WHEN Samuel Barber was 9 years old, he wrote his mother a note: NOTICE to Mother and nobody else -- Dear Mother: I have written this to tell you my worrying secret.Now don't cry when you read it because it is neither yours or my fault. I suppose I will have to tell it now without any nonsense. To begin with, I was not meant to be an athlet (sic). I was meant to be a composer, and will be I'm sure. I'll ask you one more thing -- Don't ask me to try to forget this unpleasant thing and go and play foot-ball -- please -- Some-times I've been worrying about this so much that it makes me mad (not very), Love, Sam Barber II." In this early pronouncement, Samuel Barber certainly was right. His death at 70 on Jan. 23, after a long and bitter struggle with cancer, removed from the American music scene a man who contributed a very large share, perhaps even a disproportionate share, of lyricism to the world in which he enjoyed living.

Barber was not a composer driven to write unceasingly or on any set schedule. He did not turn out numbers of quartets or concertos or symphonies. sHis entire works included one concerto for piano, one for violin, one for cello and one for oboe, one string quartet, one piano sonata and one for cello, two symphonies, three orchestral essays, as well as works in varying forms, a noble list of around 50 songs -- and four operas.

Operatic composition occupied Barber over a longer period of time than any other form. When he was 10 he wrote his first opera, "The Rose Tree," to a libretto by his family's Irish cook. His second and third operas enjoyed librettos by Barber's lifelong intimate and colleague, Gian Carlo Menotti. "A Hand of Bridge," a brief and witty chamber opera for four characters, was succeeded by the work that brought Barber the first of his two Pulitzer Prizes, "Vanessa," which enjoyed a triumphant premiere at the Metropolitan Opera in 1958. It is symbolic of the close personal and artistic relationship between Barber and Menotti that out of hundreds of opera recordings there is only one whose jacket lists the names of both composer and librettist. The RCA album of "Vanessa" carries the names Barber-Menotti.

The two men, born 16 months and several thousands of miles apart, became friends in 1927 when Menotti entered the Curtis Institute where he studied composition with Rosario Scalero, who was also Barber's teacher. In the years that followed, the two composers, whose creative careers were moving along parallel lines of success, went on walking tours of Europe. In 1943 they bought a home in Mt. Kisco, New York, where they lived for over three decades. They called it Capricorn, a title Barber gave to an ingenious concerto grosso he wrote a year after moving in.

Menotti once explained how two creators were able to compose at the same time in the same house. He said it was possible because each man had his own wing where he could work unheard by the other. By the time Barber was ready to write his first full-scale opera, nothing in his world seemed more natural than for him to turn, for a libretto, to Menotti -- who, by that time, had written the words and music for eight of his own finest operas.

With "Vanessa," which had its world premiere at the Metropolitan Opera in 1958, Barber achieved an immediate success. Menotti provided Barber with a combination of intensely romantic situations dramatic tinged with tragedy that inspired Barber to heights of lyric writing no American composer has surpassed. Barber hoped to hear either Sena Jurinac or Maria Callas in the title role, but for various reasons neither was available. But the Met gave Barber something of a dream cast headed by Elanor Steber and the young Nicolai Gedda, and for conductor, Dimitri Mitropoulos. Fortunately the entire production was recorded, creating one of the historic testaments to Barber's stature.

Another of those testaments exists in the recording of Barber's Hermit Songs, a cycle of 10 marvelous songs that had its premiere at the Library of Congress in 1953 when Barber served as pianist for the new young star named Leontyne Price. Out of all of Barber's vocal works, however, the one that gives us a rare glimpse of the composer as singer is the recording he made in 1935 of his masterful setting of Matthew Arnold's moving poem, "Dover Beach." Barber wrote the music when he was 21, for baritone and string quartet.

One of the remarkable aspects of Barber's music is his equal mastery of both vocal and instrument writing, a dual gift not common to his other American colleagues, Virgil Thomson, Aaron Copland, William Schuman, or Roy Harris. With his First Symphony, written in 1936, he demonstrated a command of formal design, contrapuntal technique, and melodic inspiration. His violin concerto, played here with searing beauty in Isaac Stern's recent concerto marathon, reiterated these same gifts, while his piano concerto, which won the 1963 Pulitzer Prize, showed Barber's mastery of conventional forms in new usage.

There is no musical idiom in which Barber did not speak with authority. His oratorio, "The Prayers of Kierkegaard," has not yet found its rightful place in our choral concerts, but its power, enhanced by previously unheard beauties, will eventually win proper recognition.

The ballet score which Barber wrote for Martha Graham in 1946 was first known as "The Serpent Heart." It reappeared as "Cave of the Heart," but is most widely known as the orchestral suite, "Medea," in which Medea's Meditation and Dance of Vengeance are prime examples of Barber's gifts for intense dramatic expression.

When the publishing house of G. Schirmer asked Barber for a piano sonata in 1949 to mark the firm's centenary, it was rewarded with a work that quickly became the most frequently played American sonata. Following its titantic premiere at the hands of Vladimir Horowitz, it was taken up by every American pianist able to cope with the immense demands found in each of its movements. Before long it was one of the required works on virtually every American competition for pianists.

Conductors of chamber choruses know the rare fragrance of the "Three Reincarnations" Barber wrote as settings of haunting Irish poetry as well. Barber was fortunate among contemporary composers in the frequency with which his works were performed. On one occasion in the early 1950s he wrote to a conductor who wanted to play his newly orchestrated version of "Souvenirs," but wanted to omit the third section entitled "A Corner of the Ballroom/pas de deux." The conductor, who was not identified, had said that the work was too long with that episode. Barber wrote him: "As much as I am in favor of brevity, I cannot agree with your conviction that the pas de deux be cut. For purely musical reasons, it is the only cantabile piece in the suite, the only one with a melodic line, and if you speak of public success, I have never noticed the public to object to that! . . .

"Believe me, I admire your musicianship and vast experience, but your statement (that dance music cannot be of longer duration that 14 1/2 minutes in a symphony concert) seems arbitrary and untenable. What if Maestro Eugene should say that 15 1/2 minutes were the maximum duration, or Maestro Wilhelm 22, or Maestro Arturo 3? The thought is too terrifying to pursue . . . If you do not have my complete blessings, you do have my very best wishes."

It should be recorded that while the conductor in question was obviously neither Ormandy, Furtwangler, or Toscanini, neither was it Koussevitzky, who died a year before the work was written, nor Reiner, who gave the premiere of "Souvenirs" in Chicago in 1953.

Barber was exactly right, of course, about the public's love for a "long melodic line". That is one reason why his popular and haunting Adagio for Strings is heard many times a year, both in its original form for string quartet and in the enlargement for string orchestra that Barber made at the request of Arturo Toscanini: The Adagio illustrates in a peculiarly poignant way its composer's gift for creating an unforgettable melody, one that yields pride of place to no music, and for developing that melody with inspired mastery. Composers of whom that can be said are few in any generation.