  Theories of stellar orbit diffusion in disk galaxies predict different rates
of increase of the velocity dispersions parallel and perpendicular to the disk
plane, and it is therefore of interest to measure the different velocity
dispersion components in galactic disks of different types. We show that it is
possible to extract the three components of the velocity ellipsoid in an
intermediate-inclination disk galaxy from measured line-of-sight velocity
dispersions on the major and minor axes. On applying the method to observations
of the Sb galaxy NGC 488, we find evidence for a higher ratio of vertical to
radial dispersion in NGC 488 than in the solar neighbourhood of the Milky Way
(the only other place where this quantity has ever been measured). The
difference is qualitatively consistent with the notion that spiral structure
has been relatively less important in the dynamical evolution of the disk of
NGC 488 than molecular clouds.
