A musical and epistolary message from the past, sealed in a small time capsule for half a century, is being made public today by the Library of Congress. The material includes previously unknown recordings of two American classics and a letter "to the People of these United States of America in ... 1988."

The letter, written by Andre' Kostelanetz, one of the best-known classical conductors of the time, makes some shrewd predictions about the future of American music. It was sealed, with three custom-made 78-rpm records, in a film can with a bronze plaque, which was marked not to be opened until March 3, 1988.

Nobody knows how long the library has had the material, though it acquired Kostelanetz's papers during the 1970s. The material was rediscovered some time ago by George Kipper, a technician who was sorting through the library's collection of instantaneous acetate recordings from the 78-rpm era.

Kostelanetz is presumed to be the conductor on the records, which contain George Gershwin's "Rhapsody in Blue" and the "Cloudburst" movement from Ferde Grofe''s "Grand Canyon Suite." The recording sounds like "a good studio performance" by an unknown orchestra, according to Sam Brylawski, a reference librarian in the library's recorded-sound reference center.

Kostelanetz said he selected these items from the "the profusion of compositions" available "because, in my opinion, they incorporate musically the essence of the American spirit ... an originality of rhythm and melody ... typical of the fast-changing American scene."

"It is impossible to anticipate ... the favorite forms of presentation for these numbers fifty years from now," he concluded. "But I write this with the feeling that the compositions themselves will hold a place in the hearts of music lovers and that the names of Gershwin and Grofe' will be accorded prominent space when the history of American music is being written."

Other than what he says in the letter, there are no clues as to why Kostelanetz chose to seal this material for the future, but if he wanted to enhance his reputation as a musical prophet, he succeeded. "Rhapsody in Blue" and the "Grand Canyon Suite" are still among the most popular pieces of American music.

Besides the letter and the records, the can contained needles with which to play the records. Kostelanetz could hardly have imagined the exact formats of the tape cassette and the compact disc -- not to mention video recording. But he said in the letter that he expected the next half-century to be "fraught with changes too vast to contemplate from the distant vantages of the year of A.D. 1938."

"What marvels science will bring about in the interval exceeds even our imagination of the present day," he wrote.

"It is unlikely, however, that the characteristics and inherent qualities that have distinguished America and Americans, through the centuries, shall have been swept away."

Kostelanetz was 37 years old when he put away the Gershwin material for posterity less than a year after the composer's death, and that may have been part of his motivation. If Kostelanetz was thinking he might die young like Gershwin, who was almost exactly his contemporary, he was mistaken. He lived until January 1980, nearly long enough to see his time capsule opened, and was active as a pops conductor well into the 1970s.

Before sealing his message to the future, he had already become a part of music history. He pioneered classical music broadcasts on CBS beginning in 1930, and he conducted the New York Philharmonic's concerts in Central Park for years. But he may be best remembered for the music he commissioned from American composers -- including Aaron Copland's "A Lincoln Portrait," the "New England Triptych" of William Schuman, Jerome Kern's "Mark Twain Suite" and the "Hudson River Suite" of Grofe'. He also commissioned the Ogden Nash verses that are frequently recited at performances of Saint-Sae ns' "Carnival of the Animals."