He stands on the sidewalk with his feet together and his body swaying, bending, literally squeezing the notes out of his harmonica. Beside him stands a shoebox-size amplifier which is wired to the mike he somehow shelters in his hands along with the small Hohner Marine Band Special 20.

Sometimes Charlie Sayles wears a whole bandoleer of 10-hole harmonicas, one for every key. He prefers them to chromatics. When he was here for the 1976 Smithsonian Folk Festival he was fooling around one night with Robert Jeffery, a blues pianist, improvising stuff, and he had to keep whipping out this one and that one till they found the best key to jam in.

(The Smithsonian's Ralph Rinzler had discovered him that year, tying himself into knots on the sidewalks of Greenwich Village while his amplifier growled and spat and crooned and wept the funkiest blues this side of Sonny Terry.)

"It's not easy, performing on the street," said Sayles, who has been staying at Rinzler's house while on a two-month grant to teach harmonica to prisoners at Lorton. "It's hardest when nobody notices you. A street musician is a threat to some people, standing up there, maybe dressed funny. It can be a little scary. So they hustle past like you weren't there."

He made a living at it for several years, averaging around $20 a day and up to $150 on a really great one. He doesn't play every day. It takes a certain resolve. He lives alone ("I'm just me") in Philadelphia now, moved there from New York. He started playing in his Atlanta days, accompanying a guitar-playing friend.

"I never would have had the guts to start in by myself."

A harmonica isn't as easy to play as you might think, not the way he plays it. It took him four years to pick up all those tricks you don't read about in the instruction book: the Wobble, the Wah-Wah, talking techniques, note bending. Sayles still spends maybe five hours a day listening to tapes of the great players and imitating them.

He was working with a rock 'n' roll number by Magic Dick, an impossibly complicated series of growls, pops and sleek glissandos. He listened intently, then repeated it exactly, down to the last subtle vibrato. ". . . Then he does this thing . . . " and went on to the next measure.

As a macho teen-ager, his quick mind unengaged by the woodworking and auto mechanics courses in high school at Salem, Mass., Sayles didn't think much of music either. "Heard Dylan when I was 13 and I liked that, but it didn't take. Then I was in the service four years, in Vietnam and Germany, and when I was just about getting out I had a buddy who played one, so I bought one to mess with, but it still didn't happen till my cousin wanted someone to play along with him. That was 10 years ago."

He experiments constantly with microphones and amplifiers. He works on new songs -- "I got about 50 that I made, I play 'em pretty much the same but not note for note." Now he burst into "Woody," which started with the Woody Woodpecker theme and immediately took off in a sparkling rocket shower of sound.

"Every two months I throw down $50 or $60 for some new harps. The notes go dead. Usually if one goes, four or five others go too."

Absently, he bent an E, curving it down all the way to a D. "The most important thing about music," he said in his slight New England accent, "is that you're giving a person something for nothing. You might not hit a million people, but even if you make one person feel good, imagine what that means. The musician gives out something that took him years to learn, and you pay five dollars to hear it. That's nothing if it makes you feel good."