Jeremy Bentham might have put it this way: What is the purpose of free, outdoor, symphony concerts?

Simply this: to bring the world's greatest music to the greatest number. And, to feature some of the world's greatest artists performing it.

To wit, that is exactly what the National Symphony did last night in its fourth and final concert this summer at the U.S. Capitol to a crowd of 25,000.

On the podium was a veteran conductor -- Erich Leinsdorf, a favorite with both the orchestra and audiences.

And as the featured soloist in Beethoven's mighty warhorse, the "Emperor" Concerto, the young Polish pianist Emmanuel Ax etched a performance that was sharply outlined, a bit stark, but not without its moments of tenderness or lyricism.

The concert was distinguished by the Leinsdorf "style" -- no-nonsense music-making, clean articulation of line, no fussiness with tempos.

He opened with a vigorous reading of Carl Maria von Weber's "Oberon" Overture, using appropriately for an outdoor setting, a huge orchestra. The NSO played very well with tight ensemble, Leinsdorf urging the orchestra to play louder than he would in the concert hall.

But both soloist and conductor were too "buttoned up" for the "Emperor," and thus the poetry, indeed the passion of the slow movement, was not captured. The big outer movements, however, had all the bravado and imperial grandeur that justly earned the concerto its sobriquet.

Emmanuel Ax proved to be a sensitive pianist, but this was not the occasion to evalute properly his musicality.

The reason was the sound. In spite of elaborate means to amplify the stage, even at the foot of the stage the orchestra sounded "tinny" and pinched, while the speakers used to carry the sound across the lawn often did so at the expense of clarity and smoothness, especially during crescendos and loud passages. The piano, too, sounded unattractively scratchy in loud passages in the upper register, and was barely audible in quiet episodes in the concerto. This was an engineering problem though, not a musical one.

These drawbacks did not daunt the enthusiasm of the audience, or the effervescense of master of ceremonies Willard Scott, Channel 4's weatherman. Even as the rain began to pour during Leinsdorf's poised reading of Tchaikovsky's "Romeo and Juliet," the crowd stayed on, with umbrellas aloft until the close of "Bolero" by Ravel.