A Harvard professor has discovered 33 previously unknown compositions by Johann Sebastian Bach in a Yale University library. All the pieces are for organ and were composed in the years between 1700 and 1707 or 1708, when Bach was in his late teens and early twenties.

"Nobody, including myself, was prepared for this," said Christoph Wolff, chairman of Harvard's music department, who discovered the Bach works. "It only shows that the harder you look, the more you find. The rather embarrassing and amusing thing is that it took someone from Harvard to show Yale what they have."

Professor Harold E. Samuel of Yale said it was "without question the most significant find of Bach's music in the 20th century."

Since 1873, a bound manuscript containing the unknown Bach works has been the property of Yale and most recently has been kept in Yale's Beinecke Rare Book and Manuscript Library. The manuscript includes a total of 83 works by various composers, including several other members of the prolific and highly musical Bach family.

The existence of the manuscript and its general contents (organ music of the late 17th and early 18th centuries) had long been known, but the fact that many of the unattributed works were by J.S. Bach had not been noticed.

The manuscript has now become "priceless," a Harvard spokesman said, although it is not in Johann Sebastian Bach's handwriting, which would have made it even more valuable.

Wolff identified the composer as Bach "primarily on internal, stylistic evidence."

Wolff had been researching intermittently in the Yale library for several years. With a colleague in Leipzig, he is planning to publish a new, complete list of Bach's works for the 300th anniversary of Bach's birth, which will be celebrated in 1985.

"The most important composers in the book, besides J.S. Bach, are his uncles Johann Michael and Johann Christoph the elder, who died in 1703," Wolff said. "This manuscript contains the largest known group of works by Johann Michael. I was originally excited about that; they are at the very beginning. We have previously known only some half dozen of his pieces; discovering 26 new ones more than quadruples his known repertoire. He is important as one of the people who in- fluenced the young Johann Sebastian."

The newly discovered J.S. Bach works increase by approximately 20 percent the body of his known chorale preludes -- organ works based on hymn melodies -- and they will shed new light on his development as a composer during a formative period of his life.

The manuscript originally belonged to Johann Christian Heinrich Rinck, an organist who was a student of Johann Christian Kittel, who had been Bach's last organ student. After Rinck's death in 1846, an extensive collection of his organ manuscripts was bought by an American musician, Lowell Ma- son, who gave the collection to Yale.

According to Wolff, these early Bach works are in "basically the same format as similar works by people of the older generation in the same collection. But they stand out in that they focus on bold features, harmonic daring and detailed polyphonic elaboration."

Besides the chorale preludes, he said, the manuscript contains "5 1/2 previously identified works by Johann Sebastian Bach." The "half-work" is an introduction to a piece that is already known.

Wolff has already played the newly discovered works on an organ -- he is undoubtedly the first person to play this music in more than a century -- and he said that "it was a rather unreal thing. The quality of these pieces is of a rather broad range; some are clearly experimental and perhaps of more academ- ic than genuinely musical inter- est.

"But I would say with confidence that the overall quality of the pieces is remarkable -- with respect to what we know of the other early works of Johann Sebastian Bach and in comparison to the other mater- ial contained right in that manuscript."

The newly discovered works will be published in facsimile next year by the Yale University Press. A scholarly edition (including other works that present variant readings of previously known compositions) will be published later as part of the complete, critical collection of Bach's music now being prepared.