  Using the u-plane integral of Moore and Witten, we derive a simple expression
for the Donaldson invariants of $\Sigma_g \times S^2$, where $\Sigma_g$ is a
Riemann surface of genus g. This expression generalizes a theorem of Morgan and
Szabo for g=1 to any genus g. We give two applications of our results: (1) We
derive Thaddeus' formulae for the intersection pairings on the moduli space of
rank two stable bundles over a Riemann surface. (2) We derive the eigenvalue
spectrum of the Fukaya-Floer cohomology of $\Sigma_g \times S^1$.
