"Ah, Belinda, I am prest" is from Henry Purcell's Dido and Aeneas. This is the first of Dido's tragic arias in this opera. She laments how she is filled with torment and will languish her grief. It is a perfect example of a passacaglia, a form of continuous variation. The bass line consists of a repeating four bar phrase the repeats throughout the entire piece. It moves C-Bnat-C-G-A-E-F-G-G-C. This repeats from msr. 1-44, with a slight alteration at msr. 32, where an extra C and Bnat have been added to the beginning of the line. This theme changes slightly at msr. 45 when the song changes keys and the bass line moves down a fourth. The theme returns to its original key in msr. 53. But at msr. 77, the bass line moves down an octave and the tenor takes over the original bass line, so that the aria finishes with the variation in doubled in the lower parts.
The theme coincides with the vocal line in some places and not in others. In the very beginning of the song, the first vocal phrase enters midway through the bass theme and ends with the theme. (its second time through) This kind of pattern with the vocal line entering in the middle of the bass them and finishing with it is seen throughout the song. But there are places where the vocal line doesn't begin nor end with the theme, like in msr. 11-17.
The vocal line is embellished in many ways to portray Dido's lamenting. She sings many B naturals, which clash with the harmony, adding tension. Purcell uses dotted rhythms, which to me seem like it's here trying to sing without crying, breaking up the line slightly. Purcell also uses A naturals and F sharps, like in msr. 45-47 in the vocal line and harmony to bring out "languish" and "grief". I also like the long line he gives to "languish" in msr. 49-52-the drawing out of the melody on this word really gives the listener the feeling of aching and grieving.
Purcell brilliantly uses the bass line to portray his theme in this song. It is a great example of passacaglia-such a fun word to say-variation. How he manages to write a beautiful aria over a continuous repeating bass line is incredible and is worth noticing.