In a dressing room of the Smithsonian's Museum of Historyand Technology, Harry Truman was helping Abraham Lincoln attach his beard.

"As president I feel a great responsibility to get it right," said actor Vincent Clark of his upcoming reading of the Gettysburg Address. "But I have a real advantage over John [MacDonald, who portrayed Truman] because nobody really knows how Lincoln sounded."

In gearing up forthe part, Clark said he went to a local cmemtery to performthe address, and practiced behind the bar in his apartment,which he said was the closest thing to a podium he could find.

An hour later, Clark was behind the podium intoning with measured precision before a crowd of weary tourists. Unlike the original reading, however, Clark was under the glaring lights of a local television crew.

At the conclusion of the speech an old man in the back of the crowd said, "Youshould have Patric Henry, 'Give me liberty or give me death.'" A Smithsonian staffer turned toward him, "Oh, he'll be here tomorrow."

Time periods were compressed yesterday as live performers and craftsmen reenacted American history surrounded by the museum's impenetrable glass cases housing curios of the past. Roving musicians, artisans, and a vetriloquist were stationed throughout the museum as a part of the Smithsonian's Fourth of July Celebration, which began yesterday and will continue through Sunday.

The cameras also followed MacDonald into the railroad exihibit, where the actor portrayed Truman, as well as Theodore Roosevelt, William Howard Taft and Franklin Roosevelt, as they delivered whistle-stop speeches through the heartland of America.

Once in therailroad exhibit, MacDonald assumed a number of roles. First he was a railroad worker, shouting, "Make way for the president's train," as he circulated small American flags amongthe crowd of about 30 spectators. He then climbed behind apodium between two trains and began his performance, with only different voices and a change of jackets to distinguish between his performance as Truman and other presidents and presidential candidates who had pioneered the whistle-stop format.

Meanwhile, back at the We The People Hall, where Lincoln had just held forth, Uncle Sam, in the person of John Rusk, was handing out pictures of himself in all of his red,white and blue glory. Although he is not a professional actor like MacDonald and Clark, Rusk really got into his part. "That's the name I was born with 73 years ago," said UncleSam of the words John Rusk, "but I like Uncle Sam better."

Rusk explained that even though he was never in the service, he has spent the past 33 years traveling throughout the United States impersonating the famed recruiter.

Rusk saidthat as a child, "I was always the tallest and the skinniest, so I always carreid the flag." He said this chore followed through elementary school, the cub scouts, the boy scouts and into adulthood when he became active in Shriner activities.

"My greatest honor came in 1976 when I was declared Uncle Sam of All North America by the Shirners' Imperial Council," said Rusk, a retired government building inspector.

MacDonald, Clark and Uncle Sam were part of an entourage ofseveral hundred Smithsonian-sponsored impersnators, musicians and craftsmen who will be roaming the halls of History and Technology this weekend.

The patriots of Northern Virginia fife and drum corp was rained out of its scheduled performance on the Mall yesterday and marched instead under neon lights in the pre-fab motif of the Restaurant Carousel snackbar at the museum. Weary tourists resignedly snapped their Instamatics as the adolescent musicians, clad in Revolution-era costumes, played a brief program of high-pitched battlesongs featuring the drum section in a redition of "Drums Along the Potomac." Nearby, Styrofoam salad plates and plastic-wrapped desserts rotated in a mechanical cafeteria line.

Sword-maker Luther Sowers, a 40-year-old retired schoolteacher who now works full-time fashioning swords in Salisbury, N.C., stood behind a display of hilts and blades and explained the good-luck charms of his craft. He engraves various talismans and good-luck symbols into his replicas of historical swords. "If he had things that meant good luck on his sword," he explained, "it would give a guy a lot more confidence."

Sowers' best customers are the state and national parks that use his weapons in "living history" exhibitions and displays, where tourists can run their fingers along the blade and/or clutch the hilt of a sword.

Museum regulations,however, limited Sowers' ability to demonstrate the sword-making process, which he said involves "a lot of grit, and flame and fire.

"The whole idea of living history," said Sowers, "is if you're wearing a suit of armor and somebody comes up and says, 'Hey, is that helment heavy?' you can take it off and say, 'Here, try it on.'"