Pierre Boulez is not the first man to grasp fully the musical potential of the fire siren, but he may be the second.The first was Edgard Varese, who used this instrument first in "Ameriques," the composition for enormous orchestra that occupied him in 1920 and 1921 after he moved to Brooklyn from his native France.

The siren (as well as another odd instrument from "Ameriques" oddly but accurately called the "lion's roar") turns up again in a 1931 Varese work "Ionisation," a revolutionary piece composed entirely for percussion instruments (plus siren), which gave the avant-garde composers of the time (including such diverse types as John Cage and Carlos Chayez) a whole sound concept to play with.

For a bit over half a century, Varese has been known (to those who know him at all) as the wild man of 20th-century music - a much-contested title, but one to which he has been most clearly entitled. Now, charter members of the Varese fan club (there must be at least a half-dozen, including Frank Zappa) may rejoice - a bit ambivalently, perhaps - because their idol is about to become recognized as the classic they have always known he was. With the release of a brilliant album entitled "Boulez Conducts Varese" (Columbia M 34552), Varese seems clearly destined to have the kind of belated boom that the long-playing record has previously bestowed on such diverse composers as Antonio Vivaldi, Gustav Mahler and Charles Ives.

What Boulez has done is to take three of Varese's compositions - "Arcana" (1926-1927) plus the two pieces with fire sirens, "Ameriques" and "Ionisation" - and, by his acute musical perceptions and the pure virtuoso skill of the Philharmonic, integrate them into the mainstream of classical music.

It is probably a question primarily of vision. The notes that a composer puts on paper are a sort of Rorschach ink-blot in which (beyond the basic data of what notes and chords to play, how loud and how fast) the performer sees largely what he wants or expects to see. Varese has not been exactly neglected on records, but in earlier recordings he has usually been performed by musicians who saw in him a wild-eyed bomb-tosser, and that's how they made the music come out. Boulez, himself a composer who has marched along some of the paths, where Varese went first, machete in hand, sees him in a different perspective - as one of modern music's grand old men, a link between the music of today and the music of the past. And with the enormous and very skilled resources of the New York Philharmonic responding superbly to the slightest nuances of his vision, Boulez is able to translate what he sees very precisely into sound for all the rest of us to hear.

The result is a Varese who has roots, and one whom a large audience can assimilate much more easily than the wild man of earlier records. In this interpretation, both of the large orchestral pieces can be seen as firmly rooted in the work of Stravinsky. "Arcana," most notably, comments on the climactic dance from Stravinsky's "Firebird" the way Beethoven does with Mozart, for example, in his Third Piano Concerto, and goes beyond Stravinsky as Beethoven did with Mozart in his Fourth. Boulez links Varese to the classical tradition via the lineage of Rimsky-Korsakov, and suddenly it is earier to see where his music came from and how he advanced the tradition. It is also easier to enjoy the composer, partly because we can see where he fits in but also because the performance is brilliant and the sound overwhelming.

For hard-core Varese fans, another new issue will be equally exciting. It is called "The Varese Record" (Finnadar SR 9018) and it is a reissue, with additions, of the first long-playing record devoted to Varese's music, issued 27 years ago by the small EMS label, which won a "best recording of the year" award at the 1951 Audio Fair. The performance of "Density 21.5" for unaccompanied flute by flutist Rene Le Roy has been surpassed, and most of the other items on the record ("Ionisation," "Octandre" and "Integrales") exist in other recordings of comparable or higher quality, but these performances (with Frederic Waldman conducting the Juilliard Percussion Ensemble and the New York Wind Ensemble) were supervised by the composer and therefore have a special authority, and the record is unquestionably a permanent landmark in the Varese discography. It still sounds remarkably good after all those years.

Even for those who own a treasured copy of the EMS original (long a collector's item), the Finnadar reissue is specially interesting because it contains more than 9 minutes of Varese music otherwise unavailable in this form: the three original tape segments of his "Deserts" for tape and orchestra, which he did not compose until three years after the EMS record was issued. There are the composer's first extant work for tape, his predestined medium, and although they wre composed to be played with an orchestra participating, they stand on their own as interesting independent works. A final attraction on this disc is the excellent liner notes, which include Frank Zappa's nostalgic essay: "Edgard Varese, Idol of My Youth." In its way, this is a more important landmark than the Boulez recording, but the Boulez will do more for the composer's estate.