  Jets from active galactic nuclei are thought to play a role in the evolution
of their host and local environments, but a detailed prescription is limited by
the understanding of the jets themselves. Proper motion studies of compact
bright components in radio jets can be used to produce model-independent
constraints on their Lorentz factor, necessary to understand the quantity of
energy deposited in the inter-galactic medium. We present our initial work on
the jet of radio-galaxy 3C~78, as part of CAgNVAS (Catalogue of proper motions
in Active galactic Nuclei using Very Large Array Studies), with a goal of
constraining nature of jet plasma on larger ($>100$ parsec) scales. In 3C~78 we
find three prominent knots (A, B and C), where knot B undergoes subluminal
longitudinal motion ($\sim0.6c$ at $\sim$ 200 pc), while knot C undergoes
extreme (apparent) backward motion and eventual forward motion ($\sim-2.6c$,
$0.5c$, at $\sim$ 300 pc). Assuming knots are shocks, we infer the bulk speeds
from the pattern motion of Knots B and C. We model the spectral energy
distribution (SED) of the large-scale jet and observe that a physically
motivated two-zone model can explain most of the observed emission. We also
find that the jet profile remains approximately conical from parsec to
kiloparsec scales. Using the parsec-scale speed from VLBI studies ($\sim0.1c$)
and the derived bulk speeds, we find that the jet undergoes bulk acceleration
between the parsec and the kiloparsec scales providing the first direct
evidence of jet acceleration in a conical and matter-dominated jet.
