"There are billions of dollars worth of paintings and sculpture here, the only Leonardo in the whole country," says the museum director. "Jenkins, it's your responsibility to protect them . . . "

jenkins, in his new uniform and badge, spies the vandal from across the room, lunges at him as the villain raises a knife in front of the painting, and wrestles him to the ground, taking a stab wound but saving an invaluable work of art.

"You'll be presented a citation as soon as you're back on your feet," the director is saying st the guard's bedside. "And what's more, you'll have weekends off for life. Jenkins, you're a hero."

"Thank you, sir."

The real trouble with eing a museum guard is that almost nothing happens. Not for days, weeks, months, years. And when something does happens, it's hardly over as a guard expected it to be.

A few weeks ago, for example, a gun was fired at the National Gallery of Art. A half dozen guards rushed to the cafeteria and found an out-of-town, off-duty policeman who had sat down for lunch with a loaded .45 loose in his hip pocket. The weapon had accidently discharged, "grazing his fanny" and blowing the chair apart, the administrator reported.

The guards went back to work - standing around - which seems to be a museum guard's unremitting destiny.

Charles Moore for instance, had stood around the National Gallery for five years. Then one day "I happened to be in galleries 88 and 90 when she came in."

A vandal?

Hardly. It was Jacqueline Onassis, then the "inquiring photographer" for the Washington Times-Herald. She inquired as to his favorite painting and took his picture. Moore thought it nice, but not exactly the golden moment that a guard is trained for.

Louis Carroll, another guard, had stood around the National Gallery for some years waiting too. Then a man came in and scrapped, a tiny bit of paint off a Vermeer with his finger-nail, Carroll rushed up. The man said it was an accident, Carroll took action.

"I asked him to keep his hands off the paintings."

Carroll says he hasn't had any problem since then. That was almost 10 years ago.

Tucked away in the National Gallery of Art is a little known pistol range where guards can practice; how much they do Gallery officials won't say, nor did they even mention the range's existence until asked about it. Yet for all their preparation, Gallery guards rarely are called into action.

The boredom never ends and many guards quit because of it. They speak in hushed tones when they speak at all. Often they live for the next 15-minute break. They can even die - a National Air and Space Museum guard did this year after being shot by a fellow gurad while "horsing around," according to District police. But mostly it is unbroken tediumfor Washington museum guards, the Smithsonian's uniformed day-shift of 400 guards, the National Gallery's 195, or the 20 in plainclothes at the Corcoran, the city's oldest private art-museum.

Moore, 62, has had a typically eventful career at the National Gallery. Every few years something happens. That leads him to believe that "you have to be special breed" to succeed in the job, a belief backed up by Robert Burke, director of protection services for the Smithsonian, who says that "courtesy, fairness and a calm demeanor" are essential traits. Supervisors look for those characteristics in guard candidates during one or two months of on-the-job training and then, for those chosen to go on, in a two-week formal training program.

Moore, who got a break when he was promoted from a gallery guard to duty at the door on the Constitution Avenue side of the National Gallery, remembers once when it helped not to get rattled. It was when the Queen Mother walked in on him.

"Everybody was out on the curb waiting for her but they missed her," he says. "She walked in with Mrs. Eisenhower and Jim Hagerty, the press secretary. Mr. Finley, the director, was back in the office, I called him; he nearly had a fit."

Five years later President Kennedy walked in. "He stood right over there," Moore says. "It was right after the Inauguration. He must have stood there 20 minutes. He joked with a couple of old ladies. He whispered in their ears; they whispered in his ear."

Such memorials help sustain Corp. Moore. Yet his finest moment was still to come. Recalled to guard duty when the Mona Lisa arrived in December, 1942, he ended up on "What's My Line." Nobody guessed he was Mona Lisa's guard.

Simply because it's difficult to tell much about a person by looking at him, a guard faces an almost impossible task when it comes to identifying a potential troublemaker in a crowd before he makes trouble. Moore regets that two much visitors came through his door undetected.

One of these was a man who sprayed red paint on a 15th-century Florentine sculpture. Another was a woman who reportedly scratched a painting, then dared to return a second time and put a nailfile to a Gauguin.

"We weren't expecting to see her again," says Moore, recalling that rare and unfortunate occasion when she slipped by while carrying "close to seven pieces of luggage and a shopping bag."

In another curious infiltration that Moore had nothing to do with, a man was discovered after closing time in a phone booth, recalls former director John Walker in his book, "Self-Potrait with Donors." The man slumped to the floor and said he had a heart attack. Guards summoned a taxi but as they were putting him in it his coat fell open revealing a pistol. They searched him and found tools for cutting wire and glass.

At first Walker thought Prince, the gallery's after-hours guard dog for a time, bad sniffed out the villain. but Prince had missed that opportunity, keeping intact his record, which Moore remembers as uninspiring. Prince slipped a lot on the marble floors, put fear into the cleaning men who worked at night, and was soon dismissed, according to Walker, as "a disaster."

Without Prince, guard duty became humdrum again. Moore gets a lot of the same questions, such as, "Where's the nearest restroom?" "Do they think I'm going to send them to the farthest?" he asks.

Being police is a large part of being a guard. During this year's King Tut exhibit, Moore listened attentively to a visitor's long reminiscene about an earlier Egyptian exhibition. Finally, the man asked a question: Had the treasures deterioted any since the last time they were here in 1961?

"The truth of the matter is . . . " began Moore.

"They're not as good as they were?" asked the man.

". . . I haven't seen the exhibit," Moore said.