  We present an \xmm mosaic observation of the hot ($kT\sim6.5$ keV) and nearby
($z=0.0881$) relaxed cluster of galaxies A478. We derive precise gas density,
gas temperature, gas mass and total mass profiles up to $12\arcmin$ (about half
of the virial radius $R_{200}$). The gas density profile is highly peaked
towards the center and the surface brightness profile is well fitted by a sum
of three $\beta$--models. The derived gas density profile is in excellent
agreement, both in shape and in normalization, with the published Chandra
density profile (measured within $5\arcmin$ of the center). Projection and PSF
effects on the temperature profile determination are thoroughly investigated.
The derived radial temperature structure is as expected for a cluster hosting a
cooling core, with a strong negative gradient at the cluster center. The
temperature rises from $\sim2$ keV up to a plateau of $\sim6.5$ keV beyond 2'
(i.e. $r>208\rm{kpc}=0.1 R_{200}$, $R_{200}=2.08$ Mpc being the virial radius).
  From the temperature profile and the density profile and under the hypothesis
of hydrostatic equilibrium, we derived the total mass profile of A478 down to
0.01 and up to 0.5 the virial radius. We tested different dark matter models
against the observed mass profile. The Navarro, Frenk & White
(\cite{navarro97}) model is significantly preferred to other models. It leads
to a total mass of $M_{200}=1.1\times 10^{15}$ M$_\odot$ for a concentration
parameter of $c=4.2\pm0.4$. The gas mass fraction slightly increases with
radius. The gas mass fraction at a density contrast of $\delta=2500$ is
$\fgas=0.13\pm0.02$, consistent with previous results on similar hot and
massive clusters. We confirm the excess of absorption in the direction of
A478.[abridged]
