  The central mass distribution of clusters of galaxies can be inferred from
gravitationally lensed arcs with known redshifts. For the Abell cluster 2218,
this method yields a core mass which is larger by a factor of $2.5 \pm 0.5$
than the value deduced from X-ray observations, under the assumptions that the
gas is supported by thermal pressure and that the cluster is spherical. We show
that a non-thermal pressure support is the most plausible explanation for this
discrepancy. Such a pressure can be naturally provided by strong turbulence and
equipartition magnetic fields ($\sim 50\mu$G) that are tangled on small spatial
scales (<10 kpc). The turbulent and magnetic pressures do not affect the
measured Sunyaev-Zel'dovich effect for this cluster. Intracluster magnetic
fields with a comparable magnitude ($\sim 10^{1-2} \mu$G) have already been
detected by Faraday rotation in other clusters. If generic, a small-scale
equipartition magnetic field should affect the structure of cooling flows and
must be included in X-ray determinations of cluster masses.
