FEW JAZZ instrumentalists successfully make the switch to singing and songwriting; the new demands of repeating hooks and lyrical content usually prove too much for them. But after two sparkling acoustic guitar albums for Windham Hill Records, Michael Hedges shows he's the exception to the rule with an impressive vocal album of original songs, "Watching My Life Go By."

He succeeds because his jazz instrumentals were always rooted in folk forms and because he supported himself singing in bars while he studied classical guitar.

Hedges doesn't try to write normal pop songs but works in the open-ended form of Joni Mitchell's late '70s albums: extended lines, elliptical imagery and melodic variations. His songs tend to be internal musings that leap from thought to thought without sketching in the connections.

Sometimes this can be maddeningly esoteric; at other times it can inspire flashes of insight."Holiday" moves from a Fourth of July picnic to meditations on patriotism to a forlorn rendition of "My Country 'Tis of Thee"; "I Want You" moves from contorted verses to the succinct title plea of the chorus hook.

Hedges' voice has the sustained quality of a harmonica as his acoustic guitar surrounds it in a swarm of notes.

MICHAEL HEDGES -- "Watching My Life Go By" (Open Air, OA-0303); appearing Friday with Liz Story at Lisner Auditorium.