The music ensemble that calls itself the Wondrous Machine specializes in music of the 18th and early 19th centuries and performs on instruments of the period. Last night's program at St. Paul's Church in Alexandria focused on chamber music of Haydn and Mozart and, in particular, on some of the less usual instrumental combinations those composers wrote.

It so happened that the Mozart pieces, the G Major Duo for Violin and Viola, K. 423 and the oboe quartet are among Mozart's more inspired compositions, while Haydn, unfortunately, was represented by a Baryton Trio arranged for oboe, viola and cello, and a Divertimento for horn, violin and cello, neither vintage Haydn and neither accorded the sort of joyous performance that might have disguised its shortcomings.

The group itself changes its composition from concert to concert to accommodate the varied repertoire it plays. And while this permits programming flexibility, it has resulted in an ensemble that hasn't quite jelled.

The best playing of the evening was in the concluding Mozart quartet where violinist Mary Price sounded more comfortable playing second fiddle to the oboe than she had earlier in the evening in leading roles. Stanley King managed his 18th century reproduction oboe with style and jocularity. And, if he didn't always reach the top notes, he made good music of the others.

The last movement of the Mozart Duo also was played with warmth and charm and a nice sense of dialogue by Price and violist Melissa Graybeal. The Haydn Divertimento, however, congenial enough music, was beset by horn troubles, and the opening Haydn Trio was charming but rather dull.