  We model the kiloparsec-scale synchrotron emission from jets in 10
Fanaroff-Riley Class I radio galaxies for which we have sensitive,
high-resolution imaging and polarimetry from the Very Large Array. We assume
that the jets are intrinsically symmetrical, axisymmetric, decelerating,
relativistic outflows and we infer their inclination angles and the spatial
variations of their flow velocities, magnetic field structures and emissivities
using a common set of fitting functions. The inferred inclinations agree well
with independent indicators. The spreading rates increase rapidly, then
decrease, in a flaring region. The jets then recollimate to form conical outer
regions at distance r0 from the active galactic nucleus (AGN). The flaring
regions are homologous when scaled by r0. At ~0.1r0, the jets brighten abruptly
at the onset of a high-emissivity region and we find an outflow speed of ~0.8c,
with a uniform transverse profile. Jet deceleration first becomes detectable at
~0.2r0 and the outflow often becomes slower at its edges than it is on-axis.
Deceleration continues until ~0.6r0, after which the outflow speed is usually
constant. The dominant magnetic-field component is longitudinal close to the
AGN and toroidal after recollimation, but the field evolution is initially much
slower than predicted by flux-freezing. In the flaring region, acceleration of
ultrarelativistic particles is required to counterbalance the effects of
adiabatic losses and account for observed X-ray synchrotron emission, but the
brightness evolution of the outer jets is consistent with adiabatic losses
alone. We interpret our results as effects of the interaction between the jets
and their surroundings. (Slightly abridged).
