Observations of jet dissipation

We summarise the recent progress of a project to determine physical conditions in the jets of low-luminosity (FR I) radio galaxies. We model the jets as intrinsically symmetrical, relativistic, decelerating flows. By fitting to deep, high-resolution VLA images in total intensity and linear polarization, we have been able to derive for the first time the three-dimensional distributions of velocity, rest-frame emissivity and magnetic-field structure. We describe our results for four sources and outline the implications for theories of jet deceleration. We also summarise a conservation-law analysis of 3C 31, which gives profiles of internal pressure, density, Mach number and entrainment rate along the jets, again for the first time. Finally, we discuss the applicability of adiabatic models to FR I jets and identify the regions where distributed particle acceleration is necessary