Here is my transcription of the tune. A two-part design with closely similar endings—rising figures in bars 7-8 and 17-18. Part B has a double ending, a "standard" position for the cadence in 15-16 and the ending in 17-18 that we hear as nearly the same as bars 7-8. The figure in bars 10 and 14 is particularly expressive as it carries the text "tourbillon."
Two graphs: the first divides the tune in half, with a three-part Ursatz in 1-8 and a different one in 9-18. In the latter, the background ^2 is assumed at bar 12, which I grant is not entirely satisfactory as it forces the ^1 to appear at the beginning of the repeated figure (13-14) that held ^3 the first time around (9-10). An alternative would be to imply ^2 in bar 15 and ^1 in bar 16, with the latter only sounded in retrospect in bar 18. Again, not entirely satisfactory.
The second graph draws on the common "one-too-far" figure that is often used in, for example, country dances, at the beginning of the B-section, as here. The association of "one-too-far" with the text at that moment (in one of the verses, at least) "tourbillon" seems apt. The result of isolating ^3 in this way is a reading that seems more in tune, so to speak, with the shapes of the melody.
