On a raw day in October 1969, a day during the Autumn of Discontent that launched the great Moratorium March, youth power protests took possession of the headlines and the streets.

Students from all over the country lined the avenues and boulevards of Washington. Many of them marched down Constitution Avenue, signs in hands, democracy in action. Many poured into the National Museum of History and Technology.

As dozens of the noisy, weary protesters filed into the first-floor entrance area, security guards broke into a mild sweat. Violence? Bloodshed? What to expect?

Within moments, the answer became clear: the unexpected. The protesters grew quiet, as if hypnotized by the Foucault pendulum swinging slowly, methodically before them. In a day of upheaval, it came to represent serenity.

"When they started to fall asleep," said George Eklund, curator of physical sciences and the man who oversees the pendulum, "the guards got even more worried. They called me up and said, 'Should we stop the pendulum to get rid of them?' I just said, 'Heavens no.' "

Since the museum opened in 1964 -- it has since been renamed the National Museum of American History -- the Foucault pendulum has been a constant in a city of constant change. At least until yesterday. It was then that the 240-pound brass bob on its steel line was stilled, except for cleaning, for the first time in 23 years.

At 4 a.m., workers halted the moving sculpture and began constructing a wooden fortress around it. By 10 a.m., as the curious began to pour into the museum, the last nails were pounded in.

A reinterpretation of existing museum space and a new exhibit are the factors stopping the graceful swing of the Foucault pendulum, one of 30 in the United States, seen by an estimated 5 million visitors last year. Yet this elegant illustrator of the Earth's rotation is not gone for good.

"We're just moving it up a floor," said Eklund, adding that the pendulum is beloved not only by visitors but by the museum staff as well. "We've got a lot of bang for buck as far as the pendulum is concerned." The new pendulum exhibition, he said, should open this fall.

The new first-floor exhibition, "A Material World," will illustrate "the business of modifying materials into artifacts," according to Eklund. Bob Norton, in charge of exhibition production, expects it to be finished "by April at the latest."

When the pendulum is moved up a floor, its 73-foot cord will shorten somewhat, but its educational mission -- examining two phenomena, the loss of energy and the Earth's rotation -- will remain. Eklund, in fact, is convinced the move is a good one: A clear glass or acrylic panel to be cut into the first-floor ceiling will allow visitors to look up and see the swaying bob from a new perspective.

Named for the French physicist Jean Foucault in 1851, this device was the first to offer physical proof of the Earth's daily rotation. The pendulum's line of swing appears to rotate clockwise. Not so. Actually, it is the Earth that is rotating counterclockwise. The pegs at the base of the pendulum -- the entire set is knocked over by the bob every 33 hours -- demonstrate the properties of revolution and motion.

Even with its heavy weight and long swing, the Foucault pendulum, Eklund explained, is sensitive to external forces. Because it loses energy with each swing, a electromagnet the size of a doughnut is installed at the top of the cord to give it a little kick. Air currents are troublesome, and so is vibration of the building. Heavy traffic in front of the museum in the daytime results in a greater variation in the bob's swing than at night.

Most people who trekked to the museum over the years to see the pendulum weren't there for the mechanics of the device, however. They came for the beauty of the thing, and to clap and whistle every time the mighty bob fells a defenseless peg.

The museum recognizes the fact that it might have to field a few complaints. "We acknowledge that there might be negative reaction," said Susan Foster, public affairs specialist. "The pendulum is well known, and consequently a lot of people have seen it or came to see it. But we feel the new exhibit will be just as wonderful and beneficial. And also, we're not tearing down the pendulum, we're just moving it to a different place."

Foster said the museum will also turn visitors in the direction of other Foucault pendulums -- there is one at the National Academy of Sciences here, and others in Philadelphia and Boston -- if they are dead set on seeing one during the summer months. "We're here to increase public awareness," she said. "It would only stand to reason that if someone is curious about scientific principles of a pendulum, we would direct them to another."

Curious the visitors were yesterday, attempting to peer through clear plastic peepholes in the wooden fortress, attempting to explain the move to their children.

Henry T. Sloane is one museum employe who admits wistfulness at the pendulum's summer departure. He is the one who for 23 years has polished the brass bob daily with a diaper, the one who many times set up the pegs each morning. As the museum doors opened yesterday, he eyed the wooden barrier and spoke warmly of the device.

"The pendulum's a very interesting thing; that's why I liked to take care of it," he said. "You can tell time by it, you know. There's a dial in that floor, but I don't know how to use it."

He paused, watching the gaggles of children stream in, many of them rushing past the plywood barricade, unaware of what rested within.

"Yes," he said, "some people used to think that was the only thing in here." He paused again. "Everybody sure liked it."