We make use of extensive HST optical and VLA radio observations of the Seyfert galaxy Markarian 78 to analyse the properties of the jet flow. Our starting assumption is that the jet's power can be inferred from the energy stored in the radio lobe, and the jet's momentum flux can be inferred from the momentum of the ionized gas. Using the properties of three regions, we derive a jet flow power of ∼1040.5 erg s−1 and momentum flux of ∼1033.5 dyne. Assuming the jet contains both thermal and relativistic material, we find that the thermal component dominates both the jet's luminosity and its momentum flux. We find the jet to be mildly transonic, with speed only a few times that of the [OIII] velocities. The jet is, however, quite dense, with sufficient ram pressure to accelerate the ionized gas to the observed velocities. Over the region lifetime, the jet can provide the thermal component of the lobes, and likely does this by entraining ISM material en route. These jet properties seem, at least to us, to be eminently plausible, suggesting an approach of this kind may be appropriate for other radio quiet objects.