  A d.c. current can be pumped through a quantum dot by periodically varying
two independent parameters $X_1$ and $X_2$, like a gate voltage or magnetic
field. We present a formula that relates the pumped current to the parametric
derivatives of the scattering matrix $S(X_1,X_2)$ of the system. As an
application we compute the statistical distribution of the pumped current in
the case of a chaotic quantum dot.
