  A magnetic field rotating on the free surface of a ferrofluid layer is shown
to induce considerable fluid motion toward the direction the field is rolling.
The measured flow velocity i) increases with the square of the magnetic field
amplitude, ii) is proportional to the thickness of the fluid layer, and iii)
has a maximum at a driving frequency of about 3 kHz. The pumping speed can be
estimated with a two-dimensional flow model.
