SHHH. THERE are people in the basement of Washington Cathedral rubbing their crayons all over the art work. Say what?

It's true, but you can dismiss the visions of vandalism. In fact, the London Brass Rubbing Center is in the business of promoting such seemingly odd behavior. And they charge for use of the crayons.

The brass rubbing center has occupied a series of small rooms next to the cathedral's lower-level giftshop for the past week or two. Herein can be found tables laden with composition copies, cast from flat, low-relief brass engravings in the floors and walls of Great Britain's medieval churches.

The stout of back and microscopic of talent can enter and be welcome; can hand over green money and be handed a piece of silver wax, black paper and instructions.

You know about rubbings. Every elementary school graduate who put a quarter under a piece of construction paper and rubbed over it with a crayon knows about rubbings - pretty patterns on paper, produced in a pinch by persons of paltry artistic persuasion. First-grade stuff, no?

"It sounded a bit silly to us too, a first," says Andrew Dodwell, whose British accent is genuine - unlike the 40 memorial "brasses" he and Richard Etches brought to the States to found the center. But the idea is apparently not unprofitable: Of an estimated 150 daily visitors to the center since it opened, Dodwell says, 70 have done rubbings. And few were first-graders.

Though they look like antique metal, the "brasses" are actually a mixture of powdered metal and resin. Even those who frequent the London Brass Rubbing Center's four locations in England don't get to rub the real thing.

The "real thing" are the estimated 4,000 memorial plaques, laid in the floors and walls of medieval England's churches to commemorate the passing of knights, kings, priests, ladies and merchants. Until 15 or so years ago, the only visitors who did more than look at the engraved plates were "a few Oxford types," Dodwell says, who came to do rubbings for presumably academic reasons.

Which was fine, until it caught on. The growing number of visitors taping paper over the brasses and rubbing graphite or colored wax over the paper began to take its toll on the irreplaceable engravings. The churches began to hedge when visitors asked permission to do a rubbing or two.

Working with the churches, the London Brass Rubbing Center people collected dozens of casts of the originals into their first location at St. James Church, Picadilly, in 1974. It was, Dodwell says, the perfect solution: The real brasses were saved from the ravages of rubbing, and the churches began receiving royalties as Britishers and visitors flocked to the centers and Dodwell and his partners were in business.

Four centers later, when it was found that 60 per cent of the clientele was American, Dodwell says, they asked the Washington Cathedral for space. (The host church also receives royalties. The Washington Cathedral's share, like that of the other churches, is 7 per cent.)

The business has been healthy, Dodwell says, though he declines to be specific."Let's just say we're doing well enough to come over here at our own risk," he says. "But I'm not driving a Rolls Royce."

Dodwell attributes the appeal of brass rubbingto two things:

First is the historic aspect, the stories told by the brass monuments - some Dodwell said, are rather interesting. Second reason, Dodwell says, is that "there's absolutely no need for artistic ability" to create a good-looking drawing, suitable for framing, giving away as a gift or even turning into a T-shirt design.

In her book "Rubbing Craft," artist-printmaker Cecily Firestein once refers to the craft as "instant gratification." The book (Quick Fox paperback, $4.95) gives step-by-step instructions on how to find such instant joy in letter-boxes, doors, gravestones, manhole covers and brasses, and how to use the designs to make jewelry, T-shirts, needlepoint and woodwork.

The center provides all the colored wax (silver, gold or various hues) and paper (black or white) you need, and shows you how to do it. All you have to do yourself is pick a monument, which range in size from about 8 x 10 inches to more than six feet long. They cost from $1 to $16 to rub, averaging about $4. And after you pick a subject?

Ay, there's the rub.