On a recent misty gray morning, Jack Knox stood in the middle of a small fenced-in area at Yeager Gum's farm north of Leesburg. In the pen with Knox were three panting sheep, and into the pen came a black-and-white dog and its owner, there to learn a lesson in herding livestock.

"Get back!" shouted Knox at the dog, though in his thick Scottish accent Knox seemed to be saying, "Get bach," instead. "Come by . . . away to me . . . that'll do," Knox told the dog. While the sheep milled about, the dog eyed Knox and the owner looked both attentive and slightly confused.

Using jargon mystifying to most, Knox was imparting a lesson far more complex than learning verbal commands. The instructor for 16 people and more dogs at a two-day clinic sponsored by the United States Border Collie Club was there to teach the owners how to bring discipline to their dogs' strong herding instinct.

"It's all training and building confidence," Knox told the dog owners and spectators who had gathered outside the pen. The training is not just for the dogs, but for their handlers as well. Knox once was a shepherd in Scotland, where the dogs have been bred for the last 400 years along its border with England.

Some of those who paid $55 each to attend the clinic, are seeking the kind of success Jack Knox has had in sheepdog trials throughout the country. Some want their dogs to help bring in the livestock each evening. Others, content to watch their dogs herd the chickens, are merely fascinated with the talent of the border collies and the process of handling their herding instincts.

"Listen to this boy. He's confident, he's positive that his dog's going to learn a lesson," said Knox, as 12-year-old Garrick Terry of Hamilton took his 2-year-old dog Jean into the field to move the sheep.

Garrick, whose mother Candace Terry raises and trains border collies and helped organize the clinic, trained the dog himself. Most days after school, Garrick and Jean take the family's flock of sheep across several fields to a spot where they can graze. Like an old-time shepherd, Garrick stays there for an hour or so, while his dog watches the sheep. He reads, does homework, or, quite unlike an old-time shepherd, listens to a portable radio.

Unlike Garrick, who plans to enter Jean in sheepdog trials someday, Yeager Gum, whose 245-acre farm has been the site of these clinics for several years, is content to keep his dogs on the farm where they help him bring in the cows and "put the chickens in the henhouse without any problem."

"It's amazing how much time these dogs can save you," said Gum.

Others, like Elsie Maylott of Richland Acres near Sterling Park, must drive some distance to give their dogs a taste of farm life and herding livestock.

Maylott, a bibliographer for the Library of Congress for the last 27 years, became fascinated with border collies 10 years ago when she and her family moved to Loudoun County and bought a border collie puppy from a neighbor. Her puppy and its litter mates started going to a nearby lake where, Maylott said, "The dogs just naturally started herding the ducks. They would actually swim out into the water, get around behind them and move them in, and it didn't matter what time of year it was. So that kind of got me interested."

Now, Maylott has a second border collie that she brings to Jack Knox's clinics. "It's a challenge, being able to work with the dog," she said.

The dogs' owners are about as different in their goals as their border collies differ in appearance. Although there are several border collie registers in the country, the breed is not registered with the American Kennel Club and few dog shows offer confirmation classes for border collies.

Most border collie people want to keep it that way. "We're not registered with the AKC, and we don't want a standard. It's just brains before beauty," said Candace Terry, a U.S. Border Collie Club member. "If you have a standard, you'll get all pretty dogs that can't think."

Off and on during the recent clinic, Knox had his own dog, 9-year-old Jan, work the sheep in an open field, giving the small crowd a chance to see a highly trained border collie moving sheep.

With three sheep out of view, Knox, using only a series of sharp whistles, sent Jan over a distant hill to bring the sheep, very slowly, down through several gates and into a small pen set up at the bottom of a hill.

Responding to each whistle, Jan alternately raced in wide circles around the sheep, crept close behind them, lay in the grass and made sharp darts to each side of the sheep to keep them in steady progress toward the pens. From start to finish, the dog never took her eyes off the sheep.

Knox, who raises and trains border collies at his Ettrick Kennel in Lexington, Va., has competed successfully for years at national and international sheepdog trials. His dog, Dryden Creigh is the reigning reserve world champion trial dog. Knox is working on a training manual with Donald McCaig, president of the Virginia border collie association and author of the novel "Nop's Trials," soon to be a movie.

Knox now gives handlers' clinics throughout the United States. Like those who brought their dogs to the clinic, he believes "There's not a dog to touch a border collie."