  The nth symmetric product of a Riemann surface carries a natural family of
Kaehler forms, arising from its interpretation as a moduli space of abelian
vortices. We give a new proof of a formula of Manton-Nasir for the cohomology
classes of these forms. Further, we show how these ideas generalise to families
of Riemann surfaces.
  These results help to clarify a conjecture of D. Salamon on the relationship
between Seiberg-Witten theory on 3-manifolds fibred over the circle and
symplectic Floer homology.
