute.

Once adrift beneath the nylon, we were not merely abandoned to the vagaries of Zephyr. We easily steered starboard or port by pulling one of the two cords that dangled above each shoulder. A large orange arrow next to the target served as chief navigator.Nearer the ground, Kabeller brought us home by shouting his commands through a portable loudspeaker. "Left toward the barn, orange-and-white. Now!" The "Now!" was usually necessary to bring round a day-dreaming chutist.

Sure enough, just as we'd been warned, the ground arrived faster than anticipated. Seven up, seven down, everyone reasonably intact. Wrapping the cords and nylon around our arms, we stumbled toward the hangar to discard out equipment and lay-claim to our treasured first-jump certificates (suitable for framing). "Terrible lanings," Kabeller mumbled as he passed us inside. "Terrible?" I thought to myself, my feelings slightly hurt. "I'd say we did pretty well considering the size of that first step."